Publications
2020

(2020) Physical Review B. 102, 3, 035125. Abstract[All authors]
Unconventional quasiparticle excitations in condensed matter systems have become one of the most important research frontiers. Beyond twofold and fourfold degenerate Weyl and Dirac fermions, threefold, sixfold, and eightfold symmetry protected degeneracies have been predicted. However they remain challenging to realize in solid state materials. Here the charge density wave compound TaTe4 is proposed to hold eightfold fermionic excitation and Dirac point in energy bands. High quality TaTe4 single crystals are prepared, where the charge density wave is revealed by directly imaging the atomic structure and a pseudogap of about 45 meV on the surface. Shubnikovde Haas oscillations of TaTe4 are consistent with band structure calculation. Scanning tunneling microscopy/spectroscopy reveals atomic step edge states on the surface of TaTe4. This work uncovers that the charge density wave is able to induce new topological phases and sheds new light on the novel excitations in condensed matter materials.

(2020) Nature Communications. 11, 1, 3476. Abstract[All authors]
Weyl semimetals exhibit unusual surface states and anomalous transport phenomena. It is hard to manipulate the band structure topology of specific Weyl materials. Topological transport phenomena usually appear at very low temperatures, which sets challenges for applications. In this work, we demonstrate the band topology modification via a weak magnetic field in a ferromagnetic Weyl semimetal candidate, Co2MnAl, at room temperature. We observe a tunable, giant anomalous Hall effect (AHE) induced by the transition involving Weyl points and nodal rings. The AHE conductivity is as large as that of a 3D quantum AHE, with the Hall angle (Theta (H)) reaching a record value (tan Theta H=0.21) at the room temperature among magnetic conductors. Furthermore, we propose a material recipe to generate large AHE by gaping nodal rings without requiring Weyl points. Our work reveals an intrinsically magnetic platform to explore the interplay between magnetic dynamics and topological physics for developing spintronic devices.

(2020) Physical Review B. 101, 24, 245146. Abstract
FeSe0.45Te0.55 (FeSeTe) has recently emerged as a promising candidate to host topological superconductivity, with a Dirac surface state and signatures of Majorana bound states in vortex cores. However, correlations strongly renormalize the bands compared to electronic structure calculations, and there is no evidence for the expected bulk band inversion. We present here a comprehensive angle resolved photoemission (ARPES) study of FeSeTe as a function of photon energies ranging from 15100 eV. We find that although the top of the bulk valence band shows essentially no k(z), dispersion, its normalized intensity exhibits a periodic variation with k(z). We show, using ARPES selection rules, that the intensity oscillation is a signature of band inversion indicating a change in the parity going from Gamma to Z. We also present a simple realistic tightbinding model which gives insight into ARPES observations. Thus we provide direct evidence for a topologically nontrivial bulk band structure that supports protected surface states.

(2020) Physical Review B. 101, 18, 184403. Abstract[All authors]
We demonstrate an anomalous spinorbit torque induced by the broken magnetic symmetry in the antiferromagnet IrMn. We study the magnetic structure of three phases of IrMn thin films using neutron diffraction technique. The magnetic mirror symmetry M' is broken laterally in both L1(0)IrMn and L1(2)IrMn3 but not gammaIrMn3. We observe an outofplane dampinglike spinorbit torque in both L1(0)IrMn/permalloy and L1(2)IrMn3/permalloy bilayers but not in gammaIrMn3/permalloy. This is consistent with both the symmetry analysis on the effects of a broken M' on spinorbit torque and the theoretical predictions of the spin Hall effect and the RashbaEdelstein effect. In addition, the measured spinorbit torque efficiencies are 0.61 +/ 0.01, 1.01 +/ 0.03, and 0.80 +/ 0.01 for the L1(0), L1(2), and gamma phases, respectively. Our work highlights the critical roles of the magnetic asymmetry in spinorbit torque generation.

(2020) Science advances. 6, 17, 3522. Abstract[All authors]
The WiedemannFranz (WF) law has been tested in numerous solids, but the extent of its relevance to the anomalous transverse transport and the topological nature of the wave function, remains an open question. Here, we present a study of anomalous transverse response in the noncollinear antiferromagnet Mn3Ge extended from room temperature down to subkelvin temperature and find that the anomalous Lorenz ratio remains close to the Sommerfeld value up to 100 K but not above. The finitetemperature violation of the WF correlation is caused by a mismatch between the thermal and electrical summations of the Berry curvature and not by inelastic scattering. This interpretation is backed by our theoretical calculations, which reveals a competition between the temperature and the Berry curvature distribution. The data accuracy is supported by verifying the anomalous Bridgman relation. The anomalous Lorenz ratio is thus an extremely sensitive probe of the Berry spectrum of a solid.

(2020) Science advances. 6, 10, 0948. Abstract
The layered antiferromagnetic MnBi2Te4 films have been proposed to be an intrinsic quantum anomalous Hall (QAH) insulator with a large gap. It is crucial to open a magnetic gap of surface states. However, recent experiments have observed gapless surface states, indicating the absence of outofplane surface magnetism, and thus, the quantized Hall resistance can only be achieved at the magnetic field above 6 T. We propose to induce outofplane surface magnetism of MnBi2Te4 films via the magnetic proximity with magnetic insulator CrI3. A strong exchange bias of similar to 40 meV originates from the long Cre(g) orbital tails that hybridize strongly with Te p orbitals. By stabilizing surface magnetism, the QAH effect can be realized in the MnBi2Te4/CrI3 heterostructure. Moreover, the highChern number QAH state can be achieved by controlling external electric gates. Thus, the MnBi2Te4/CrI3 heterostructure provides a promising platform to realize the electrically tunable zerofield QAH effect.

(2020) Nature Materials. Abstract[All authors]
Bi2TeI is identified as a dual topological insulator. It is a weak topological insulator with metallic states at the (010) surfaces and a topological crystalline insulator at the (001) surfaces.Dual topological materials are unique topological phases that host coexisting surface states of different topological nature on the same or on different material facets. Here, we show that Bi2TeI is a dual topological insulator. It exhibits band inversions at two time reversal symmetry points of the bulk band, which classify it as a weak topological insulator with metallic states on its 'side' surfaces. The mirror symmetry of the crystal structure concurrently classifies it as a topological crystalline insulator. We investigated Bi2TeI spectroscopically to show the existence of both twodimensional Dirac surface states, which are susceptible to mirror symmetry breaking, and onedimensional channels that reside along the step edges. Their mutual coexistence on the step edge, where both facets join, is facilitated by momentum and energy segregation. Our observation of a dual topological insulator should stimulate investigations of other dual topology classes with distinct surface manifestations coexisting at their boundaries.

(2020) National Science Review. 7, 3, p. 579587 Abstract[All authors]
The search for unconventional superconductivity in Weyl semimetal materials is currently an exciting pursuit, since such superconducting phases could potentially be topologically nontrivial and host exotic Majorana modes. The layered material TaIrTe4 is a newly predicted timereversal invariant type II Weyl semimetal with the minimum number of Weyl points. Here, we report the discovery of surface superconductivity in Weyl semimetal TaIrTe4. Our scanning tunneling microscopy/spectroscopy (STM/STS) visualizes Fermi arc surface states of TaIrTe4 that are consistent with the previous angleresolved photoemission spectroscopy results. By a systematic study based on STS at ultralow temperature, we observe uniform superconducting gaps on the sample surface. The superconductivity is further confirmed by electrical transport measurements at ultralow temperature, with an onset transition temperature (Tc) up to 1.54 K being observed. The normalized upper critical field h*(T/Tc) behavior and the stability of the superconductivity against the ferromagnet indicate that the discovered superconductivity is unconventional with the pwave pairing. The systematic STS, and thickness and angulardependent transport measurements reveal that the detected superconductivity is quasi1D and occurs in the surface states. The discovery of the surface superconductivity in TaIrTe4 provides a new novel platform to explore topological superconductivity and Majorana modes.

(2020) Nature Photonics. Abstract[All authors]
Strongfielddriven electric currents in condensedmatter systems are opening new frontiers in petahertz electronics. In this regime, new challenges are arising as the roles of band structure and coherent electronhole dynamics have yet to be resolved. Here, by using highharmonic generation spectroscopy, we reveal the underlying attosecond dynamics that dictates the temporal evolution of carriers in multiband solidstate systems. We demonstrate that when the electronhole relative velocity approaches zero, enhanced constructive interference leads to the appearance of spectral caustics in the highharmonic generation spectrum. We introduce the role of the dynamical joint density of states and identify its mapping into the spectrum, which exhibits singularities at the spectral caustics. By studying these singularities, we probe the structure of multiple unpopulated high conduction bands.
2019

(2019) Physical Review X. 9, 4, 041061. Abstract[All authors]
Intrinsic anomalous Nernst effect, like its Hall counterpart, is generated by Berry curvature of electrons in solids. Little is known about its response to disorder. In contrast, the link between the amplitude of the ordinary Nernst coefficient and the meanfree path is extensively documented. Here, by studying Co3Sn2S2, a topological halfmetallic semimetal hosting sizable and recognizable ordinary and anomalous Nernst responses, we demonstrate an anticorrelation between the amplitudes of carrier mobility and the anomalous Sxy(A) (the ratio of transverse electric field to the longitudinal temperature gradient in the absence of magnetic field). We argue that the observation, paradoxically, establishes the intrinsic origin of the anomalous Nernst effect in this system. We conclude that various intrinsic offdiagonal coefficients are set by the way the Berry curvature is averaged on a grid involving the meanfree path, the Fermi wavelength, and the de Broglie thermal length.

(2019) Science advances. 5, 11, 6996. Abstract
The growing diversity of topological classes leads to ambiguity between classes that share similar boundary phenomenology. This is the status of bulk bismuth. Recent studies have classified it as either a strong or a higherorder topological insulator, both of which host helical modes on their boundaries. We resolve the topological classification of bismuth by spectroscopically mapping the response of its boundary modes to a screwdislocation. We find that the onedimensional mode, on stepedges, extends over a wide energy range and does not open a gap near the screwdislocations. This signifies that this mode binds to the screwdislocation, as expected for a material with nonzero weak indices. We argue that the small energy gap, at the time reversal invariant momentum L, positions bismuth within the critical region of a topological phase transition between a higherorder topological insulator and a strong topological insulator with nonzero weak indices.

(2019) Physical Review Letters. 123, 18, 186401. Abstract
In recent years, transition metal dichalcogenides (TMDs) have garnered great interest as topological materials. In particular, monolayers of centrosymmetric betaphase TMDs have been identified as 2D topological insulators (TIs), and bulk crystals of noncentrosymmetric gammaphase MoTe2 and WTe2 have been identified as typeII Weyl semimetals. However, angleresolved photoemission spectroscopy and STM probes of these semimetals have revealed huge, arclike surface states that overwhelm, and are sometimes mistaken for, the much smaller topological surface Fermi arcs of bulk typeII Weyl points. In this Letter, we calculate the bulk and surface electronic structure of both beta and gammaMoTe2. We find that betaMoTe2 is, in fact, a Z(4)nontrivial higherorder TI (HOTI) driven by double band inversion and exhibits the same surface features as gammaMoTe2 and gammaWTe2. We discover that these surface states are not topologically trivial, as previously characterized by the research that differentiated them from the Weyl Fermi arcs but, rather, are the characteristic split and gapped fourfold Dirac surface states of a HOTI. In betaMoTe2, this indicates that it would exhibit helical pairs of hinge states if it were bulk insulating, and in gammaMoTe2 and gammaWTe2, these surface states represent vestiges of HOTI phases without inversion symmetry that are nearby in parameter space. Using nested Wilson loops and firstprinciples calculations, we explicitly demonstrate that, when the Weyl points in gammaMoTe2 are annihilated, which may be accomplished by symmetrypreserving strain or lattice distortion, gammaMoTe2 becomes a nonsymmetryindicated, noncentrosymmetric HOTI. We also show that, when the effects of spinorbit coupling are neglected, betaMoTe2 is a nodalline semimetal with Z(2)nontrivial monopole nodal lines (MNLSM). This finding confirms that MNLSMs driven by double band inversion are the weakspinorbit coupling limit of HOTIs, implying that MNLSMs are higherorder topological semimetals with flatbandlike hinge states, which we find to originate from the corner modes of 2D "fragile" TIs.

(2019) Science. 365, 6459, p. 12861291 Abstract[All authors]
Bulksurface correspondence in Weyl semimetals ensures the formation of topological " Fermi arc" surface bands whose existence is guaranteed by bulk Weyl nodes. By investigating three distinct surface terminations of the ferromagnetic semimetal Co3Sn2S2, we verify spectroscopically its classification as a timereversal symmetrybroken Weyl semimetal. We show that the distinct surface potentials imposed by three different terminations modify the Fermiarc contour and Weyl node connectivity. On the tin (Sn) surface, we identify intraBrillouin zone Weyl node connectivity of Fermi arcs, whereas on cobalt (Co) termination, the connectivity is across adjacent Brillouin zones. On the sulfur (S) surface, Fermi arcs overlap with nontopological bulk and surface states. We thus resolve both topologically protected and nonprotected electronic properties of a Weyl semimetal.

(2019) Nature Communications. 10, 3783. Abstract
The bulk photovoltaic effect (BPVE) rectifies light into the dc current in a singlephase material and attracts the interest to design highefficiency solar cells beyond the pn junction paradigm. Because it is a hot electron effect, the BPVE surpasses the thermodynamic ShockleyQueisser limit to generate abovebandgap photovoltage. While the guiding principle for BPVE materials is to break the crystal centrosymmetry, here we propose a magnetic photogalvanic effect (MPGE) that introduces the magnetism as a key ingredient and induces a giant BPVE. The MPGE emerges from the magnetisminduced asymmetry of the carrier velocity in the band structure. We demonstrate the MPGE in a layered magnetic insulator CrI3, with much larger photoconductivity than any previously reported results. The photocurrent can be reversed and switched by controllable magnetic transitions. Our work paves a pathway to search for magnetic photovoltaic materials and to design switchable devices combining magnetic, electronic, and optical functionalities.

(2019) Nature Communications. 10, 3478. Abstract[All authors]
Surface Fermi arcs (SFAs), the unique open Fermisurfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFAmediated quantum oscillations, chiral magnetic effects, threedimensional quantum Hall effect, nonlocal voltage generation and anomalous electromagnetic wave transmission. Here, by using insitu surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology.

(2019) Nature Communications. 10, 2475. Abstract[All authors]
Weyl and Dirac fermions have created much attention in condensed matter physics and materials science. Recently, several additional distinct types of fermions have been predicted. Here, we report ultrahigh electrical conductivity in MoP at low temperature, which has recently been established as a triple point fermion material. We show that the electrical resistivity is 6 n Omega cm at 2 K with a large mean free path of 11 microns. de Haasvan Alphen oscillations reveal spin splitting of the Fermi surfaces. In contrast to noble metals with similar conductivity and number of carriers, the magnetoresistance in MoP does not saturate up to 9 T at 2 K. Interestingly, the momentum relaxing time of the electrons is found to be more than 15 times larger than the quantum coherence time. This difference between the scattering scales shows that momentum conserving scattering dominates in MoP at low temperatures.

(2019) Science advances. 5, 5, 6696. Abstract[All authors]
Spinorbit torque (SOT) offers promising approaches to developing energyefficient memory devices by electric switching of magnetization. Compared to other SOT materials, metallic antiferromagnet (AFM) potentially allows the control of SOT through its magnetic structure. Here, combining the results from neutron diffraction and spintorque ferromagnetic resonance experiments, we show that the magnetic structure of epitaxially grown L1(0)IrMn (a collinear AFM) is distinct from the widely presumed bulk one. It consists of twin domains, with the spin axes orienting toward [111] and [111], respectively. This unconventional magnetic structure is responsible for much larger SOT efficiencies up to 0.60 +/ 0.04, compared to 0.083 +/ 0.002 for the polycrystalline IrMn. Furthermore, we reveal that this magnetic structure induces a large isotropic bulk contribution and a comparable anisotropic interfacial contribution to the SOT efficiency. Our findings shed light on the critical roles of bulk and interfacial antiferromagnetism to SOT generated by metallic AFM.

(2019) Physical Review B. 99, 16, 165418. Abstract
In principle the stacking of different twodimensional (2D) materials allows the construction of 3D systems with entirely new electronic properties. Here we propose to realize topological crystalline insulators (TCI) protected by mirror symmetry in heterostructures consisting of graphene monolayers separated by twodimensional polar spacers. The polar spacers are arranged such that they can induce an alternating doping and/or spinorbit coupling in the adjacent graphene sheets. When spinorbit coupling dominates, the nontrivial phase arises due to the fact that each graphene sheet enters a quantum spinHall phase. Instead, when the graphene layers are electron and hole doped in an alternating fashion, a uniform magnetic field leads to the formation of quantum Hall phases with opposite Chern numbers. It thus has the remarkable property that unlike previously proposed and observed TCIs, the nontrivial topology is generated by an external timereversal breaking perturbation.

(2019) Science advances. 5, 4, 8575. Abstract
The spin Hall effect (SHE) is the conversion of charge current to spin current, and nonmagnetic metals with large SHEs are extremely sought after for spintronic applications, but their rarity has stifled widespread use. Here, we predict and explain the large intrinsic SHE in betaW and the A15 family of superconductors: W3Ta, Ta3Sb, and Cr3Ir having spin Hall conductivities (SHCs) of 2250, 1400, and 1210 h/e (S/cm), respectively. Combining concepts from topological physics with the dependence of the SHE on the spin Berry curvature (SBC) of the electronic bands, we propose a simple strategy to rapidly search for materials with large intrinsic SHEs based on the following ideas: High symmetry combined with heavy atoms gives rise to multiple Diraclike crossings in the electronic structure; without sufficient symmetry protection, these crossings gap due to spinorbit coupling; and gapped crossings create large SBC.

(2019) Nano Letters. 19, 1, p. 197202 Abstract[All authors]
The airstable and highmobility twodimensional (2D) Bi2O2Se semiconductor has emerged as a promising alternative that is complementary to graphene, MoS2, and black phosphorus for nextgeneration digital applications. However, the roomtemperature residual charge carrier concentration of 2D Bi2O2Se nanoplates synthesized so far is as high as about 10(19)10(20) cm(3), which results in a poor electrostatic gate control and unsuitable threshold voltage, detrimental to the fabrication of 0 highperformance lowpower devices. Here, we first present a facile approach for synthesizing 2D Bi2O2Se single crystals with ultralow carrier concentration of similar to 10(16) cm(3) and high Hall mobility up to 410 cm(2) V1 s(1) simultaneously at room temperature. With optimized conditions, these highmobility and lowcarrierconcentration 2D Bi2O2Se nanoplates with domain sizes greater than 250 m and thicknesses down to 4 layers (similar to 2.5 nm) were readily grown by using Se and Bi2O3 powders as coevaporation sources in a dual heating zone chemical vapor deposition (CVD) system. Highquality 2D Bi2O2Se crystals were fabricated into highperformance and lowpower transistors, showing excellent current modulation of >10(6), robust current saturation, and low threshold voltage of 0.4 V. All these features suggest 2D Bi2O2Se as an alternative option for highperformance lowpower digital applications.
2018

(2018) Physical Review B. 98, 20, 205419. Abstract[All authors]
We have investigated the atomic and electronic structure of the (root 3x root 3)R30 degrees SnAu2/Au(111) surface alloy. Lowenergy electron diffraction and scanning tunneling microscopy measurements show that the native herringbone reconstruction of bare Au(111) surface remains intact after formation of a longrange ordered (root 3x root 3)R30 degrees SnAu2/Au(111) surface alloy. Angleresolved photoemission and twophoton photoemission spectroscopy techniques reveal Rashbatype spinsplit bands in the occupied valence band with comparable momentum space splitting as observed for the Au(111) surface state, but with a holelike parabolic dispersion. Our experimental findings are compared with density functional theory (DFT) calculation that fully support our experimental findings Taking advantage of the good agreement between our DFT calculations and the experimental results, we are able to extract that the occupied SnAu hybrid band is of (s, d)orbital character, while the unoccupied SnAu hybrid bands are of (p, d)orbital character. Hence we can conclude that the Rashbatype spin splitting of the holelike SnAu hybrid surface state is caused by the significant mixing of Au d with Sn s states in conjunction with the strong atomic spinorbit coupling of Au. i.e., of the substrate.

(2018) 2D Materials. 5, 4, 044001. Abstract
We studied the nonlinear electric response in WTe2 and MoTe2 monolayers. When the inversion symmetry is breaking but the the timereversal symmetry is preserved, a secondorder Hall effect called the nonlinear anomalous Hall effect (NLAHE) emerges owing to the nonzero Berry curvature on the nonequilibrium Fermi surface. We reveal a strong NLAHE with a Hallvoltage that is quadratic with respect to the longitudinal current. The optimal current direction is normal to the mirror plane in these twodimensional (2D) materials. The NLAHE can be sensitively tuned by an outofplane electric field, which induces a transition from a topological insulator to a normal insulator. Crossing the critical transition point, the magnitude of the NLAHE increases, and its sign is reversed. Our work paves the way to discover exotic nonlinear phenomena in inversionsymmetrybreaking 2D materials.

(2018) Science advances. 4, 9, 8355. Abstract[All authors]
Semiconductors are essential materials that affect our everyday life in the modern world. Twodimensional semiconductors with high mobility and moderate bandgap are particularly attractive today because of their potential application in fast, lowpower, and ultrasmall/thin electronic devices. We investigate the electronic structures of a new layered airstable oxide semiconductor, Bi2O2Se, with ultrahigh mobility (similar to 2.8 x 10(5) cm(2)/V.s at 2.0 K) and moderate bandgap (similar to 0.8 eV). Combining angleresolved photoemission spectroscopy and scanning tunneling microscopy, we mapped out the complete band structures of Bi2O2Se with key parameters (for example, effective mass, Fermi velocity, and bandgap). The unusual spatial uniformity of the bandgap without undesired ingap states on the sample surface with up to similar to 50% defects makes Bi2O2Se an ideal semiconductor for future electronic applications. In addition, the structural compatibility between Bi2O2Se and interesting perovskite oxides (for example, cuprate hightransition temperature superconductors and commonly used substrate material SrTiO3) further makes heterostructures between Bi2O2Se and these oxides possible platforms for realizing novel physical phenomena, such as topological superconductivity, Josephson junction fieldeffect transistor, new superconducting optoelectronics, and novel lasers.

(2018) New Journal of Physics. 20, 073028. Abstract
The spin Hall effect (SHE), which converts a charge current into a transverse spin current, has long been believed to be a phenomenon induced by spinorbit coupling. Here, we identify an alternative mechanism to realize the intrinsic SHE through a noncollinear magnetic structure that breaks the spin rotation symmetry. No spinorbit coupling is needed even when the scalar spin chirality vanishes, different from the case of the topological Hall effect and topological SHE reported previously. In known noncollinear antiferromagnetic compounds Mn3X (X = Ga, Ge, and Sn), for example, we indeed obtain large spin Hall conductivities based on ab initio calculations.

(2018) Advanced Materials. 30, 41, 1707628. Abstract
Exotic electronic states are realized in novel quantum materials. This field is revolutionized by the topological classification of materials. Such compounds necessarily host unique states on their boundaries. Scanning tunneling microscopy studies of these surface states have provided a wealth of spectroscopic characterization, with the successful cooperation of ab initio calculations. The method of quasiparticle interference imaging proves to be particularly useful for probing the dispersion relation of the surface bands. Herein, how a variety of additional fundamental electronic properties can be probed via this method is reviewed. It is demonstrated how quasiparticle interference measurements entail mesoscopic size quantization and the electronic phase coherence in semiconducting nanowires; helical spin protection and energymomentum fluctuations in a topological insulator; and the structure of the Bloch wave function and the relative insusceptibility of topological electronic states to surface potential in a topological Weyl semimetal.

(2018) Applied Physics Letters. 112, 24, 243103. Abstract
Recent years have seen the rising importance of interface stacking in determining the electronic properties of multilayer materials stemming from the interlayer coupling; however, the stacking effects on exotic topological quantum orders largely remain to be explored. Here, we show by firstprinciples studies that bilayer Bi2Te3 host stacking is dependent on quantum spin Hall effects, with a topological phase transition induced by a change in the interlayer stacking pattern. The spinfiltered helical edge states are concomitantly switched on/off along with the changing interlayer stacking pattern. Since fewlayer Bi2Te3 has already been experimentally synthesized, the present finding opens an avenue for exploring the fundamental mechanisms and the practical implications of the quantum phenomena associated with band topology in this versatile and intriguing 2D material. Published by AIP Publishing.

(2018) Physical Review B. 95, 24, 241203. Abstract
Recently, an airstable layered semiconductor Bi2O2Se was discovered to exhibit an ultrahigh mobility in transistors fabricated with its thin layers. In this work, we explored the mechanism that induces the high mobility and distinguishes Bi2O2Se from other semiconductors. We found that the electron donor states lie above the lowest conduction band. Thus, electrons get spontaneously ionized from donor sites (e.g., Se vacancies) without involving the thermal activation, different from the donor ionization in conventional semiconductors. Consequently, the resistance decreases as reducing the temperature as observed in our measurement, which is similar to a metal but contrasts to a usual semiconductor. Furthermore, the electron conduction channels locate spatially away from ionized donor defects (Se vacancies) in different van der Waals layers. Such a spatial separation can strongly suppress the scattering caused by donor sites and subsequently increase the electron mobility, especially at the low temperature. We call this highmobility mechanism selfmodulation doping, i.e., the modulation doping spontaneously happening in a singlephase material without requiring a heterojunction. Our work paves a way to design highmobility semiconductors with layered materials.

(2018) Physical Review B. 97, 24, 241118. Abstract
Using firstprinciples calculations, we investigate the photogalvanic effect in the Weyl semimetal material TaAs. We find colossal photocurrents caused by the Weyl points in the band structure in a wide range of laser frequency. Our calculations reveal that the photocurrent is predominantly contributed by the threeband transition from the occupied Weyl band to the empty Weyl band via an intermediate band away from the Weyl cone, for excitations both by linearly and circularly polarized light. Therefore, it is essential to sum over all threeband transitions by considering a full set of Bloch bands (both Weyl bands and trivial bands) in the firstprinciples band structure while it does not suffice to only consider the twoband direct transition within a Weyl cone. The calculated photoconductivities are well consistent with recent experiment measurements. Our work provides a firstprinciples calculation on nonlinear optical phenomena of Weyl semimetals and provides a deeper understanding of the photogalvanic effects in complexed materials.

(2018) New Journal of Physics. 20, 043008. Abstract[All authors]
Threedimensional topological semimetals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of spacetime and thus new types of fermionic excitations that explicitly violate Lorentzinvariance have been proposed, the socalled typeII Dirac fermions. We investigate the electronic spectrum of the transitionmetal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a typeII Dirac node to occur in this material. A hole and an electronlike Fermi surface dominate the semimetal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of typeII Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Diracfermion excitations.

(2018) Nature Physics. 14, 3, p. 242251 Abstract
The recent demonstrations of electrical manipulation and detection of antiferromagnetic spins have opened up a new chapter in the story of spintronics. Here, we review the emerging research field that is exploring the links between antiferromagnetic spintronics and topological structures in real and momentum space. Active topics include proposals to realize Majorana fermions in antiferromagnetic topological superconductors, to control topological protection and Dirac points by manipulating antiferromagnetic order parameters, and to exploit the anomalous and topological Hall effects of zeronetmoment antiferromagnets. We explain the basic concepts behind these proposals, and discuss potential applications of topological antiferromagnetic spintronics.

(2018) Physical Review B. 97, 7, 075429. Abstract
We introduce a class of twodimensional (2D) materials that possess coexisting ferroelectric and topologically insulating orders. Such ferroelectric topological insulators (FETIs) occur in noncentrosymmetric atomic layer structures with strong spinorbit coupling (SOC). We showcase a prototype 2D FETI in an atomically thin bismuth layer functionalized by CH2OH, which exhibits a large ferroelectric polarization that is switchable by a ligand molecule rotation mechanism and a strong SOC that drives a band inversion leading to the topologically insulating state. An external electric field that switches the ferroelectric polarization also tunes the spin texture in the underlying atomic lattice. Moreover, the functionalized bismuth layer exhibits an additional quantum order driven by the valley splitting at the K and K' points in the Brillouin zone stemming from the symmetry breaking and strong SOC in the system, resulting in a remarkable state of matter with the simultaneous presence of the quantum spin Hall and quantum valley Hall effect. These phenomena are predicted to exist in other similarly constructed 2D FETIs, thereby offering a unique quantum material platform for discovering novel physics and exploring innovative applications.

(2018) Physical Review B. 97, 4, 041101. Abstract
Noncentrosymmetric metals are anticipated to exhibit a dc photocurrent in the nonlinear optical response caused by the Berry curvature dipole in momentum space. Weyl semimetals (WSMs) are expected to be excellent candidates for observing these nonlinear effects because they carry a large Berry curvature concentrated in small regions, i.e., near the Weyl points. We have implemented the semiclassical Berry curvature dipole formalism into an ab initio scheme and investigated the secondorder nonlinear response for two representative groups of materials: the TaAsfamily typeI WSMs and the MoTe2family typeII WSMs. Both types of WSMs exhibited a Berry curvature dipole in which typeII Weyl points are usually superior to the typeI WSM because of the strong tilt. Corresponding nonlinear susceptibilities in several materials promise a nonlinear Hall effect in the dc field limit, which is within the experimentally detectable range.

(2018) Advances in Physics. 3, 1, 1414631. Abstract
The realization of Dirac and Weyl physics in solids has made topological materials one of the main focuses of condensed matter physics. Recently, the topic of topological nodal line semimetals, materials in which Dirac or Weyllike crossings along special lines in momentum space create either a closed ring or line of degeneracies, rather than discrete points, has become a hot topic in topological quantum matter. Here, we review the experimentally confirmed and theoretically predicted topological nodal line semimetals, focusing in particular on the symmetry protection mechanisms of the nodal lines in various materials. Three different mechanisms: a combination of inversion and timereversal symmetry, mirror reflection symmetry, and nonsymmorphic symmetry and their robustness under the effect of spin orbit coupling are discussed. We also present a new Weyl nodal line material, the Tesquare net compound KCu. Finally, we discuss potential experimental signatures for observing exotic properties of nodal line physics.[GRAPHICS].
2017

(2017) Nat Commun. 8, 1642. Abstract[All authors]
The peculiar band structure of semimetals exhibiting Dirac and Weyl crossings can lead to spectacular electronic properties such as large mobilities accompanied by extremely high magnetoresistance. In particular, two closely neighboring Weyl points of the same chirality are protected from annihilation by structural distortions or defects, thereby significantly reducing the scattering probability between them. Here we present the electronic properties of the transition metal diphosphides, WP2 and MoP2, which are typeII Weyl semimetals with robust Weyl points by transport, angle resolved photoemission spectroscopy and first principles calculations. Our single crystals of WP2 display an extremely low residual lowtemperature resistivity of 3 n Omega cm accompanied by an enormous and highly anisotropic magnetoresistance above 200 million % at 63 T and 2.5 K. We observe a large suppression of charge carrier backscattering in WP2 from transport measurements. These properties are likely a consequence of the novel Weyl fermions expressed in this compound.

(2017) Phys. Rev. Lett.. 119, 18, Abstract
Noncollinear antiferromagnets, such as Mn3 Sn and Mn3 Ir, were recently shown to be analogous to ferromagnets in that they have a large anomalous Hall effect. Here we show that these materials are similar to ferromagnets in another aspect: the charge current in these materials is spin polarized. In addition, we show that the same mechanism that leads to the spinpolarized current also leads to a transverse spin current, which has a distinct symmetry and origin from the conventional spin Hall effect. We illustrate the existence of the spinpolarized current and the transverse spin current by performing ab initio microscopic calculations and by analyzing the symmetry. We discuss possible applications of these novel spin currents, such as an antiferromagnetic metallic or tunneling junction.

(2017) Physical Review B. 96, 16, 165143. Abstract[All authors]
Topological insulators represent unusual topological quantum states, typically with gapped bulk band structure but gapless surface Dirac fermions protected by timereversal symmetry. Recently, a distinct kind of topological insulator resulting from nonsymmorphic crystalline symmetry was proposed in the KHgX (X = As, Sb, Bi) compounds. Unlike regular topological crystalline insulators, the nonsymmorphic glidereflection symmetry in KHgX guarantees the appearance of an exotic surface fermion with hourglass shape dispersion (where two pairs of branches switch their partners) residing on its (010) side surface, contrasting to the usual twodimensional Dirac fermion form. Here, by using highresolution angleresolved photoemission spectroscopy, we systematically investigate the electronic structures of KHgSb on both (001) and (010) surfaces and reveal the unique ingap surface states on the (010) surface with delicate dispersion consistent with the ``hourglass Fermion'' recently proposed. Our experiment strongly supports that KHgSb is a nonsymmorphic topological crystalline insulator with hourglass fermions, which serves as an important step to the discovery of unique topological quantum materials and exotic fermions protected by nonsymmorphic crystalline symmetry.

(2017) Physical Review B. 96, 16, 165113. Abstract
A Weyl semimetal discovered recently, NbP, exhibits two groups of Weyl points with one group lying inside the k(z) = 0 plane and the other group staying away from this plane. All Weyl points have been assumed to be type I, in which the Fermi surface (Fs) shrinks into a point as the Fermi energy crosses the Weyl point. In this paper, we have revealed that the second group of Weyl points are actually type II, which are found to be touching points between the electron and hole pockets in the FS. Corresponding Weyl cones are strongly tilted along a line approximately 17 degrees off the k(z) axis in the k(x)  k(z) (or k(y)  k(z)) plane, violating the Lorentz symmetry but still giving rise to Fermi arcs on the surface. Therefore, NbP exhibits both typeI (k(z) = 0 plane) and typeII (k(z) not equal 0 plane) Weyl points.

(2017) Physical Review Letters. 119, 13, 136401. Abstract
We predict the existence of triple point fermions in the band structure of several halfHeusler topological insulators by ab initio calculations and the Kane model. We find that many halfHeusler compounds exhibit multiple triple points along four independent C3 axes, through which the doubly degenerate conduction bands and the nondegenerate valence band cross each other linearly nearby the Fermi energy. When projected from the bulk to the (111) surface, most of these triple points are located far away from the surface (Gamma) over bar point, as distinct from previously reported triple point fermion candidates. These isolated triple points give rise to Fermi arcs on the surface, that can be readily detected by photoemission spectroscopy or scanning tunneling spectroscopy.

(2017) Nature. 547, 7663, p. 324+ Abstract[All authors]
The conservation laws, such as those of charge, energy and momentum, have a central role in physics. In some special cases, classical conservation laws are broken at the quantum level by quantum fluctuations, in which case the theory is said to have quantum anomalies(1). One of the most prominent examples is the chiral anomaly(2,3), which involves massless chiral fermions. These particles have their spin, or internal angular momentum, aligned either parallel or antiparallel with their linear momentum, labelled as left and right chirality, respectively. In three spatial dimensions, the chiral anomaly is the breakdown (as a result of externally applied parallel electric and magnetic fields(4)) of the classical conservation law that dictates that the number of massless fermions of each chirality are separately conserved. The current that measures the difference between left and righthanded particles is called the axial current and is not conserved at the quantum level. In addition, an underlying curved spacetime provides a distinct contribution to a chiral imbalance, an effect known as the mixed axialgravitational anomaly(1), but this anomaly has yet to be confirmed experimentally. However, the presence of a mixed gaugegravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field(5,6), even in flat spacetime, suggesting that such types of mixed anomaly could be experimentally probed in condensed matter systems known as Weyl semimetals(7). Here, using a temperature gradient, we observe experimentally a positive magnetothermoelectric conductance in the Weyl semimetal niobium phosphide (NbP) for collinear temperature gradients and magnetic fields that vanishes in the ultraquantum limit, when only a single Landau level is occupied. This observation is consistent with the presence of a mixed axialgravitational anomaly, providing clear evidence for a theoretical concept that has so far eluded experimental detection.

(2017) Journal of the American Chemical Society. 139, 24, p. 81068109 Abstract
We report superconductive iridium pnictides BaxIr4X12 (X = As and P) with a filled skutterudite structure, demonstrating that Ba filling dramatically alters their electronic properties and induces a nonmetalto metal transition with increasing, the Ea content x. The highest superconducting transition temperatures are 4.8 and 5.6 K observed for BaxIr4As12 and BaxIr4P12, respectively. The superconductivity in BaxIr4X12 can be classified into the BardeenCooperSchrieffer type with intermediate coupling.

(2017) Physical Review B. 95, 23, 235104. Abstract
We have found Dirac nodal lines (DNLs) in the band structures of metallic rutile oxides IrO2, OsO2, and RuO2 and have revealed a large spin Hall conductivity contributed by these nodal lines, which explains a strong spin Hall effect (SHE) of IrO2 discovered recently. Two types of DNLs exist. The first type forms DNL networks that extend in the whole Brillouin zone and appears only in the absence of spinorbit coupling (SOC), which induces surface states on the boundary. Because of SOCinduced band anticrossing, a large intrinsic SHE can be realized in these compounds. The second type appears at the Brillouin zone edges and is stable against SOC because of the protection of nonsymmorphic symmetry. Besides reporting these DNL materials, our work reveals the general relationship between DNLs and the SHE, indicating a way to apply Dirac nodal materials for spintronics.

(2017) Physical Review B. 95, 23, 235158. Abstract
In this work, we construct a generalized Kane model with a coupling term between itinerant electron spins and local magnetic moments of antiferromagnetic ordering in order to describe the lowenergy effective physics in a large family of antiferromagnetic halfHeusler materials. The topological properties of this generalized Kane model are studied and a large variety of topological phases, including the Dirac semimetal phase, Weyl semimetal phase, nodal line semimetal phase, typeB triple point semimetal phase, topological mirror (or glide) insulating phase, and antiferromagnetic topological insulating phase, are identified in different parameter regions of our effective models. In particular, we find that the system is always driven into the antiferromagnetic topological insulator phase once a bulk band gap is open, irrespective of the magnetic moment direction, thus providing a robust realization of antiferromagentic topological insulators. Furthermore, we discuss the possible realization of these topological phases in realistic antiferromagnetic halfHeusler materials. Our effective model provides a basis for the future study of physical phenomena in this class of materials.

(2017) Physical Review Letters. 118, 23, 236403. Abstract[All authors]
The Ta181 quadrupole resonance [nuclear quadrupole resonance (NQR)] technique is utilized to investigate the microscopic magnetic properties of the Weyl semimetal TaP. We find three zerofield NQR signals associated with the transition between the quadrupole split levels for Ta with I = 7/2 nuclear spin. A quadrupole coupling constant, nu(Q) = 19.250 MHz, and an asymmetric parameter of the electric field gradient, eta = 0.423, are extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spinlattice relaxation rate (1/T1T) is measured for the f(2) line (+/ 5/2 +/ 3/2 transition). We find that there exist two regimes with quite different relaxation processes. Above T* approximate to 30 K, a pronounced (1/T1T) proportional to T2 behavior is found, which is attributed to the magnetic excitations at the Weyl nodes with temperaturedependent orbital hyperfine coupling. Below T*, the relaxation is mainly governed by a Korringa process with 1/T1T = const, accompanied by an additional T1/2type dependence to fit our experimental data. We show that Ta NQR is a novel probe for the bulk Weyl fermions and their excitations.

High electron mobility and quantum oscillations in nonencapsulated ultrathin semiconducting Bi2O2Se(2017) Nature Nanotechnology. 12, 6, p. 530+ Abstract[All authors]
Highmobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new twodimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research(19). However, airstable ultrathin semiconducting materials with superior performances remain elusive at present(10). Here, we report ultrathin films of nonencapsulated layered Bi2O2Se, grown by chemical vapour deposition, which demonstrate excellent air stability and highmobility semiconducting behaviour. We observe bandgap values of similar to 0.8 eV, which are strongly dependent on the film thickness due to quantumconfinement effects. An ultrahigh Hall mobility value of > 20,000 cm(2) V1 s(1) is measured in asgrown Bi2O2Se nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition(11) and at the LaAlO3SrTiO3 interface(12), making the detection of Shubnikovde Haas quantum oscillations possible. Topgated fieldeffect transistors based on Bi2O2Se crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm(2) V1 s(1)), large current on/off ratios (> 10(6)) and nearideal subthreshold swing values (similar to 65 mV dec(1)) at room temperature. Our results make Bi2O2Se a promising candidate for future highspeed and lowpower electronic applications.

(2017) Advanced Materials. 29, 18, 1605965. Abstract[All authors]
A pressureinduced topological quantum phase transition has been theoretically predicted for the semiconductor bismuth tellurohalide BiTeI with giant Rashba spin splitting. In this work, evolution of the electrical transport properties in BiTeI and BiTeBr is investigated under high pressure. The pressuredependent resistivity in a wide temperature range passes through a minimum at around 3 GPa, indicating the predicted topological quantum phase transition in BiTeI. Superconductivity is observed in both BiTeI and BiTeBr, while resistivity at higher temperatures still exhibits semiconducting behavior. Theoretical calculations suggest that superconductivity may develop from the multivalley semiconductor phase. The superconducting transition temperature, Tc, increases with applied pressure and reaches a maximum value of 5.2 K at 23.5 GPa for BiTeI (4.8 K at 31.7 GPa for BiTeBr), followed by a slow decrease. The results demonstrate that BiTeX (X = I, Br) compounds with nontrivial topology of electronic states display new ground states upon compression.

(2017) Physical Review X. 7, 2, 21016. Abstract[All authors]
The higher the energy of a particle is above equilibrium, the faster it relaxes because of the growing phase space of available electronic states it can interact with. In the relaxation process, phase coherence is lost, thus limiting highenergy quantum control and manipulation. In onedimensional systems, high relaxation rates are expected to destabilize electronic quasiparticles. Here, we show that the decoherence induced by relaxation of hot electrons in onedimensional semiconducting nanowires evolves nonmonotonically with energy such that above a certain threshold hot electrons regain stability with increasing energy. We directly observe this phenomenon by visualizing, for the first time, the interference patterns of the quasionedimensional electrons using scanning tunneling microscopy. We visualize the phase coherence length of the onedimensional electrons, as well as their phase coherence time, captured by crystallographic FabryPerot resonators. A remarkable agreement with a theoretical model reveals that the nonmonotonic behavior is driven by the unique manner in which onedimensional hot electrons interact with the cold electrons occupying the Fermi sea. This newly discovered relaxation profile suggests a highenergy regime for operating quantum applications that necessitate extended coherence or long thermalization times, and may stabilize electronic quasiparticles in one dimension.

(2017) Advanced Materials. 29, 19, 1606202. Abstract[All authors]
The search for highly efficient and lowcost catalysts is one of the main driving forces in catalytic chemistry. Current strategies for the catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfides in the hydrogen evolution reaction (HER). Here, the study proposes and demonstrates a different principle that goes beyond local site optimization by utilizing topological electronic states to spur catalytic activity. For HER, excellent catalysts have been found among the transitionmetal monopnictidesNbP, TaP, NbAs, and TaAswhich are recently discovered to be topological Weyl semimetals. Here the study shows that the combination of robust topological surface states and large room temperature carrier mobility, both of which originate from bulk Dirac bands of the Weyl semimetal, is a recipe for high activity HER catalysts. This approach has the potential to go beyond graphene based composite photocatalysts where graphene simply provides a high mobility medium without any active catalytic sites that have been found in these topological materials. Thus, the work provides a guiding principle for the discovery of novel catalysts from the emerging field of topological materials.

(2017) ChemistryA European Journal. 23, 19, p. 46804686 Abstract
Quasi twodimensional (2D) oxidebased honeycomb lattices have attracted great attention for displaying specific electronic instabilities, which give rise to unconventional bonding patterns and unexpected magnetic exchange couplings. The synthesis of AgRuO3, another representative exhibiting unique structural properties, is reported here. The stacking sequence of the honeycomb layers (Ru2O6) differs from analogous precedents; in particular, the intercalating silver atoms are shifted from the middle of the interspaces and cap the void octahedral sites of the (Ru2O6) slabs from both sides. This way, charge neutral, giant 2D molecules of Ag/Ru2O6/Ag result; a feature that significantly enhances the overall 2D character of AgRuO3. Measurements of magnetization have revealed extremely strong magnetic exchange coupling to be present, surviving to a temperature as high as 673K, which is the temperature of thermal decomposition. No indication for longrange magnetic order has, however, been observed. Theoretical analyses confirm the pronounced 2D character of the electronic system, and in particular reveal the interhoneycomb layer coupling J(c) to be distinctly weak.

(2017) Physical Review B. 95, 12, 121109. Abstract
The class of topological semimetals comprises a large pool of compounds. Together they provide a wide platform to realize exotic quasiparticles, for example, Dirac, nodalline Dirac, and Weyl fermions. In this Rapid Communication, we report the Berry phase, Fermisurface topology, and anisotropic magnetoresistance of HfSiS which has recently been predicted to be a nodalline semimetal. This compound contains a large carrier density, higher than most of the known semimetals. Massive amplitudes of de Haasvan Alphen and Shubnikovde Haas oscillations up to 20 K in 7 T assist us in witnessing a nontrivial piBerry phase, which is a consequence of topological Diractype dispersion of bands originating from the hybridization of p(x) + p(y) and d(x2y2) orbitals of squarenet plane of Si and Hf atoms, respectively. Furthermore, we establish the threedimensional Fermi surface which consists of very asymmetric water caltroplike electrons and barley seedlike hole pockets which account for the anisotropic magnetoresistance in HfSiS.

(2017) Scientific Reports. 7, 43394. Abstract[All authors]
NbP is a recently realized Weyl semimetal (WSM), hosting Weyl points through which conduction and valence bands cross linearly in the bulk and exotic Fermi arcs appear. The most intriguing transport phenomenon of a WSM is the chiral anomalyinduced negative magnetoresistance (NMR) in parallel electric and magnetic fields. In intrinsic NbP the Weyl points lie far from the Fermi energy, making chiral magnetotransport elusive. Here, we use Gadoping to relocate the Fermi energy in NbP sufficiently close to the W2 Weyl points, for which the different Fermi surfaces are verified by resultant quantum oscillations. Consequently, we observe a NMR for parallel electric and magnetic fields, which is considered as a signature of the chiral anomaly in condensedmatter physics. The NMR survives up to room temperature, making NbP a versatile material platform for the development of Weyltronic applications.

(2017) Physical Review B. 95, 7, 75128. Abstract
We have carried out a comprehensive study of the intrinsic anomalous Hall effect and spin Hall effect of several chiral antiferromagnetic compounds Mn3X (X = Ge, Sn, Ga, Ir, Rh and Pt) by ab initio band structure and Berry phase calculations. These studies reveal large and anisotropic values of both the intrinsic anomalous Hall effect and spin Hall effect. The Mn3X materials exhibit a noncollinear antiferromagnetic order which, to avoid geometrical frustration, forms planes of Mn moments that are arranged in a Kagometype lattice. With respect to these Kagome planes, we find that both the anomalous Hall conductivity (AHC) and the spin Hall conductivity (SHC) are quite anisotropic for any of these materials. Based on our calculations, we propose how to maximize AHC and SHC for different materials. The band structures and corresponding electron filling, that we show are essential to determine the AHC and SHC, are compared for these different compounds. We point out that Mn3Ga shows a large SHC of about 600 (h/e)(Omega cm)(1). Our work provides insights into the realization of strong anomalous Hall effects and spin Hall effects in chiral antiferromagnetic materials.

(2017) New Journal of Physics. 19, 15008. Abstract
Recent experiments revealed that Mn3Sn and Mn3Ge exhibit a strong anomalous Hall effect at room temperature, provoking us to explore their electronic structures for topological properties. By ab. initio band structure calculations, we have observed the existence of multiple Weyl points in the bulk and corresponding Fermi arcs on the surface, predicting antiferromagnetic Weyl semimetals in Mn3Ge and Mn3Sn. Here the chiral antiferromagnetism in the Kagometype lattice structure is essential to determine the positions and numbers of Weyl points. Our work further reveals a new guiding principle to search for magnetic Weyl semimetals among materials that exhibit a strong anomalous Hall effect.

(2017) Physical Review B. 95, 3, 35114. Abstract[All authors]
We present a quasiparticle interference study of clean and Mn surfacedoped TaAs, a prototypical Weyl semimetal, to test the screening properties as well as the stability of Fermi arcs against Coulomb and magnetic scattering. Contrary to topological insulators, the impurities are effectively screened in Weyl semimetals. The adatoms significantly enhance the strength of the signal such that theoretical predictions on the potential impact of Fermi arcs can be unambiguously scrutinized. Our analysis reveals the existence of three extremely short, previously unknown scattering vectors. Comparison with theory traces them back to scattering events between large parallel segments of spinsplit trivial states, strongly limiting their coherence. In sharp contrast to previous work [R. Batabyal et al., Sci. Adv. 2, e1600709 (2016)], where similar but weaker subtle modulations were interpreted as evidence of quasiparticle interference originating from Femi arcs, we can safely exclude this being the case. Overall, our results indicate that intra as well as interFermi arc scattering are strongly suppressed and may explain whyin spite of their complex multiband structuretransport measurements show signatures of topological states in Weyl monopnictides.

(2017) Nature Communications. 8, 13942. Abstract[All authors]
The rareearth monopnictide LaBi exhibits exotic magnetotransport properties, including an extremely large and anisotropic magnetoresistance. Experimental evidence for topological surface states is still missing although band inversions have been postulated to induce a topological phase in LaBi. In this work, we have revealed the existence of surface states of LaBi through the observation of three Dirac cones: two coexist at the corners and one appears at the centre of the Brillouin zone, by employing angleresolved photoemission spectroscopy in conjunction with ab initio calculations. The odd number of surface Dirac cones is a direct consequence of the odd number of band inversions in the bulk band structure, thereby proving that LaBi is a topological, compensated semimetal, which is equivalent to a timereversal invariant topological insulator. Our findings provide insight into the topological surface states of LaBi's semimetallicity and related magnetotransport properties.

(2017) Physical Review B. 95, 3, 35102. Abstract
Topological Dirac semimetals (DSMs) exhibit nodal points through which energy bands disperse linearly in threedimensional (3D) momentum space, a 3D analog of graphene. The first experimentally confirmed DSMs with a pair of Dirac points (DPs), Na3Bi and Cd3As2, show topological surface Fermi arc states and exotic magnetotransport properties, boosting the interest in the search for stable and nontoxic DSM materials. Based on densityfunctional theory and dynamical meanfield theory calculations, we predict a family of palladium and platinum oxides to be robust 3D DSMs with three pairs of Dirac points that are well separated from bulk bands. The Fermi arcs at the surface display a Lifshitz transition upon a continuous change of the chemical potential. Corresponding oxides are already available as highquality single crystals, an excellent precondition for the verification of our predictions by photoemission and magnetotransport experiments, extending DSMs to the versatile family of transitionmetal oxides.

(2017) Annual Review of Condensed Matter Physics. 8, p. 337354 Abstract
Topological insulators and topological semimetals are both new classes of quantum materials, which are characterized by surface states induced by the topology of the bulk band structure. Topological Dirac or Weyl semimetals show linear dispersion around nodes, termed the Dirac or Weyl points, as the threedimensional analog of graphene. We review the basic concepts and compare these topological states of matter from the materials perspective with a special focus on Weyl semimetals. The TaAs family is the ideal materials class to introduce the signatures of Weyl points in a pedagogical way, from Fermi arcs to the chiral magnetotransport properties, followed by hunting for the typeII Weyl semimetals in WTe2, MoTe2, and related compounds. Many materials are members of big families, and topological properties can be tuned. As one example, we introduce the multifunctional topological materials, Heusler compounds, in which both topological insulators and magnetic Weyl semimetals can be found. Instead of a comprehensive review, this article is expected to serve as a helpful introduction and summary by taking a snapshot of the quickly expanding field.
2016

(2016) Physical Review B. 94, 24, 245135. Abstract
In this work, we studied timereversalbreaking topological phases as a result of the interplay between antiferromagnetism and inverted band structures in antiferromagnetic double perovskite transitionmetal Sr2FeOsO6 films. By combining the firstprinciples calculations and analytical models, we demonstrate that the quantum anomalous Hall phase and chiral topological superconducting phase can be realized in this system. We find that to achieve timereversalbreaking topological phases in antiferromagnetic materials, it is essential to break the combined symmetry of time reversal and inversion, which generally exists in antiferromagnetic structures. As a result, we can utilize an external electric gate voltage to induce the phase transition between topological phases and trivial phases, thus providing an electrically controllable topological platform for future transport experiments.

(2016) New Journal of Physics. 18, 113038. Abstract
We investigated the electronic structure of the layered transitionmetal dichalcogenides VS2 and VSe2 by firstprinciples calculations. Both compounds exhibit metalinsulator transitions when crossing over from the bulk to the twodimensional monolayer. In the monolayer limit, the Coulomb interaction is enhanced due to the dimension reduction, leading to the insulating state. Moreover, these monolayers are found to be ferromagnetic, supplying excellent candidates for ferromagnetic insulators. When increasing the thickness, the fewlayer structure turns metallic and presents large anomalous Hall conductivity (similar to 100 S/cm), which oscillates with respect to the thickness due to the size effect. Our findings presents profound materials, such as ferromagnetic insulators and anomalous Hall ferromagnets, for the spintronic application.


(2016) Physical Review Letters. 117, 14, 146401. Abstract
Tantalum arsenide is a member of the noncentrosymmetric monopnictides, which are putative Weyl semimetals. In these materials, threedimensional chiral massless quasiparticles, the socalled Weyl fermions, are predicted to induce novel quantum mechanical phenomena, such as the chiral anomaly and topological surface states. However, their chirality is only well defined if the Fermi level is close enough to the Weyl points that separate Fermi surface pockets of opposite chirality exist. In this Letter, we present the bulk Fermi surface topology of high quality single crystals of TaAs, as determined by angledependent Shubnikovde Haas and de Haasvan Alphen measurements combined with ab initio bandstructure calculations. Quantum oscillations originating from three different types of Fermi surface pockets were found in magnetization, magnetic torque, and magnetoresistance measurements performed in magnetic fields up to 14 T and temperatures down to 1.8 K. Of these Fermi pockets, two are pairs of topologically nontrivial electron pockets around the Weyl points and one is a trivial hole pocket. Unlike the other members of the noncentrosymmetric monopnictides, TaAs is the first Weyl semimetal candidate with the Fermi energy sufficiently close to both types of Weyl points to generate chiral quasiparticles at the Fermi surface.

(2016) Scientific Reports. 6, 33859. Abstract[All authors]
Weyl semimetals are often considered the 3Danalogon of graphene or topological insulators. The evaluation of quantum oscillations in these systems remains challenging because there are often multiple conduction bands. We observe de Haasvan Alphen oscillations with several frequencies in a single crystal of the Weyl semimetal niobium phosphide. For each fundamental crystal axis, we can fit the raw data to a superposition of sinusoidal functions, which enables us to calculate the characteristic parameters of all individual bulk conduction bands using Fourier transform with an analysis of the temperature and magnetic fielddependent oscillation amplitude decay. Our experimental results indicate that the band structure consists of Dirac bands with low cyclotron mass, a nontrivial Berry phase and parabolic bands with a higher effective mass and trivial Berry phase.

(2016) Science advances. 2, 9, e1600759. Abstract
There has been considerable interest in spinorbit torques for the purpose of manipulating the magnetization of ferromagnetic elements for spintronic technologies. Spinorbit torques are derived from spin currents created from charge currents in materials with significant spinorbit coupling that propagate into an adjacent ferromagnetic material. A key challenge is to identify materials that exhibit large spin Hall angles, that is, efficient chargetospin current conversion. Using spin torque ferromagnetic resonance, we report the observation of a giant spin Hall angle theta(eff)(SH) of up to similar to 0.35 in (001)oriented singlecrystalline antiferromagnetic IrMn3 thin films, coupled to ferromagnetic permalloy layers, and theta(eff)(SH) that is about three times smaller in (111)oriented films. For (001)oriented samples, we show that the magnitude of theta(eff)(SH) can be significantly changed by manipulating the populations of various antiferromagnetic domains through perpendicular field annealing. We identify two distinct mechanisms that contribute to qeff SH: the first mechanism, which is facetindependent, arises from conventional bulk spindependent scattering within the IrMn3 layer, and the second intrinsic mechanism is derived from the unconventional antiferromagnetic structure of IrMn3. Using ab initio calculations, we show that the triangular magnetic structure of IrMn3 gives rise to a substantial intrinsic spin Hall conductivity that is much larger for the (001) than for the (111) orientation, consistent with our experimental findings.

(2016) 2D Materials. 3, 3, 35022. Abstract
We report the existence of the quantum spin Hall effect (QSHE) in monolayers of transitionmetal carbides MC(M. =. Zr, Hf). Under ambient conditions, the ZrC monolayer exhibits QSHE with an energy gap of 54 meV, in which topological helical edge states exist. Enhanced d(xy)d(xy) interaction induces band inversion, resulting in nontrivial topological features. By applying inplane strain, the HfC monolayer can be tuned from a trivial insulator to a quantum spin Hall insulator with an energy gap of 170 meV, three times that of the ZrC monolayer. The strong stability of MC monolayers provides a new platform for QSHE and spintronic applications.

(2016) Nature Communications. 7, 12924. Abstract[All authors]
Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host nontrivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angleresolved photoemission spectroscopy and ab initio calculations on rareearth halfHeusler compounds LnPtBi (Ln = Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with nontrivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are noncentrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.

(2016) 2D Materials. 3, 3, 35018. Abstract
Quantum spin Hall (QSH) insulates exist in special twodimensional (2D) semiconductors, possessing the quantized spinHall conductance that are topologically protected from backscattering. Based on the firstprinciples calculations, we predict a novel family of QSH insulators in 2D tantalum carbide halides TaCX (X = Cl, Br, and I) with unique rectangular lattice and large direct energy gaps. The mechanism for 2D QSH effect originates from an intrinsic dd band inversion in the process of chemical bonding. Further, stain and intrinsic electric field can be used to tune the electronic structure and enhance the energy gap. TaCX nanoribbon, which has the singleDiraccone edge states crossing the bulk band gap, exhibits a linear dispersion with a high Fermi velocity comparable to that of graphene. These 2D materials with considerable nontrivial gaps promise great application potential in the new generation of dissipationless electronics and spintronics.

(2016) Physical Review Letters. 117, 14, 146403. Abstract
Since their discovery, topological insulators are expected to be ideal spintronic materials owing to the spin currents carried by surface states with spinmomentum locking. However, the bulk doping problem remains an obstacle that hinders such an application. In this work, we predict that a newly discovered family of topological materials, the Weyl semimetals, exhibits a large intrinsic spin Hall effect that can be utilized to generate and detect spin currents. Our ab initio calculations reveal a large spin Hall conductivity in the TaAs family of Weyl materials. Considering the low charge conductivity of semimetals, Weyl semimetals are believed to present a larger spin Hall angle (the ratio of the spin Hall conductivity over the charge conductivity) than that of conventional spin Hall systems such as the 4d and 5d transition metals. The spin Hall effect originates intrinsically from the bulk band structure of Weyl semimetals, which exhibit a large Berry curvature and spinorbit coupling, so the bulk carrier problem in the topological insulators is naturally avoided. Our work not only paves the way for employing Weyl semimetals in spintronics, but also proposes a new guideline for searching for the spin Hall effect in various topological materials.

(2016) Physical Review B. 94, 5, 54517. Abstract[All authors]
The discovery of superconductivity in hafnium pentatelluride HfTe5 under high pressure is reported. Two structural phase transitions and metallization with superconductivity developing at around 5 GPa are observed. A maximal critical temperature of 4.8 K is attained at a pressure of 20 GPa, and superconductivity persists up to the maximum pressure of the study (42 GPa). The combination of electrical transport and crystal structure measurements as well as theoretical electronic structure calculations enables the construction of a phase diagram of HfTe5 under high pressure.

(2016) Science advances. 2, 8, e1600709. Abstract
Fermi arcs are the surface manifestation of the topological nature of Weyl semimetals, enforced by the bulkboundary correspondence with the bulk Weyl nodes. The surface of tantalum arsenide, similar to that of other members of the Weyl semimetal class, hosts nontopological bands that obscure the exploration of this correspondence. We use the spatial structure of the Fermi arc wave function, probed by scanning tunneling microscopy, as a spectroscopic tool to distinguish and characterize the surface Fermi arc bands. We find that, as opposed to nontopological states, the Fermi arc wave function is weakly affected by the surface potential: it spreads rather uniformly within the unit cell and penetrates deeper into the bulk. Fermi arcs reside predominantly on tantalum sites, from which the topological bulk bands are derived. Furthermore, we identify a correspondence between the Fermi arc dispersion and the energy and momentum of the bulk Weyl nodes that classify this material as topological. We obtain these results by introducing an analysis based on the role the Bloch wave function has in shaping quantum electronic interference patterns. It thus carries broader applicability to the study of other electronic systems and other physical processes.

(2016) Physical Review B. 93, 24, 245148. Abstract
The electronic and magnetic properties of distorted monoclinic double perovskite Sr2CeIrO6 were examined based on both experiments and firstprinciples density functional theory calculations. From the calculations we conclude that lowspinstate Ir4+ (5d(5)) forms a rare weakly antiferromagnetic (AFM) orbital ordered state derived from alternating occupation of slightly mixed c(g)(pi) symmetry states in the presence of spinorbit coupling (SOC). This orbital ordering is caused due to the competition between the comparable strength of JahnTeller structural distortion and SOC. We found both electronelectron correlation and SOC are required to drive the experimentally observed AFMinsulating ground state. Electronic structure investigation suggests that this material belongs to the intermediateSOC regime, by comparing our results with the other existing iridates. This single active site double perovskite provides a rare platform with a prototype geometrically frustrated fcc lattice where among the different degrees of freedom (i.e., spin, orbital, and lattice) SOC, structural distortion, and Coulomb correlation energy scales compete and interact with each other.

(2016) Physical Review B. 93, 24, 241106. Abstract
Topological insulators are characterized by an inverted band structure in the bulk and metallic surface states on the surface. In LaBi, a semimetal with a band inversion equivalent to a topological insulator, we observe surfacestatelike behavior in the magnetoresistance. The electrons responsible for this pseudotwodimensional transport, however, originate from the bulk states rather topological surface states, which is witnessed by the angledependent quantum oscillations of the magnetoresistance and ab initio calculations. As a consequence, the magnetoresistance exhibits strong anisotropy with large amplitude (similar to 10(5)%).

(2016) Physical Review B. 93, 20, 205303. Abstract
Starting from the threedimensional Dirac semimetal in Na3Bi, we found a topological insulator (TI) in the known compound of NaBaBi by extra pressure. The TI of NaBaBi can be viewed as the distorted version of Na3Bi with breaking inversion symmetry. When the exchangecorrelation energy is considered in generalized gradient approximation (GGA), the TI phase has a band inversion between the Bip and Nas orbitals. Since GGA often overestimates the band inversion, we also performed more accurate calculations by using hybrid functional theory (HSE). From HSE calculations we found that NaBaBi exhibits as a trivial insulator at zero pressure, and the other TI phase with pd inversion can be achieved by pressure. Though both of two TI phases have Diracconetype surface states, they have opposite spin textures. In the upper cone, a lefthanded spin texture exists for the sp inverted phase (similar to a common TI, e.g., Bi2Se3), whereas a righthanded spin texture appears for the pd inverted phase. This work presents a prototype model of a TI exhibits righthanded spin texture.

(2016) Physical Review B. 93, 20, 205102. Abstract
We report on the pressure evolution of the Fermi surface topology of the Weyl semimetal NbP, probed by Shubnikovde Haas oscillations in the magnetoresistance combined with ab initio calculations of the band structure. Although we observe a drastic effect on the amplitudes of the quantum oscillations, the frequencies only exhibit a weak pressure dependence up to 2.8 GPa. The pressureinduced variations in the oscillation frequencies are consistent with our bandstructure calculations. Furthermore, we can relate the changes in the amplitudes to small modifications in the shape of the Fermi surface. Our findings show evidence of the stability of the electronic band structure of NbP and demonstrate the power of combining quantumoscillation studies and bandstructure calculations to investigate pressure effects on the Fermi surface topology in Weyl semimetals.

(2016) Nature Communications. 7, 11615. Abstract[All authors]
Weyl semimetals (WSMs) are topological quantum states wherein the electronic bands disperse linearly around pairs of nodes with fixed chirality, the Weyl points. In WSMs, nonorthogonal electric and magnetic fields induce an exotic phenomenon known as the chiral anomaly, resulting in an unconventional negative longitudinal magnetoresistance, the chiralmagnetic effect. However, it remains an open question to which extent this effect survives when chirality is not welldefined. Here, we establish the detailed Fermisurface topology of the recently identified WSM TaP via combined angleresolved quantumoscillation spectra and bandstructure calculations. The Fermi surface forms bananashaped electron and hole pockets surrounding pairs of Weyl points. Although this means that chirality is illdefined in TaP, we observe a large negative longitudinal magnetoresistance. We show that the magnetoresistance can be affected by a magnetic fieldinduced inhomogeneous current distribution inside the sample.

(2016) Physical Review B. 93, 16, 161116. Abstract
Using firstprinciples densityfunctional theory, we have investigated the electronic and magnetic properties of recently synthesized and characterized 5d doubleperovskites Sr2BOsO6(B = Y, In, Sc). The electronic structure calculations show that in all compounds the Os5+ (5d(3)) site is the only magnetically active one, whereas Y3+, In3+, and Sc3+ remain in nonmagnetic states with Sc/Y and In featuring d(0) and d(10) electronic configurations, respectively. Our studies reveal the important role of closedshell (d(10)) versus openshell (d(0)) electronic configurations of the nonmagnetic sites in determining the overall magnetic exchange interactions. Although the magnetic Os5+ (5d(3)) site is the same in all compounds, the magnetic superexchange interactions mediated by nonmagnetic Y/In/Sc species are strongest for Sr2ScOsO6, weakest for Sr2InOsO6, and intermediate in the case of the Y (d(0)) due to different energy overlaps between Os5d and Y/In/Scd states. This explains the experimentally observed substantial differences in the magnetic transition temperatures of these materials, despite an identical magnetic site and underlying magnetic ground state. Furthermore, shortrange OsOs exchange interactions are more prominent than longrange OsOs interactions in these compounds, which contrasts with the behavior of other 3d5d double perovskites.

(2016) Science advances. 2, 4, e1501870. Abstract[All authors]
It is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that the noncolinear triangular antiferromagnet Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals; the magnitude of the anomalous conductivity is similar to 500 (ohm.cm)(1) at 2 K and similar to 50 (ohm.cm)(1) at room temperature. The angular dependence of the anomalous Hall effect measurements confirms that the small residual inplane magnetic moment has no role in the observed effect except to control the chirality of the spin triangular structure. Our theoretical calculations demonstrate that the large anomalous Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect of 1100 (/e) (ohm.cm)(1), comparable to that of platinum. The present results pave the way toward the realization of room temperature antiferromagnetic spintronics and spin Hall effectbased data storage devices.

(2016) Physical Review B. 93, 12, 121105. Abstract[All authors]
The Weyl semimetal NbP was found to exhibit topological Fermi arcs and exotic magnetotransport properties. Here, we report on magnetic quantumoscillation measurements on NbP and construct the threedimensional Fermi surface with the help of bandstructure calculations. We reveal a pair of spinorbitsplit electron pockets at the Fermi energy and a similar pair of hole pockets, all of which are strongly anisotropic. The Weyl points that are located in the k(z) approximate to pi/c plane are found to exist 5 meV above the Fermi energy. Therefore, we predict that the chiral anomaly effect can be realized in NbP by electron doping to drive the Fermi energy to the Weyl points.

(2016) Nature Communications. 7, 11038. Abstract[All authors]
Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transitionmetal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressuredriven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed domeshaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.

(2016) Physical Review B. 93, 4, 41108. Abstract
We study the interaction effect in a threedimensional Dirac semimetal and find that two competing orders, chargedensitywave orders and nematic orders, can be induced to gap the Dirac points. Applying a magnetic field can further induce an instability towards forming these ordered phases. The chargedensitywave phase is similar to that of aWeyl semimetal, while the nematic phase is unique for Dirac semimetals. Gapless zero modes are found in the vortex core formed by nematic order parameters, indicating the topological nature of nematic phases. The nematic phase can be observed experimentally using scanning tunneling microscopy.

(2016) Nature Materials. 15, 1, p. 27+ Abstract[All authors]
Topological Weyl semimetals (TWSs) represent a novel state of topological quantum matter(14) which not only possesses Weyl fermions (massless chiral particles that can be viewed as magnetic monopoles in momentum space) in the bulk and unique Fermi arcs generated by topological surface states, but also exhibits appealing physical properties such as extremely large magnetoresistance and ultrahigh carrier mobility(58). Here, by performing angleresolved photoemission spectroscopy (ARPES) on NbP and TaP, we directly observed their band structures with characteristic Fermi arcs of TWSs. Furthermore, by systematically investigating NbP, TaP and TaAs from the same transition metal monopnictide family, we discovered their Fermiology evolution with spinorbit coupling (SOC) strength. Our experimental findings not only reveal the mechanism to realize and finetune the electronic structures of TWSs, but also provide a rich material base for exploring many exotic physical phenomena (for example, chiral magnetic effects, negative magnetoresistance, and the quantum anomalous Hall effect) and novel future applications(3,4,911).
2015

(2015) Nature Communications. 6, 10167. Abstract[All authors]
Gold surfaces host special electronic states that have been understood as a prototype of Shockley surface states. These surface states are commonly employed to benchmark the capability of angleresolved photoemission spectroscopy (ARPES) and scanning tunnelling spectroscopy. Here we show that these Shockley surface states can be reinterpreted as topologically derived surface states (TDSSs) of a topological insulator (TI), a recently discovered quantum state. Based on band structure calculations, the Z(2)type invariants of gold can be welldefined to characterize a TI. Further, our ARPES measurement validates TDSSs by detecting the dispersion of unoccupied surface states. The same TDSSs are also recognized on surfaces of other wellknown noble metals (for example, silver, copper, platinum and palladium), which shines a new light on these longknown surface states.

(2015) Nano Letters. 15, 12, p. 78677872 Abstract
Topological insulators (TIs) are promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, currently realized twodimensional (2D) TIs, quantum spin Hall (QSH) insulators, suffer from ultrahigh vacuum and extremely low temperature. Thus, seeking for desirable QSH insulators with high feasibility of experimental preparation and large nontrivial gap is of great importance for wide applications in spintronics. On the basis of the firstprinciples calculations, we predict a novel family of 2D QSH insulators in transitionmetal halide MX (M = Zr, Hf; X = Cl, Br, and I) monolayers, especially, which is the first case based on transitionmetal halidebased QSH insulators. MX family has the large nontrivial gaps of 0.120.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV), and larger than ZrTe5 (0.1 eV) monolayers and graphenebased sandwiched heterstructures (3070 meV). Their corresponding 3D bulk materials are weak topological insulators from stacking QSH layers, and some of bulk compounds have already been synthesized in experiment. The mechanism for 2D QSH effect in this system originates from a novel dd band inversion, significantly different from conventional band inversion between sp, pp, or dp orbitals. The realization of pure layered MX monolayers may be prepared by exfoliation from their 3D bulk phases, thus holding great promise for nanoscale device applications and stimulating further efforts on transition metalbased QSH materials.

(2015) ACS Catalysis. 5, 12, p. 70637067 Abstract
Exotic and robust metallic surface states of topological insulators (TIs) have been expected to provide a promising platform for novel surface chemistry and catalysis. However, it is still not fully known how TIs affect the activity of catalysts. In this work, we study the effects of topological surface states (TSSs) on the activity of transition metal clusters (Au, Ag, Cu, Pt, and Pd), which are supported on a TI Bi2Se3 substrate. It was found the adsorption energy of oxygen on the supported catalysts can be always enhanced due to the TSSs. However, it does not necessarily mean an increase of the activity in catalytic oxidation reaction. Rather, the enhanced adsorption behavior in the presence of TSSs exhibits dual effects, determined by the intrinsic reactivity of these catalysts with oxygen. For the Au case, the activity of catalytic oxidation can be improved because the TSSs can enhance the dissociation rate of dioxygen. In contrast, a negative effect is found for the Pt and Pd clusters since the TSSs will suppress the desorption process of reaction products. We also found that the effect of TSSs on the activity of hydrogen evolution reaction (HER) is quite similar (i.e., the metals with original weak reactivity can gain a positive effect from TSSs). The present work can pave a way for more rational design and selection of catalysts when using TIs as substrates.

(2015) Physical Review B. 92, 16, 165421. Abstract
We studied the squareoctagonal lattice of the transition metal dichalcogenide MX2 (with M =Mo,W; X = S, Se, and Te), as an isomer of the normal hexagonal compound of MX2. By bandstructure calculations, we observe the graphenelike Dirac band structure in a rectangular lattice of MX2 with nonsymmorphic space group symmetry. Two bands with van Hove singularity points cross each at the Fermi energy, leading to two Dirac cones that locate at opposite momenta. Spinorbit coupling can open a gap at these Dirac points, inside which gapless topological edge states exists as the quantum spin Hall (QSH) effect, the 2D topological insulator.

(2015) Physical Review B. 92, 16, 161107. Abstract
We investigate the orthorhombic phase (Td) of the layered transitionmetal dichalcogenide MoTe2 as a Weyl semimetal candidate. MoTe2 exhibits four pairs of Weyl points lying slightly above (similar to 6meV) the Fermi energy in the bulk band structure. Different from its cousin WTe2, which was recently predicted to be a typeII Weyl semimetal, the spacing between each pair of Weyl points is found to be as large as 4% of the reciprocal lattice in MoTe2 (six times larger than that of WTe2). When projected onto the surface, the Weyl points are connected by Fermi arcs, which can be easily accessed by angleresolved photoemission spectroscopy due to the large Weyl point separation. In addition, we show that the correlation effect or strain can drive MoTe2 from a typeII to a typeI Weyl semimetal.

(2015) Physical Review B. 92, 11, 115428. Abstract
Very recently the topological Weyl semimetal (WSM) state was predicted in the noncentrosymmetric compounds NbP, NbAs, TaP, and TaAs and soon led to photoemission and transport experiments to verify the presumed topological properties such as Fermi arcs (unclosed Fermi surfaces) and the chiral anomaly. In this work we have performed fully ab initio calculations of the surface band structures of these four WSM materials and revealed the Fermi arcs with spinmomentumlocked spin texture. On the (001) polar surface, the shape of the Fermi surface depends sensitively on the surface terminations (cations or anions), although they exhibit the same topology with arcs. The anion (P or As) terminated surfaces are found to fit recent photoemission measurements well. Such surface potential dependence indicates that the shape of the Fermi surface can be sensitively manipulated by depositing guest species (such as K atoms), as we demonstrate. On the polar surface of a WSM without inversion symmetry, Rashbatype spin polarization naturally exists in the surface states and leads to strong spin texture. By tracing the spin polarization of the Fermi surface, one can distinguish Fermi arcs from trivial Fermi circles. The four compounds NbP, NbAs, TaP, and TaAs present an increasing amplitude of spinorbit coupling (SOC) in band structures. By comparing their surface states, we reveal the evolution of topological Fermi arcs from the spindegenerate Fermi circle to spinsplit arcs when the SOC increases from zero to a finite value. Our work presents a comprehensive understanding of the topological surface states of WSMs, which will especially be helpful for future spinrevolved photoemission and transport experiments.

(2015) ACS applied materials & interfaces. 7, 34, p. 1922619233 Abstract
The quantum spin Hall (QSH) effect predicted in silicene has raised exciting prospects of new device applications compatible with current microelectronic technology. Efforts to explore this novel phenomenon, however, have been impeded by fundamental challenges imposed by silicene's small topologically nontrivial band gap and fragile electronic properties susceptible to environmental degradation effects. Here we propose a strategy to circumvent these challenges by encapsulating silicene between transitionmetal dichalcogenides (TMDCs) layers. Firstprinciples calculations show that such encapsulated silicene exhibit a twoordersofmagnitude enhancement in its nontrivial band gap, which is driven by the strong spin orbit coupling effect in TMDCs via the proximity effect. Moreover, the cladding TMDCs layers also shield silicene from environmental gases that are detrimental to the QSH state in freestanding silicene. The encapsulated silicene represents a novel twodimensional topological insulator with a robust nontrivial band gap suitable for roomtemperature applications, which has significant implications for innovative QSH device design and fabrication.

(2015) Nature Physics. 11, 9, p. 728+ Abstract[All authors]
Threedimensional (3D) topological Weyl semimetals (TWSs) represent a state of quantum matter with unusual electronic structures that resemble both a '3D graphene' and a topological insulator. Their electronic structure displays pairs of Weyl points (through which the electronic bands disperse linearly along all three momentum directions) connected by topological surface states, forming a unique arclike Fermi surface (FS). Each Weyl point is chiral and contains half the degrees of freedom of a Dirac point, and can be viewed as a magnetic monopole in momentum space. By performing angleresolved photoemission spectroscopy on the noncentrosymmetric compound TaAs, here we report its complete band structure, including the unique Fermiarc FS and linear bulk band dispersion across the Weyl points, in agreement with the theoretical calculations1,2. This discovery not only confirms TaAs as a 3DTWS, but also provides an ideal platform for realizing exotic physical phenomena (for example, negative magnetoresistance, chiral magnetic effects and the quantum anomalous Hall effect) which may also lead to novel future applications.

(2015) Nature Physics. 11, 8, p. 645+ Abstract[All authors]
Recent experiments have revealed spectacular transport properties in semimetals, such as the large, nonsaturating magnetoresistance exhibited by WTe2 (ref.1). Topological semimetals with massless relativistic electrons have also been predicted(2) as threedimensional analogues of graphene(3). These systems are known as Weyl semimetals, and are predicted to have a range of exotic transport properties and surface states(47), distinct from those of topological insulators(8,9). Here we examine the magnetotransport properties of NbP, a material the band structure of which has been predicted to combine the hallmarks of a Weyl semimetal(10,11) with those of a normal semimetal. We observe an extremely large magnetoresistance of 850,000% at 1.85 K (250% at room temperature) in a magnetic field of up to 9 T, without any signs of saturation, and an ultrahigh carrier mobility of 5 x 10(6) cm(2) V1 s(1) that accompanied by strong Shubnikovde Haas (SdH) oscillations. NbP therefore presents a unique example of a material combining topological and conventional electronic phases, with intriguing physical properties resulting from their interplay.

(2015) Physical Review B. 91, 23, 235306. Abstract
The search for inversionasymmetric topological insulators (IATIs) persists as an effect for realizing new topological phenomena. However, so far only a few IATIs have been discovered and there is no IATI exhibiting a large band gap exceeding 0.6 eV. Using firstprinciples calculations, we predict a series of new IATIs in saturated Group IIIBi bilayers. We show that all these IATIs preserve extraordinary large bulk band gaps, which are well above room temperature, allowing for viable applications in roomtemperature spintronic devices. More importantly, most of these systems display large bulk band gaps that far exceed 0.6 eV and, part of them even are up to similar to 1 eV, which are larger than any IATIs ever reported. The nontrivial topological situation in these systems is confirmed by the identified band inversion of the band structures, Z(2) topological invariants, and an explicit demonstration of the topological edge states. Owning to their asymmetric structures, remarkable Rashba spin splitting is produced in both the valence and conduction bands of these systems. These predictions strongly revive these new systems as excellent candidates for IATIbased novel applications.

(2015) Carbon. 87, p. 418423 Abstract
Graphene is the first model system of twodimensional topological insulator (TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in graphene, however, has eluded direct experimental detection because of its extremely small energy gap due to the weak spinorbit coupling. Here we predict by ab initio calculations a giant (three orders of magnitude) proximity induced enhancement of the TI energy gap in the graphene layer that is sandwiched between thin slabs of Sb2Te3 (or MoTe2). This gap (1.5 meV) is accessible by existing experimental techniques, and it can be further enhanced by tuning the interlayer distance via compression. We reveal by a tightbinding study that the QSH state in graphene is driven by the KaneMele interaction in competition with Kekule deformation and symmetry breaking. The present work identifies a new family of graphenebased TIs with an observable and controllable bulk energy gap in the graphene layer, thus opening a new avenue for direct verification and exploration of the longsought QSH effect in graphene. (C) 2015 Elsevier Ltd. All rights reserved.

(2015) Scientific Reports. 5, 10435. Abstract
Recent theoretical studies employing densityfunctional theory have predicted BaBiO3 (when doped with electrons) and YBiO3 to become a topological insulator (TI) with a large topological gap (similar to 0.7 eV). This, together with the natural stability against surface oxidation, makes the BismuthOxide family of special interest for possible applications in quantum information and spintronics. The central question, we study here, is whether the holedoped Bismuth Oxides, i.e. Ba1xKxBiO3 and BaPb1xBixO3, which are "highTc" bulk superconducting near 30 K, additionally display in the further vicinity of their Fermi energy EF a topological gap with a Diractype of topological surface state. Our electronic structure calculations predict the Kdoped family to emerge as a TI, with a topological gap above EF. Thus, these compounds can become superconductors with holedoping and potential TIs with additional electron doping. Furthermore, we predict the BismuthOxide family to contain an additional Dirac cone below EF for further hole doping, which manifests these systems to be candidates for both electronand holedoped topological insulators.

(2015) Physical Review B. 91, 16, 165435. Abstract
One important feature of surface states in topological insulators is the socalled "spinmomentum locking," which means that electron spin is oriented along a fixed direction for a given momentum and forms a texture in the momentum space. In this work, we study spin textures of two typical topological insulators in Hgbased chalcogenides, namely, HgTe and HgS, based on both the firstprinciples calculation and the eightband Kane model. We find opposite helicities of spin textures between these two materials, originating from the opposite signs of spinorbit couplings. Based on the effective Kane model, we present a physical picture to understand opposite spin textures in these two materials with the help of the relationship between spin textures and mirror Chern numbers. We also reveal the existence of gapless states at the interface between HgTe and HgS due to the opposite spin textures and opposite mirror Chern numbers.

(2015) ANGEWANDTE CHEMIEINTERNATIONAL EDITION. 54, 18, p. 54175420 Abstract
Local environments and valence electron counts primarily determine the electronic states and physical properties of transitionmetal complexes. For example, squareplanar coordination geometries found in transitionmetal oxometalates such as cuprates are usually associated with the d(8) or d(9) electron configuration. In this work, we address an unusual squareplanar single oxoanionic [IrO4](4) species, as observed in Na4IrO4 in which IrIV has a d5 configuration, and characterize the chemical bonding through experiments and by ab initio calculations. We find that the IrIV center in groundstate Na4IrO4 has squareplanar coordination geometry because of the weak Coulomb repulsion of the Ir5d electrons. In contrast, in its 3d counterpart Na4CoO4, the CoIV center is tetrahedrally coordinated because of strong electron correlation. Na4IrO4 may thus serve as a simple yet important example to study the ramifications of Hubbardtype Coulomb interactions on local geometries.

(2015) Physical Review Letters. 114, 11, 117201. Abstract[All authors]
Cd3As2 is a candidate threedimensional Dirac semimetal which has exceedingly high mobility and nonsaturating linear magnetoresistance that may be relevant for future practical applications. We report magnetotransport and tunnel diode oscillation measurements on Cd3As2, in magnetic fields up to 65 T and temperatures between 1.5 and 300 K. We find that the nonsaturating linear magnetoresistance persists up to 65 T and it is likely caused by disorder effects, as it scales with the high mobility rather than directly linked to Fermi surface changes even when approaching the quantum limit. From the observed quantum oscillations, we determine the bulk threedimensional Fermi surface having signatures of Dirac behavior with a nontrivial Berry phase shift, very light effective quasiparticle masses, and clear deviations from the bandstructure predictions. In very high fields we also detect signatures of large Zeeman spin splitting (g similar to 16).

(2015) Physical Review B. 91, 9, 94107. Abstract
We have performed ab initio bandstructure calculations on more than 2000 halfHeusler compounds in order to search for new candidates for topological insulators. Herein, LiAuS and NaAuS are found to be the strongest topological insulators with the bulk band gaps of 0.20 and 0.19 eV, respectively, different from the zero bandgap feature reported in other Heusler topological insulators. Due to the inversion asymmetry of the Heusler structure, their topological surface states on the top and bottom surfaces exhibit ptype and ntype carriers, respectively. Thus, these materials may serve as an ideal platform for the realization of topological magnetoelectric effects as polar topological insulators. Moreover, these topological surface states exhibit the righthand spin texture in the upper Dirac cone, which distinguishes them from currently known topological insulator materials. Their topological nontrivial character remains robust against inplane strains, which makes them suitable for epitaxial growth of films.

(2015) Zeitschrift fur Anorganische und Allgemeine Chemie. 641, 2, p. 197205 Abstract[All authors]
Double perovskites Sr2BOsO6 (B = Y, In, and Sc) were prepared from the respective binary metal oxides, and their structural, magnetic, and electronic properties were investigated. At room temperature all these compounds crystallize in the monoclinic space group P2(1)/n. They contain magnetic osmium (Os5+, t(2g)(3)) ions and are antiferromagnetic insulators with Neel temperatures TN = 53 K, 26 K, and 92 K for B = Y, In, and Sc, respectively. Powder neutron diffraction studies on Sr2YOsO6 and Sr2InOsO6 showed that the crystal structures remain unchanged down to 3 K. The Y and In compounds feature a type I antiferromagnetic spin structure with ordered Os moments of 1.91 mu B and 1.77 mu B, respectively. The trend in TN does not simply follow the development of the lattice parameters, which suggests that d(0) compared to d(10) ions on the B site favor a somewhat different balance of exchange interactions in the frustrated Os5+ fcclike lattice.
2014

(2014) Physical Review Letters. 113, 25, 256401. Abstract
Recently, the longsough quantum anomalous Hall effect was realized in a magnetic topological insulator. However, the requirement of an extremely low temperature (approximately 30 mK) hinders realistic applications. Based on ab initio band structure calculations, we propose a quantum anomalous Hall platform with a large energy gap of 0.34 and 0.06 eV on honeycomb lattices comprised of Sn and Ge, respectively. The ferromagnetic (FM) order forms in one sublattice of the honeycomb structure by controlling the surface functionalization rather than dilute magnetic doping, which is expected to be visualized by spin polarized STM in experiment. Strong coupling between the inherent quantum spin Hall state and ferromagnetism results in considerable exchange splitting and, consequently, an FM insulator with a large energy gap. The estimated meanfield Curie temperature is 243 and 509 K for Sn and Ge lattices, respectively. The large energy gap and high Curie temperature indicate the feasibility of the quantum anomalous Hall effect in the nearroomtemperature and even roomtemperature regions.

(2014) Physical Review B. 90, 16, 165140. Abstract
Topological insulators are known for their metallic surface states, a result of strong spinorbit coupling, that exhibit unique surface transport phenomenon. However, these surface transport phenomena are buried in the presence of metallic bulk conduction. We synthesized very high quality Bi2Te2Se single crystals by using a modified Bridgman method that possess high bulk resistivity of >20 Omega cm below 20 K, whereas the bulk is mostly inactive and surface transport dominates. The temperature dependence of resistivity follows an activation law like a gap semiconductor in temperature range 20300 K. To extract the surface transport from that of the bulk, we designed a special measurement geometry to measure the resistance and found that singlecrystal Bi2Te2Se exhibits a crossover from bulk to surface conduction at 20 K. Simultaneously, the material also shows strong evidence of weak antilocalization in magnetotransport owing to the protection against scattering by conducting surface states. This simple geometry facilitates finding evidence of surface transport in topological insulators, which are promising materials for future spintronic applications.

(2014) ACS Nano. 8, 10, p. 1044810454 Abstract
We predict a family of robust twodimensional (2D) topological insulators in van der Waals heterostructures comprising graphene and chalcogenides BiTeX (X = Cl, Br, and I). The layered structures of both constituent materials produce a naturally smooth interface that is conducive to proximityinduced topological states. Firstprinciples calculations reveal intrinsic topologically nontrivial bulk energy gaps as large as 7080 meV, which can be further enhanced up to 120 meV by compression. The strong spinorbit coupling in BiTeX has a significant influence on the graphene Dirac states, resulting in the topologically nontrivial band structure, which is confirmed by calculated nontrivial Z2 index and an explicit demonstration of metallic edge states. Such heterostructures offer a unique Dirac transport system that combines the 2D Dirac states from graphene and 1D Dirac edge states from the topological insulator, and it offers ideas for innovative device designs.

(2014) MRS Bulletin. 39, 10, p. 859866 Abstract
Ternary semiconducting or metallic halfHeusler compounds with an atomic composition 1:1:1 are widely studied for their flexible electronic properties and functionalities. Recently, a new material property of halfHeusler compounds was predicted based on electronic structure calculations: the topological insulator. In topological insulators, the metallic surface states are protected from impurity backscattering due to spinmomentum locking. This opens up new perspectives in engineering multifunctional materials. In this article, we introduce halfHeusler materials from the crystallographic and electronic structure point of view. We present an effective model Hamiltonian from which the topological state can be derived, notably from a nontrivial inverted band structure. We discuss general implications of the inverted band structure with a focus on the detection of the topological surface states in experiments by reviewing several exemplary materials. Special attention is given to superconducting halfHeusler materials, which have attracted ample attention as a platform for noncentrosymmetric and topological superconductivity.

(2014) Physical Review B. 90, 10, 100505. Abstract
Timereversal breaking topological superconductors are new states of matter which can support Majorana zero modes at the edge. In this Rapid Communication, we propose a different realization of onedimensional topological superconductivity and Majorana zero modes. The proposed system consists of a monolayer of transitionmetal dichalcogenides M X2 (M = Mo, W; X = S, Se) on top of a superconducting substrate. Based on firstprinciples calculations, we show that a zigzag edge of the monolayer M X2 terminated by a metal atom M has edge states with strong spinorbit coupling and spontaneous magnetization. By proximity coupling with a superconducting substrate, topological superconductivity can be induced at such an edge. We propose NbS2 as a natural choice of substrate, and estimate the proximity induced superconducting gap based on firstprinciples calculation and a low energy effective model. As an experimental consequence of our theory, we predict that Majorana zero modes can be detected at the 120 degrees corner of a M X2 flake in proximity to a superconducting substrate.

(2014) EPL. 107, 5, 57006. Abstract
The topological surface states of mercury telluride (HgTe) are studied by ab initio calculations assuming different strains and surface terminations. For the Teterminated surface, a single Dirac cone exists at the Gammapoint. The Dirac point shifts up from the bulk valence bands into the energy gap when the substrateinduced strain increases. At the experimental strain value (0.3%), the Dirac point lies slightly below the bulk valence band maximum. A lefthanded spin texture was observed in the upper Dirac cone, similar to that of the Bi2Se3type topological insulator. For the Hgterminated surface, three Dirac cones appear at three timereversalinvariant momenta, excluding the Gammapoint, with nontrivial spin textures. Copyright (C) EPLA, 2014

(2014) Physical Review B. 90, 7, 75438. Abstract
Topological insulators represent a paradigm shift in surface physics. The most extensively studied Bi2Se3type topological insulators exhibit layered structures, wherein neighboring layers are weakly bonded by van der Waals interactions. Using firstprinciples densityfunctional theory calculations, we investigate the impact of the stacking sequence on the energetics and band structure properties of three polymorphs of Bi2Se3, Bi2Te3, and Sb2Te3. Considering their ultrathin films up to 6 nm as a function of its layer thickness, the overall dispersion of the band structure is found to be insensitive to the stacking sequence, while the band gap is highly sensitive, which may also affect the critical thickness for the onset of the topologically nontrivial phase. Our calculations are consistent with both experimental and theoretical results, where available. We further investigate tribological layer slippage, where we find a relatively low energy barrier between two of the considered structures. Both the stackingdependent band gap and low slippage energy barriers suggest that polymorphic stacking modification may offer an alternative route for controlling the properties of this new state of matter.

(2014) Physical Review B. 89, 21, 214414. Abstract
Using densityfunctional theory calculations, we investigated the electronic structure and magnetic exchange interactions of the ordered 3d5d double perovskite Sr2FeOsO6, which has recently drawn attention for interesting antiferromagnetic transitions. Our study reveals the vital role played by longrange magnetic exchange interactions in this compound. The competition between the ferromagnetic nearestneighbor OsOFe interaction and antiferromagnetic nextnearestneighbor OsOFeOOs interaction induces strong frustration in this system, which explains the lattice distortion and magnetic phase transitions observed in experiments.

(2014) Physical Review Letters. 112, 14, 147202. Abstract[All authors]
Magnetic properties and spin dynamics have been studied for the structurally ordered double perovskite Sr2CoOsO6. Neutron diffraction, muonspin relaxation, and acsusceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature TN1 = 108 K, cobalt (3d(7), S = 3/2) and osmium (5d(2), S = 1) moments fluctuate dynamically, while their average effective moments undergo longrange order. In the second AFM phase below TN2 = 67 K, cobalt moments first become frozen and induce a noncollinear spincanted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at T approximate to 5 K. Ab initio calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin dynamics, making Sr2CoOsO6 distinct from previously reported doubleperovskite compounds.

(2014) Hyperfine Interactions. 226, 3Jan, p. 289297 Abstract
The insulating and antiferromagnetic double perovskite Sr2FeOsO6 has been studied by Fe57 Mossbauer spectroscopy between 5 and 295 K. The iron atoms are essentially in the Fe3 + high spin and thus the osmium atoms in the Os state. Two magnetic phase transitions, which according to neutron diffraction studies occur below T (N) = 140 K and T (2) = 67 K, give rise to magnetic hyperfine patterns, which differ considerably in the hyperfine fields and thus, in the corresponding ordered magnetic moments. The evolution of hyperfine field distributions, average values of the hyperfine fields, and magnetic moments with temperature suggests that the magnetic state formed below T (N) is strongly frustrated. The frustration is released by a magnetostructural transition which below T (2) leads to a different spin sequence along the cdirection of the tetragonal crystal structure.

(2014) Physical Review B. 89, 4, 41409. Abstract
Based on firstprinciples calculations, we predict Bi2TeI, a stoichiometric compound that is synthesized, to be a weak topological insulator (TI) in layered subvalent bismuth telluroiodides. Within a bulk energy gap of 80 meV, two Diracconelike topological surface states exist on the side surface perpendicular to the BiTeI layer plane. These Dirac cones are relatively isotropic due to the strong interlayer coupling, distinguished from those of previously reported weak TI candidates. Moreover, with chemically stable cladding layers, the BiTeIBi2BiTeI sandwiched structure is a robust quantum spin Hall system, which can be obtained by simply cleaving the bulk Bi2TeI.

(2014) Nanoscale. 6, 13, p. 74747479 Abstract
Developing graphenebased nanoelectronics hinges on opening a band gap in the electronic structure of graphene, which is commonly achieved by breaking the inversion symmetry of the graphene lattice via an electric field (gate bias) or asymmetric doping of graphene layers. Here we introduce a new design strategy that places a bilayer graphene sheet sandwiched between two cladding layers of materials that possess strong spinorbit coupling (e.g., Bi2Te3). Our ab initio and tightbinding calculations show that a proximity enhanced spinorbit coupling effect opens a large (44 meV) band gap in bilayer graphene without breaking its lattice symmetry, and the band gap can be effectively tuned by an interlayer stacking pattern and significantly enhanced by interlayer compression. The feasibility of this quantumwell structure is demonstrated by recent experimental realization of highquality heterojunctions between graphene and Bi2Te3, and this design also conforms to existing fabrication techniques in the semiconductor industry. The proposed quantumwell structure is expected to be especially robust since it does not require an external power supply to open and maintain a band gap, and the cladding layers provide protection against environmental degradation of the graphene layer in its device applications.
2013

(2013) Nano Letters. 13, 12, p. 62516255 Abstract
Topological insulators (TIs) represent a new quantum state of matter characterized by robust gapless states inside the insulating bulk gap. The metallic edge states of a twodimensional (2D) TI, known as the quantum spin Hall (QSH) effect, are immune to backscattering and carry fully spinpolarized dissipationless currents. However, existing 2D TIs realized in HgTe and InAs/GaSb suffer from small bulk gaps (

(2013) EPL. 104, 3, 30001. Abstract
The binary compounds FeSi, RuSi, and OsSi are chiral insulators crystallizing in the space group P2(1)3 which is cubic. By means of ab initio calculations we find for these compounds a nonvanishing electronic Berry phase, the sign of which depends on the handedness of the crystal. There is thus the possibility that the Berry phase signals the existence of a macroscopic electric polarization due to the electrons. We show that this is indeed so if a small external magnetic field is applied in the [111] direction. The electric polarization is oscillatory in the magnetic field and possesses a signature that distinguishes the handedness of the crystal. Our findings add to the discussion of topological classifications of insulators and are significant for spintronics applications, and in particular, for a deeper understanding of skyrmions in insulators. Copyright (C) EPLA, 2013

(2013) Physical Review B. 88, 19, 195128. Abstract
The surface band bending tunes considerably the surface band structures and transport properties in topological insulators. We present a direct measurement of the band bending on the Bi2Se3 by using the bulk sensitive angularresolved hard xray photospectroscopy (HAXPES). We tracked the depth dependence of the energy shift of Bi and Se core states. We estimate that the band bending extends up to about 20 nm into the bulk with an amplitude of 0.230.26 eV, consistent with profiles previously deduced from the binding energies of surface states in this material.

(2013) Nature Physics. 9, 11, p. 709711 Abstract
Topological insulators are a new class of quantum materials that are characterized by robust topological surface states (TSSs) inside the bulk insulating gap(1,2), which hold great potential for applications in quantum information and spintronics as well as thermoelectrics. One major obstacle is the relatively small size of the bulk bandgap, which is typically around 0.3 eV for the known topological insulator materials (ref. 3 and references therein). Here we demonstrate through ab initio calculations that a known superconductor BaBiO3 (BBO) with a Tc of nearly 30 K (refs 4,5) emerges as a topological insulator in the electrondoped region. BBO exhibits a large topological energy gap of 0.7 eV, inside which a Dirac type of TSSs exists. As the first oxide topological insulator, BBO is naturally stable against surface oxidization and degradation, distinct from chalcogenide topological insulators(68). An extra advantage of BBO lies in its ability to serve as an interface between TSSs and superconductors to realize Majorana fermions for future applications in quantum computation(9).

(2013) Physical Review Letters. 111, 16, 167205. Abstract[All authors]
The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic ironosmium layers along the tetragonal c axis. Neutron powder diffraction experiments, Fe57 Mossbauer spectra, and density functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating ironosmium distances finetunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example of a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.

(2013) Physical Review Letters. 111, 13, 136804. Abstract
The search for largegap quantum spin Hall (QSH) insulators and effective approaches to tune QSH states is important for both fundamental and practical interests. Based on firstprinciples calculations we find twodimensional tin films are QSH insulators with sizable bulk gaps of 0.3 eV, sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and by external strain. The mechanism for the QSH effect in this system is band inversion at the Gamma point, similar to the case of a HgTe quantum well. With surface doping of magnetic elements, the quantum anomalous Hall effect could also be realized.

(2013) Inorganic Chemistry. 52, 11, p. 67136719 Abstract
In the exploration of new osmium based double perovskites, Sr2FeOsO6 is a new insertion in the existing family. The polycrystalline compound has been prepared by solid state synthesis from the respective binary oxides Powder Xray diffraction (PXRD) analysis shows the structure is pseudocubic at room temperature, whereas low temperature synchrotron data refinements reveal the structure to be tetragonal, space group I4/m. Heat capacity and magnetic measurements of Sr2FeOsO6 indicated the presence of two magnetic phase transitions at T1 = 140 K and T2 = 67 K. Band structure calculations showed the compound as a narrow energy gap semiconductor, which supports the experimental results obtained from the resistivity measurements The present study documents significant structural and electronic effects of substituting Fe3+ for Cr3+ ion in Sr2CrOsO6.

(2013) Journal Of PhysicsCondensed Matter. 25, 20, 206006. Abstract
We investigate the structural stability and magnetic properties of the cubic, tetragonal and hexagonal phases of Mn(3)Z (Z = Ga, Sn and Ge) Heusler compounds using firstprinciples densityfunctional theory. We propose that the cubic phase plays an important role as an intermediate state in the phase transition from the hexagonal to the tetragonal phases. Consequently, Mn3Ga and Mn3Ge behave differently from Mn3Sn, because the relative energies of the cubic and hexagonal phases are different. This result agrees with experimental observations for these three compounds. The weak ferromagnetism of the hexagonal phase and the perpendicular magnetocrystalline anisotropy of the tetragonal phase obtained in our calculations are also consistent with experiment.

(2013) Journal Of PhysicsCondensed Matter. 25, 15, 155601. Abstract
We propose the concept of 'topological Hamiltonian' for topological insulators and superconductors in interacting systems. The eigenvalues of the topological Hamiltonian are significantly different from the physical energy spectra, but we show that the topological Hamiltonian contains the information of gapless surface states, therefore it is an exact tool for topological invariants.

(2013) Nano Letters. 13, 3, p. 10731079 Abstract
In Dirac materials, like graphene or topological insulators, massless pseudorelativistic electrons promise new, very fast electronic devices by utilizing the partial suppression of backscattering. However, the semimetal nature of graphene makes the realization of practical field effect transistors difficult, due to small onoff current ratios. Here, we propose a new concept, based on Dirac states inside the conduction (or valence) band of a lightly doped wide band gap semiconductor. With the application of a gate voltage, the Dirac states become populated; that is, the Fermi level is switched between the "classical" highresistivity semiconducting and the relativistic highmobility metallic range. We demonstrate by theoretical calculations that such a transition can be realized, for example, in thin anatase nanowires, which have been synthesized before. Tadoped anatase nanowires offer an excellent possibility to build field effect transistors with high speed and good onoff ratio. Guidelines for finding similar "Dirac semiconductors" are provided.

(2013) Physica Status SolidiRapid Research Letters. 7, 2Jan, p. 148150 Abstract
Although topological surface states are known to be robust against nonmagnetic surface perturbations, their band dispersions and spatial distributions are still sensitive to surface defects. Taking Bi2Se3 as an example, we demonstrate that Se vacancies modify the surface band structures considerably. When large numbers of Se vacancies exist on the surface, topological surface states may sink down from the first to the second quintuple layer and get separated from the vacancies. We simulated scanning tunnelling microscopy images to distinguish surfaces with Se and Bi terminations. (C) 2013 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim


(2013) Physica Status SolidiRapid Research Letters. 7, 2Jan, p. 91100 Abstract
Topological insulators (TIs) are a new quantum state of matter which have gapless surface states inside the bulk energy gap [14]. Starting with the discovery of twodimensional TIs, the HgTebased quantum wells [5, 6], many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families [7], the HgTe family and the Bi2Se3 family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials [810]. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and thermoelectrics, and give examples of compound classes where both good thermoelectric properties and topological insulators can be found. (C) 2013 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

(2013) Physical Review Letters. 110, 1, 16403. Abstract
Gas molecule doping on the topological insulator Bi2Se3 surface with existing Se vacancies is investigated using firstprinciples calculations. Consistent with experiments, NO2 and O2 are found to occupy the Se vacancy sites, remove vacancydoped electrons, and restore the band structure of a perfect surface. In contrast, NO and H2 do not favor passivation of such vacancies. Interestingly we have revealed a NO2 dissociation process that can well explain the speculative introduced "photondoping" effect reported by recent experiments. Experimental strategies to validate this mechanism are presented. The choice and the effect of different passivators are discussed. This step paves the way for the usage of such materials in device applications utilizing robust topological surface states. DOI: 10.1103/PhysRevLett.110.016403
2012

(2012) Physical Review Letters. 109, 11, 116406. Abstract
We report the discovery of weak topological insulators by ab initio calculations in a honeycomb lattice. We propose a structure with an odd number of layers in the primitive unit cell as a prerequisite for forming weak topological insulators. Here, the singlelayered KHgSb is the most suitable candidate for its large bulk energy gap of 0.24 eV. Its side surface hosts metallic surface states, forming two anisotropic Dirac cones. Although the stacking of evenlayered structures leads to trivial insulators, the structures can host a quantum spin Hall layer with a large bulk gap, if an additional single layer exists as a stacking fault in the crystal. The reported honeycomb compounds can serve as prototypes to aid in the finding of new weak topological insulators in layered smallgap semiconductors.

(2012) Reports on Progress in Physics. 75, 9, 96501. Abstract
Recently, topological insulator materials have been theoretically predicted and experimentally observed in both 2D and 3D systems. We first review the basic models and physical properties of topological insulators, using HgTe and Bi2Se3 as prime examples. We then give a comprehensive survey of topological insulators which have been predicted so far, and discuss the current experimental status.

(2012) Nano Letters. 12, 7, p. 34603465 Abstract
A phosphorus (P) donor has been extensively studied in bulk Si to realize the concept of Kane quantum computers. In most cases the quantum bit was realized as an entanglement between the donor electron spin and the nonzero nuclei spin of the donor impurity mediated by the hyperfine coupling between them. The donor ionization energies and the spin lattice relaxation time limited the temperatures to a few kelvin in these experiments. Here, we demonstrate by means of ab initio density functional theory calculations that quantum confinement in thin Si nanowires (SiNWs) results in (i) larger excitation energies of donor impurity and (ii) a sensitive manipulation of the hyperfine coupling by external electric field. We propose that these features may allow to realize the quantum bit (qubit) experiments at elevated temperatures with a strength of electric fields applicable in current fieldeffect transistor technology. We also show that the strength of quantum confinement and the presence of strain induced by the surface termination may significantly affect the ground and excited states of the donors in thin SiNWs, possibly allowing an optical readout of the electron spin.

(2012) Physical Review B. 85, 16, 165125. Abstract
We propose new topological insulators in ceriumfilled skutterudite (FS) compounds based on ab initio calculations. We find that two compounds, CeOs4As12 and CeOs4Sb12, are zero gap materials with band inversions between Osd and Cef orbitals, similar to HgTe. Both compounds are predicted to become topological Kondo insulators at low temperatures, which are Kondo insulators in the bulk but with robust Dirac surface states on the boundary. Furthermore, this family of topological insulators has more unique features. Due to similar lattice parameters there will be a good proximity effect with other superconducting FS compounds, which may realize Majorana fermions. Additionally, the experimentally observed antiferromagnetic phase of CeOs4Sb12 at very low temperature provides a way to realize the massive Dirac fermion with topological magnetoelectric effects.

2011

(2011) Physical Review Letters. 106, 15, 156402. Abstract
We investigate a new class of ternary materials such as LiAuSe and KHgSb with a honeycomb structure in AuSe and HgSb layers. We demonstrate the band inversion in these materials similar to HgTe, which is a strong precondition for existence of the topological surface states. In contrast with graphene, these materials exhibit strong spinorbit coupling and a small direct band gap at the Gamma point. Since these materials are centrosymmetric, it is straightforward to determine the parity of their wave functions, and hence their topological character. Surprisingly, the compound with strong spinorbit coupling (KHgSb) is trivial, whereas LiAuSe is found to be a topological insulator.
2010

(2010) Physical Review B. 82, 16, 161108. Abstract
A different class of threedimensional topological insulator, ternary rareearth chalcogenides, is theoretically investigated with ab initio calculations. Based on both bulk bandstructure analysis and the direct calculation of topological surface states, we demonstrate that LaBiTe3 is a topological insulator. La can be substituted by other rare earth elements, which provide candidates for novel topological states such as quantum anomalous Hall insulator, axionic insulator, and topological Kondo insulator. Moreover, YBiTe3 and YSbTe3 are found to be normal insulators. They can be used as protecting barrier materials for both LaBiTe3 and Bi2Te3 families of topological insulators for their wellmatched lattice constants and chemical composition.

(2010) Nano Letters. 10, 9, p. 37913795 Abstract
Due to the proximity to an embedding medium with low dielectric constant (e.g., oxides), semiconductor nanowires have higher impurity ionization energy than their bulk counterparts, resulting lower. free carrier density. Using ab initio calculations within density functional theory, we propose a way to reduce the ionization energy in nanowires by fabricating a special cross section with appropriate engineering of doping and an applied gate voltage. We demonstrate on a phosphorusdoped silicon nanowire that the ionization energy can be effectively tuned and the impurity backscattering can also be reduced. For instance, even without special engineering of doping, the free carrier density may increase by 40% in a Silicon nanowire with 15 nm diameter and special cross section. Our proposal has profound implications to fabricate nanowire devices With high carrier density.

(2010) EPL. 90, 3, 37002. Abstract
We predict a new class of threedimensional topological insulators in thalliumbased IIIVVI2 ternary chalcogenides, including TlBiQ(2) and TlSbQ(2) (Q = Te, Se and S). These topological insulators have robust and simple surface states consisting of a single Dirac cone at the G point. The mechanism for topological insulating behavior is elucidated using both firstprinciple calculations and effective field theory models. Remarkably, one topological insulator in this class, TlBiTe2, is also a superconductor when doped with ptype carriers. We discuss the possibility that this material could be a topological superconductor. Another material, TlSbS2, is on the border between topological insulator and trivial insulator phases, in which a topological phase transition can be driven by pressure.

(2010) Physical Review B. 81, 4, 41307. Abstract
We investigate the crossover regime from threedimensional topological insulators Bi2Te3 and Bi2Se3 to twodimensional topological insulators with quantum spin Hall effect when the layer thickness is reduced. Using both analytical models and firstprinciples calculations, we find that the crossover occurs in an oscillatory fashion as a function of the layer thickness, alternating between topologically trivial and nontrivial twodimensional behavior.
2009

(2009) Physical Review Letters. 103, 26, 266102. Abstract
Atomic motion through excitation of extended surface electronic states on Ge(001) is studied using extraction of electrons by scanning tunneling microscopy and density functional theory. Singleelectron excitation into the surface states nonlocally alters the tilting orientation of the surface Ge dimer, and the change rate depends on the excitation energy. Theoretical investigations identify the excited electronic states for the dimer motion, and clarify the strong coupling between the surface state electrons and a local vibrational mode of the dimer for changing the tilting orientation.


(2009) Physical Review B. 79, 23, 235437. Abstract
Using firstprinciples methods we studied structural and electronic properties of asymmetric heterogeneous XSi (X=Ge, Sn, and Pb) dimers on the Si(001) surface and their scatterings for the quasionedimensional pi* electrons. The XSi dimer with impurity atom X at the lower position scatters more strongly the pi* electrons than that with X at the upper position. Calculated scattering potentials can be qualitatively explained by the difference in porbital energy between Si and the lower atom of the XSi dimer. We predict that the amplitude of electronic standing waves changes significantly between the two oppositely buckled XSi dimers in differential conductance images of scanning tunneling microscopy. This suggests the possibility of fabricating atomic switches to control the conduction of pi* electrons along the dimer row. Our proposed atomic switches could be achieved by flipping the impurity dimers on the Si(001) surface using the method developed in recent experiments [K. Sagisaka et al., Phys. Rev. Lett. 91, 146103 (2003)]. Finally, we proposed the model for dimerflipping mechanism, which can explain previous experiment [K. Sagisaka and D. Fujita, Phys. Rev. B 71, 245319 (2005)].

(2009) Applied Physics Letters. 94, 19, 193106. Abstract
Our firstprinciples calculations indicate the possibility of preparing spinpolarized scanning tunneling microscopy (SPSTM) probes from Fedoped capped carbon nanotubes (CNTs). The structural stability, magnetic moment, and electronic property of hybrid systems are found to depend on the Fe adsorption site, which is attributed to the hybridization between Fe 3d and C 2p orbitals. The CNTs with Fe atoms adsorbed at the tiptop are demonstrated to be promising candidates for the SPSTM probe, with a high spin polarization leading to a completely spinpolarized current at lower voltages. In contrast, the CNTs encapsulating Fe atom are basically nonmagnetic, and thus useless for the SPSTM probe application in nature.

(2009) Surface Science. 603, 5, p. 781787 Abstract
Surface motion of a topological defect between p(2 x 2) and c(4 x 2) structures, a "kink", across buckled SnGe and SiGe dimers on Ge(0 0 1) surfaces was investigated using scanning tunneling microscopy. Energy thresholds of pi' electrons for flipping these dimers in the kink are obtained by analyzing the kink Surface motion. Electronic states of these systems and energy barriers for flipping the dimers are examined by firstprinciples calculations for considering elementary processes of the electronicallyexcited flip motion of the dimers. We propose that the flip motion is caused by a resonant scattering of the pi' electrons with localized electronic states at the kink. (c) 2009 Elsevier B.V. All rights reserved.
2008

(2008) Journal of the American Chemical Society. 130, 50, p. 1701217015 Abstract
We investigate the electronic structures and electron emission properties of alkalidoped boronnitride nanotubes (BNNTs) using densityfunctional theory calculations. We find that the nearly freeelectron (NFE) state of the BNNT couples with the alkali atom states, giving rise to metallic states near the Fermi level. Unlike the cases of potassiumdoped carbon nanotubes, not only the s but the d orbital state substantially takes part in the hybridization, and the resulting metallic states preserve the freeelectronlike energy dispersion. Through firstprinciples electron dynamic simulations under applied fields, it is shown that the alkalidoped BNNT can generate an emission current 2 orders of magnitude larger than the carbon nanotube. The nodeless wave function at the Fermi level, together with the lowered work function, constitutes the major advantage of the alkalidoped BNNT in electron emission. We propose that the alkalidoped BNNT should be an excellent electron emitter in terms of the large emission current as well as its chemical and mechanical stability.

(2008) Physical Review B. 78, 8, 81401. Abstract
Scattering potentials for pi* electrons at SiGe and SnGe dimers on a Ge(001) surface are studied by scanning tunneling microscopy and ab initio calculations. Phaseshift analysis of standing waves in dl/dV images reveals that Si and Sn atoms located in the conduction path of pi* electrons form potentials with the sign opposite to each other. Densityfunctional calculations and simple calculations based on the nearlyfreeelectron model explain the observed potential structures. These results are qualitatively understood by relative porbital energy of the Si, Sn, and Ge atoms.

(2008) Physical Review B. 77, 24, 245303. Abstract
We have comparatively studied the hydrogen (H)induced metallization mechanism and characteristics for zinc oxide (ZnO) nanowires with (2110) side surface and the ZnO surface with the same index by density functional theory calculations. It is found that the ZnO surfaces and nanowires with only surface oxygen (O) atoms saturated by H (denoted as ZnOH) become metallic, while the pristine and heterolytically chemisorbed systems are semiconducting. For the ZnOH(2110) surface, the 4s states of surface Zn atoms contribute to the sawtoothlike conducting pathways along the [0001] direction, rendering the surface metallic. By contrast, in the ZnOH(2110) nanowire, apart from the 4s states of side surface Zn atoms, both the Zn4s and O2p states of the corner atoms also significantly contribute to Hinduced metallization due to the curvature effect. In this case, the linear and sawtoothlike conducting pathways exist in the corner and the sides, respectively. With the semiconductortometal transition dependent on hydrogen concentration, ZnO(2110) nanowire is proposed to be a good candidate for nanoscale chemical sensors or electronic devices for miniaturization.
2007

(2007) Applied Physics Letters. 91, 10, 103107. Abstract
Firstprinciples calculations of crystalline silicon nanotubes (SiNTs) show that nonuniformity in wall thickness can cause sizable variation in the band gap as well as notable shift in the optical absorption spectrum. A unique quantum confinement behavior is observed: the electronic wave functions of the valence band maximum and conduction band minimum are due mainly to atoms located in the thicker side of the tube wall. This is advantageous to spatially separate the doping impurities from the conducting channel in doped SiNTs. Practically, the performance of the SiNTbased transistors may be substantially improved by selective p/n doping in the thinner side of the tube wall in the spirit of modulation doping.

Theoretical study on energy levels and photophysical properties of pn block oligomers(2007) Journal of Optoelectronics and Advanced Materials. 9, 5, p. 13731376 Abstract
Recently, a novel series of oligomers consisting of thiophene as ptype unit and oxadiazole as ntype unit were successfully synthesized. In this article, we present a firstprinciples study of the electronic, and optical properties on pn diblock and triblock oligomers systematically. Theoretical studies showed changing the number of thiophene and oxadiazole unit could effectively modulate the electronic properties of pn diblock and triblock oligomers. The electronic and photophysical properties of theoretical calculation results were in consistent with observed experimental results. These results provide useful guidelines to control the band gap principle of pn hereostructure oligomers systems, and fundamental insights into understanding the electronic and photophysical properties in pn hereostructure oligomers systems.
2006

(2006) Applied Physics Letters. 89, 2, 23104. Abstract
Firstprinciples calculations are performed to study the mechanical properties, electronic structure, and uniaxialstress effects of betaSiC nanowires (NWs). It is found that the band gap of SiC NWs becomes larger as their diameter decreases because of the quantum confinement effect, but increases (decreases) slightly with increasing tensile (compressive) stress up to about 12 GPa. The calculated Young's modulus and tensile strength of SiC NWs are about 620 and 52 GPa, respectively, in accordance with the experimental data. The characteristics of their mechanical and electronic properties suggest that betaSiC NWs may be used in electronic composites as reinforcement nanomaterials or in nanoscale electronic/photoelectric devices under harsh environments. (c) 2006 American Institute of Physics.

(2006) Physical Review B. 73, 15, 155432. Abstract
The structural characteristics, bonding modes, and electronic properties of singlecrystalline silicon nanotubes (scSiNTs) are investigated by using the firstprinciples method. These pristine scSiNTs with sp(3) hybridization, constructed by the bulklike tetrahedrally coordinated Si atoms, are found to be energetically stable. The electronic property is sensitive to the external diameter, tubewall thickness, and tubeaxis orientation due to quantum confinement effects. A direct band gap is observed in SiNTs with smaller sizes. The band gap increases monotonically with decreasing tubewall thickness, in accord with the substantial blueshift observed in the experiment. It is suggested that this type of SiNTs would have promising practical applications in nanoscale lightemitting devices and electronic devices.