Publications

2010

1. Theoretical prediction of topological insulator in ternary rare earth chalcogenides

   Yan B., Zhang H. J., Liu C. X., Qi X. L., Frauenheim T. & Zhang S. C. (2010) Physical Review B. 82, 16, 161108.  Abstract

   A different class of three-dimensional topological insulator, ternary rare-earth chalcogenides, is theoretically investigated with ab initio calculations. Based on both bulk band-structure analysis and the direct calculation of topological surface states, we demonstrate that LaBiTe ₃ 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 Bi₂ Te₃ families of topological insulators for their well-matched lattice constants and chemical composition.

2. Gate-controlled donor activation in silicon nanowires

   Yan B., Frauenheim T. & Gali Á. (2010) Nano Letters. 10, 9, p. 3791-3795  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 phosphorus-doped 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.

3. Theoretical prediction of topological insulators in thallium-based III-V-VI₂ ternary chalcogenides

   Yan B., Liu C. X., Zhang H. J., Yam C. Y., Qi X. L., Frauenheim T. & Zhang S. C. (2010) EPL. 90, 3, 37002.  Abstract

   We predict a new class of three-dimensional topological insulators in thallium-based III-V-VI₂ ternary chalcogenides, including TlBiQ ₂ and TlSbQ₂ (Q=Te, Se and S). These topological insulators have robust and simple surface states consisting of a single Dirac cone at the Γ point. The mechanism for topological insulating behavior is elucidated using both first-principle calculations and effective field theory models. Remarkably, one topological insulator in this class, TlBiTe₂, is also a superconductor when doped with p-type 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.

4. Oscillatory crossover from two-dimensional to three-dimensional topological insulators

   Liu C., Zhang H., Yan B., Qi X., Frauenheim T., Dai X., Fang Z. & Zhang S. (2010) Physical Review B. 81, 4, 041307.  Abstract

   We investigate the crossover regime from three-dimensional topological insulators Bi2 Te3 and Bi2 Se3 to two-dimensional topological insulators with quantum spin Hall effect when the layer thickness is reduced. Using both analytical models and first-principles calculations, we find that the crossover occurs in an oscillatory fashion as a function of the layer thickness, alternating between topologically trivial and nontrivial two-dimensional behavior.