Twistronics
Electronic systems which have energy bands independent of momentum (called flat bands) provide a platform where electrons coulomb interactions dominate over their kinetic energy. This platform can be used to study strongly correlated phenomena like superconductivity and magnetism. 2D van der Waals (vdW) materials are currently being used as the building block to make samples to study these correlated phenomena. The vdW materials like graphene, hexagonal boron nitride or transition metal dichalcogenides are placed on top of each other in various combinations to make samples. One interesting combination is to place two or more graphene layers on top of each other at some relative twist angle. The relative rotation of these layers results in moiré superlattices with a periodicity of the order of ≤ 15nm that results in electronic flat bands. Various correlated and topological electronic states have been realized in these flat band systems.
Flat band physics was first predicted in moiré superlattices in vdW heterostructures in twisted bilayer graphene (TBG). However, the experimental realization of these systems started with the observation of superconductivity (SC), which possesses a tunable (gated) phase-space resembling high-Tc superconductors as well as other correlated phases in ‘magic angle’ (θ≈1.1˚) TBG. The twist angle driven flat bands provide an angle and gate tunable platform to study various correlated phases in a single system. The quantum anomalous hall effect is also one of the correlated phases observed in TBG samples aligned to hBN. There is a possibility to study the proximity effects of superconducting and magnetic phases in a single device. One way to study these effects is to engineer a gate defined Josephson junction in these systems which host both these phases, that also ensures ultraclean interface between them. In our group we study superconducting and magnetic correlated phases in twisted heterostructures using Josephson junctions.
One such system which we are studying is twisted tri-layer graphene (TTG) heterostructure encapsulated in hexagonal Boron Nitride (hBN) single crystals. The Moiré superlattice with periodicity ~ 9nm is formed in TTG for a relative angle of ~1.6˚ (top and middle layer ϴTM ~ 1.6˚, the middle and bottom layer ϴMB ~ -1.6˚), which results in a set of flat bands, as well as gapless Dirac bands. Recent quantum transport measurements on TTG have shown insight into the spin configuration of the SC order parameter. The SC in TTG is found to persist above the Zeeman energy in the presence of in-plane magnetic field, thereby pointing to a possible triplet pairing. In addition, an electric field tunable crossover from a weakly coupled BCS SC phase to a strongly coupled BEC SC phase was also indicated in some studies. These exotic results on TTG superconductor open a new gateway to investigate long-standing fundamental questions in condensed matter physics using this system.
Figure 1: (a) Schematic diagram of our Josephson junction device consisting of two hBN-encapsulated TTG, metal (Ti/Au) top gate which splits the sample into two regions, separation is ~70 nm. Si back gate (Vbg) and metal top gates (Vtgl & Vtgr) are used to tune displacement field and carrier density. (b) Line-cut of Rxx vs. v at D=0, inset shows the optical image of measured device. (c) Measured Rxx via dual gated phase diagram of filling factor v vs. D at 15 mK. Both top gates are varied simultaneously as a function of Si BG. Dark blue color region highlights the superconductivity pockets. (e) Fraunhofer patterns measured for electron-SC /Normal/hole-SC configuration, respectively.