Aditya Mewar, The University of Texas at Austin

"Topological Aspects of Mesoscopic Moire Materials"

Abstract: Twisted 2D materials are known to host both structural and electronic topological properties originating from the emergence of the Moire superlattice. In real space, dislocation line defects can be formed between regions of different stacking order. In the case of twisted bilayer graphene(TBLG) these dislocation lines connect the AA stacking regions and form the underlying triangular Moire lattice. It is also seen experimentally that TBLG prepared at a certain twist angle can spontaneously untwist, suggesting thermodynamic instability. Using molecular dynamics simulations we study the energetics of the untwisting process and make a connection to the topology of the dislocation network connecting supercells. On the electronic side, Transition metal dichalcogenides (TMD’s) are predicted to host topological insulator behavior and display edge states. We study quantum transport properties of these edge states with simulations on finite size tight binding systems based on the Haldane model.

In minimally twisted bilayer graphene, where the global twist is typically less than 1 degree and placed in an external electric field, it is possible to induce chiral states in the domain wall regions. The electronic domain wall arises due to the Chern numbers of the mirror symmetry related Bernal regions. Spatially these domain walls coincide with the dislocation network of the Moire lattice.

Lattice relaxation can have a dramatic effect on these properties and is relevant to any experimental realization. We simulate the mechanically equilibrated structure under the influence of classical atomic potentials and use the relaxed lattice to calculate the hoppings of an inter-layer tight binding model and study closed system and transport properties. Our research aims to shed light on the design of novel materials with controllable topological properties.