The 38th QLC seminar

Speaker：Dr. David Cortie　(ANSTO)
Date & Time : Friday, February 24, 2023.　 10:00～11:30
Place：Seminar Room (2F), IMRAM West 1st Building, Katahira campus, Tohoku University

Title：The effect of strong non-magnetic disorder and amorphization in a 3D topological insulator

Abstract：
The sub-classes of quantum insulators can be distinguished using invariants (e.g., ℤ2) — simple groups of integers related to the Berry curvature — which also encode information on their distinctive physical properties. A special case is the 3D strong topological insulator (TI) having one “strong” index ℤ2=1, which is distinct from the normal vacuum (ℤ2=0).   Novel electronic states appear at an interface where the characteristic invariant ℤ2 switches from 1 to 0. These conductive states offer prospects for quantum electronics; however, a method is needed to spatially control ℤ2 to pattern conducting channels. Recently, we have discovered that experimentally modifying Sb2Te3 single-crystal surfaces with an ion beam switches the latter topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. X-ray, electron and neutron diffraction, were used to track the structural transition, together with high-resolution scanning transmission electron microscopy. The observations in the transport are supported by density functional theory using ab initio molecular dynamics, and model Hamiltonian calculations.  The large change in conductivity is concurrent with a transition from ℤ2 = 1 → ℤ2 = 0 at a threshold disorder strength. This result also shows that although amorphous topological insulators may exist in theory [2-4], there is no universal pathway from a crystalline topological insulator to an amorphous topological insulator. Despite the topologically trivial nature of the amorphous phase, it is practically useful. In particular, this allows ion-beam treatment for subtractive lithography to pattern arrays of topological surfaces, edges and corners in single crystal surfaces, which are the building blocks of topological electronics.

Biography
David Cortie obtained his PhD in physics from the University of Wollongong, whilst based at the Australian Nuclear Science and Technology Organisation. He then did postdoctoral work at the University of British Columbia, the Max-Planck Society, the Australian National University and the Australian Nuclear Science and Technology Organisation. In 2020, he joined the Australian Nuclear Science and Technology Organisation as a neutron scientist specialising in the applications of polarised neutron reflectometry. His research focuses on the interplay between the structure, dynamics, magnetism, and transport phenomena in quantum materials.

1)    A. Bake, Q. Zhang, D. L Cortie et. al., Top-Down Patterning of Topological Surface and Edge States using a Focused Ion Beam, Nature Communications, In press, (2023),
2)    Agarwala, A., Topological insulators in amorphous systems. Physical Review Letters, 118, 236402 (2017).
3)    Mitchell, N. P., Nash, L. M., Hexner, D., Turner, A. M. & Irvine, W. T. Amorphous topological insulators constructed from random point sets. Nature Physics 14, 380-385 (2018).
4)    Wang, C., Cheng, T., Liu, Z., Liu, F. & Huang, H. Structural Amorphization-Induced Topological Order. Physical Review Letters 128, 056401 (2022).

Committee Chair : Taku J SATO (Tohoku University)