A 2200 year old Chinese proverb tells the story of a hyperbolic salesman who claims to have an unbreakable shield and an unstoppable spear, but can’t explain what will happen if you strike one with the other. As it turns out, electrons found at the surface of topological insulator (TI) materials are also “unstoppable” - they’re theoretically immune to localization by static, non-magnetic disorder (Anderson localization). Even so, pronounced resonance states are known to form around lattice defects at a TI surface, and superficially resemble the highly localized impurity states that appear around conventional semiconductor dopant atoms. I will talk about our investigations into this seeming paradox, including results from ARPES band structure mapping, STM, and exact diagonalization numerics. These analyses suggest that localization-resistance causes the TI impurity states to collectively give rise to a new conducting electron gas, representing an exciting new mechanism for band structure engineering. Strangely enough, the paradox of the shield and spear can also be resolved if you allow the shield to undergo a topological phase transition.
L. Andrew Wray is Assistant Professor of Physics. He holds a Ph.D. in Experimental Condensed Matter Physics from Princeton University (2010) and a B.A. in Physics from the University of California, Berkeley, and conducted pre-doctoral research at the Institute of Physics, Chinese Academy of Sciences. After completing his Ph.D., Wray led research projects at the Lawrence Berkeley National Laboratory (LBNL) and Stanford Institute for Materials and Energy Sciences (SIMES) prior to joining the New York University faculty in 2014. Wray’s research focuses on the discovery, characterization and manipulation of novel quantum states inside materials. His experiments have been instrumental in identifying the first realizations of topologically ordered quantum states of matter such as the topological insulator and topological superconductor. Incisive in-situ investigation of the energy and momentum profiles of quantum states is made possible by rapidly advancing capabilities at state of the art X-ray facilities. Wray maintains active involvement in proposing new X-ray science technologies and developing novel methods to simulate and analyze resonant interactions between X-rays and matter.
Host: Dong Qian dqian@sjtu.edu.cn