IAS Distinguished Lecture

New Topological States of Matter


Topological states of matter are of interest in connection to their highly robust properties. For example, in a topological insulator, surface states are robustly protected by time-reversal symmetry. The speaker reviews the basic concepts defining such topological matter with a focus on the experimental probes that revealed their topological character via the bulk-surface correspondence. Topological materials can host Dirac, Majorana or Weyl fermions as emergent quasiparticles on their boundaries, vortices or the bulk. Theory suggests that a Weyl semimetal is a gapless system (unlike gapped systems such as topological insulators and superconductors) whose low energy bulk excitations are massless Weyl fermions. The chiralities associated with the Weyl nodes can be understood as topological charges leading to split monopoles and anti-monopoles of Berry curvature in momentum space. Due to this topology a Weyl semimetal exhibits Fermi arc quasiparticles on its surface and chiral anomaly in its bulk electromagnetic transport response. These arcs (“fractional” Fermi surfaces) are discontinuous or disjoint segments of a two dimensional Fermi contour, which are terminated onto the projections of the bulk Weyl nodes on the surface. Such Fermi arc quasiparticles can live only on the boundary of a 3D crystal in conjunction with the Weyl nodes and collectively represent the realization of a new state of topological matter in recent experiments.

About the speaker

Prof. M. Zahid Hasan obtained his PhD from Stanford University in 2002. He was then a Robert H. Dicke Fellow in fundamental physics at Princeton University and held several extended visiting appointments at Bell Laboratories (Murray Hill) and Lawrence Berkeley National Laboratory. He is currently the Professor of Physics in Princeton University. He was named as an American Competitive and Innovation Fellow (2010-2015) by the US National Science Foundation.

Prof. Hasan’s research interests focus on quantum Hall-like topological phases, exotic correlated superconductors, quantum phase transitions, and topological quantum computing physics. In recent years, he, along with his team, has experimentally discovered topological-order in bulk solids (3D topological insulators) including the single-Dirac-cone series using spin-resolved and phase-sensitive spectroscopic measurements.

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