# Discovery of Weyl fermion and topological Fermi arc quasiparticles in condensed matter systems

FCMP Lecture

Date: Tuesday, Oct. 20

Time: 3:10 pm

Place: SST 106

Speaker: Zahid Hasan (Princeton University)

Title: Discovery of Weyl fermion and topological Fermi arc quasiparticles in condensed matter systems

Abstract: Topological matter can host Dirac, Majorana and Weyl fermions as quasiparticle modes on their boundaries.

First, I briefly review the basic theoretical concepts defining insulators and superconductors where topological surface state modes are robust only in the presence of a gap (Hasan & Kane; Rev. of Mod. Phys. 82, 3045 (2010)). In these systems topological protection is lost once the gap is closed turning the system into a trivial metal. A Weyl semimetal is the rare exception in this scheme which is a topologically robust metal (semimetal) whose low energy emergent excitations are Weyl fermions. In a Weyl fermion semimetal, 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. This gives a measure of the topological strength of the system. Due to this topology a Weyl semimetal is expected to exhibit 2D Fermi arc quasiparticles on its surface. These arcs ("fractional" Fermi surfaces) are discontinuous or disjoint segments of a two dimensional Fermi contour, which are terminated onto the projections of the Weyl fermion nodes on the surface (Xu, Belopolski et al., Science 349, 613 (2015) and Huang, Xu, Belopolski et al., Nature Commun. 6:7373 (2015)). I show that Fermi arc quasiparticles can only live on the boundary of a 3D crystal which collectively represents the realization of a new state of quantum matter (Xu, Belopolski, Alidoust et al., Nature Physics (2015)).

Speaker Info:

M. Zahid Hasan is a Professor of Physics at Princeton University. He obtained his Ph.D. in 2002 from Stanford University, working at SLAC and Brookhaven National Laboratory. He is an expert in the physics of quantum matter, emergent phenomena in electron systems, and advanced spectroscopic high resolution imaging techniques. His research has focused on quantum Hall-like topological phases, exotic superconductors, quantum phase transitions, and topological quantum matter. He played a pioneering role in the experimental discoveries of bulk topological insulators, helical topological superconductors, Weyl fermion materials and related new forms of quantum matter. A highly-cited researcher, he has published more than 120 papers in refereed journals and several review articles on topological matter.

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