M Zahid Hasan
Princeton University, USA
Title: Discovery of Weyl fermion and topological Fermi arc quasiparticles in condensed matter systems
Biography
Biography: M Zahid Hasan
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 (Dirac and Majorana) modes are robust only in the presence of a gap (M.Z. Hasan and C.L. 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.