Speaker: Ching-Kai Chiu (Univ. of Maryland)

Abstract

Topological states of matter start from topological insulators and superconductors, which are fermionic systems with bulk energy gaps separating the valence and conduction bands. They possess gapless boundary states that are topologically protected and are related to physical quantities, such as quantized Hall conductivity. Topological semimetals, which are another type of topological states, exhibits band crossings near the Fermi energy, which are protected by the topological invariants. In this talk, we first review topological insulators and topological semimetals and then put our focus on topological nodal line semimetals. As reflection symmetry or time-reversal inversion symmetry is preserved, with a non-contractible integral loop respecting the symmetry, Berry phase is quantized. Topological nodal lines can be enclosed in the integral loop and pi-Berry phase topologically protects the nodal lines. However, pi-Berry phase is not the sufficient condition leading to the presence of stable surface states. Their presence crucially depends on the location of the termination and the atom distribution in the unit cell. By using these new conditions we further examine if stable surface states exist in known topological nodal line materials.

Time & Date: 10:30am, Jun. 20 (Wed.), 2018

Venue: Rm. N308, UCAS Teaching Building [View Map]