Time and place: 2019.1.4, 10:00 am, Wuhan National High Magnetic Field Center B206

Presenter: Feng Liu

Title: Topological dipoles and quadrupoles

Abstract：

Topology offers us a unique dimension of designing solid-state materials. One famous example is the Quantum spin Hall effect (QSHE) where electrons of opposite spins propagate oppositely. The origin of QSHE comes from a geometric field strength in momentum space that is the so-called Berry curvature. Besides QSHE, the geometric vector potential whose curl yields the Berry curvature – the Berry connection, can induce an electric dipole even without Berry curvature. The topological electric dipoles result fractional surface charges that manifest as topological edge states, which are robust to defects and edge roughness. Furthermore, a pair of such the topological dipoles can form a high order multipole – quadrupole, which corresponds to the topological corner states. In this talk we will discuss a simple tight-binding model that possesses topological dipoles and quadrupoles in zero Berry curvature. Experimental realizations based on solid-state material and dielectric photonic crystal are proposed. Furthermore, we show that in a pure quadrupole phase, topological edge state is pseudo-spin polarized in general.