Topological phases in narrow-band systems

Abstract

I discuss several novel topological phases in correlated electron systems, realized through spin-orbit interactions and lattice effects especially narrow-band systems. The first realizes the fractional quantum Hall effect using geometric frustration and ferromagnetism to obtain a nearly flat band with a large bandgap and non-zero Chern number. This system can support this effect at high temperatures upon partial filling of the flat band. The second proposal builds upon this system: as the ground state is a fractional quantum Hall state, excitations of this state are anyons when there is an incommensurate filling. The underlying lattice allows access to a new regime in which the anyon gas can form a charged superfluid, including states with intrinsic topological order or that similar to a BCS-type state. The third proposal studies topological crystalline insulators and strain as an effective gauge field on the surface state Dirac fermions. The zero-energy Landau orbitals form a flat band where the high density of states gives rise to the interface superconductivity observed in IV-VI semiconductor multilayers at high temperatures, with non-BCS behavior. A discussion of superconductivity in flat band systems concludes and is contrasted with classic results for a typical electron gas. This work closely parallels that in references [1, 2, 3].