Topological Insulators and Wigner Solids in Thin Films of Cadmium Arsenide

Bulk cadmium arsenide (Cd₃As₂) hosts topologically non-trivial bands in its electronic structure. In thin films, it can be engineered to a variety of new topological phases. In this talk, we will discuss topological and correlated insulator states in high-quality quantum wells of Cd₃As₂, which we grow by molecular beam epitaxy. Using Landau level spectroscopy, we show that the films transition to a two-dimensional topological insulator phase at quantum well thicknesses around 20 nm. At lower thicknesses, the topological gap closes and reopens as a trivial gap. We show that new types of insulators appear near this critical thickness. Using transport measurements, we identify these new insulators as hole and electron Wigner solids, respectively, that form spontaneously at zero magnetic field. Hysteresis and voltage fluctuations point to domain motion across the pinning potential and disappear at finite temperature as thermal fluctuations overcome the potential. We discuss the interplay of topology, electron correlations, and spin-orbit coupling that gives rise to these novel insulating states.