Yongxin Zeng, UT-Austin

"Exciton condensation in electron-hole bilayers"

Abstract: Excitons are bosonic quasiparticles composed of an electron and a hole. The idea of Bose-Einstein condensation of excitons was first proposed back in the 1960s, but the pursuit of exciton condensation was revived in recent years by the study of interlayer excitons in bilayer two-dimensional materials. In this talk I will introduce the theoretical and experimental background of exciton condensation in electron-hole bilayers, and then talk about three of our works on exciton condensation and related phenomena in different bilayer electron-hole systems. In the first work [1] we study the electrical control of an ideal electron-hole bilayer with a perfectly opaque tunnel barrier. I will explain when the system becomes an exciton fluid with electrically tunable chemical potential, and discuss recent experimental progress on transition metal dichalcogenide bilayers. In the second work [2] we study the effects of an in-plane magnetic field on the quantum spin Hall phase transitions in InAs/GaSb quantum wells. We find that the interplay of excitonic coupling, interlayer hybridization, and field-induced momentum shift between conduction and valence bands leads to three density-wave states that all break translational symmetry but possess distinct topological properties. In the last work we study a nonequilibrium electron-hole bilayer in which interlayer tunneling and bias voltage are both present. We derive a nonequilibrium field theory that describes the system and find that in the large-bias limit, interlayer tunneling effectively decreases the band gap and increases the temperature for intervalley excitons. I will conclude with an outlook of future directions in this field.

[1] Y. Zeng and A.H. MacDonald. Phys. Rev. B 102, 085154 (2020)
[2] Y. Zeng, F. Xue, and A.H. MacDonald. Phys. Rev. B 105, 125102 (2022)