Exciton condensation from level repulsion: application to bilayer graphene

Abstract

Exciton condensation in semiconductors and semimetals has long been predicted but remains elusive. In a semiconductor, condensation occurs when the exciton binding energy matches the band gap. This binding energy results from a balance between Coulomb attraction, which enhances it, and kinetic energy, which suppresses it. However, reducing kinetic energy typically increases screening, weakening Coulomb attraction. Empirically, in most candidate materials, the binding energy remains below the band gap, with few external parameters capable of altering this balance. Here, we propose an in-plane electric field as a control parameter. This field induces hybridisation between even- and odd-parity excitons, and the resulting level repulsion effectively enhances binding energy. We argue that this mechanism is generic to excitons in semiconductors and illustrate it with a model of biased bilayer graphene. Bilayer graphene is chosen since it has a tunable band gap, making it an excitonic condensate candidate and moreover, the Zener tunnelling rate contains, in addition to the usual exponential decay, a non-standard oscillating component -- thanks to details of the electron dispersion. Analogous to quantum oscillations, we propose that Fourier spectrum of the current-voltage data allows for a novel test of exciton condensation. Finally, we show that the is a large excitonic gap to critical temperature ratio -- a clear prediction for STM studies.