Quadriexcitons and excitonic condensate in a symmetric electron-hole bilayer with valley degeneracy

TL;DR

This study uses quantum Monte Carlo simulations to map the phase diagram of a symmetric electron-hole bilayer with valley degeneracy, revealing how valley effects influence excitonic phases and guiding experimental device design.

Contribution

It provides the first detailed phase diagram showing the impact of valley degeneracy on excitonic and quadriexcitonic phases in electron-hole bilayers.

Findings

Valley degeneracy reduces the stability region of the excitonic condensate.

Quadriexcitons dominate at small interlayer distances.

The excitonic condensate exists within a specific density window.

Abstract

Using quantum Monte Carlo simulations we have mapped out the zero temperature phase diagram of a symmetric electron-hole bilayer with twofold valley degeneracy, as function of the interlayer distance $d$ and in-layer density $n$. We find that the effect of the valley degeneracy is to shrink the region of stability of the excitonic condensate, in favor of quadriexcitons at small $d$ and of the four-component plasma at large $d$, with minor effects on the value of the excitonic condensate fraction. The enclosure of the condensate in a density window possibly explains why anomalous tunnelling conductivity, interpreted as signature of condensation, is observed only between two finite values of carrier density in graphene bilayers. Our phase diagram may provide directions to select device parameters for future experiments.