Theory of Two-Dimensional Spatially Indirect Equilibrium Exciton Condensates

TL;DR

This paper develops a theoretical framework for understanding equilibrium exciton condensates in bilayer two-dimensional materials, revealing phase transitions and measurable interaction properties.

Contribution

It introduces a mean-field and bosonic exciton model for bilayer exciton condensates in 2D materials, analyzing phase diagrams and collective modes.

Findings

Phase transition between single and dual condensate states

Derived expressions for exciton-exciton interaction strength

Predicted measurable capacitance signatures

Abstract

We present a theory of bilayer two-dimensional electron systems that host a spatially indirect exciton condensate when in thermal equilibrium. Equilibrium bilayer exciton condensates (BXCs) are expected to form when two nearby semiconductor layers are electrically isolated, and when the conduction band of one layer is brought close to degeneracy with the valence band of a nearby layer by varying bias or gate voltages. BXCs are characterized by spontaneous inter-layer phase coherence and counterflow superfluidity. The bilayer system we consider is composed of two transition metal dichalcogenide monolayers separated and surrounded by hexagonal boron nitride. We use mean-field-theory and a bosonic weakly interacting exciton model to explore the BXC phase diagram, and time-dependent mean-field theory to address condensate collective mode spectra and quantum fluctuations. We find that a phase transition occurs between states containing one and two condensate components as the layer separation and the exciton density are varied, and derive simple approximate expressions for the exciton-exciton interaction strength which we show can be measured capacitively.