Heterobilayers of 2D materials as a platform for excitonic superfluidity

TL;DR

This paper proposes stable 2D heterostructures with overlapping bands as ideal platforms for excitonic superfluidity, identifying specific material pairs that enable equilibrium condensation without external doping or voltage.

Contribution

It introduces intrinsically stable 2D heterostructures with doubly-indirect bands as new platforms for excitonic condensation, overcoming limitations of monolayers and other bilayers.

Findings

Identified candidate heterostructures for excitonic condensation.

Predicted equilibrium excitonic superfluidity without external doping.

Potential applications in superfluid transport and dissipationless devices.

Abstract

Excitonic condensate has been long-sought within bulk indirect-gap semiconductors, quantum wells, and 2D material layers, all tried as carrying media. Here we propose intrinsically stable 2D semiconductor heterostructures with doubly-indirect overlapping bands as optimal platforms for excitonic condensation. After screening hundreds of 2D materials, we identify candidates where spontaneous excitonic condensation mediated by purely electronic interaction should occur, and hetero-pairs Sb2Te2Se/BiTeCl, Hf2N2I2/Zr2N2Cl2, and LiAlTe2/BiTeI emerge promising. Unlike monolayers, where excitonic condensation is hampered by Peierls instability, or other bilayers, where doping by applied voltage is required, rendering them essentially non-equilibrium systems, the chemically-specific heterostructures predicted here are lattice-matched, show no detrimental electronic instability, and display broken type-III gap, thus offering optimal carrier density without any gate voltages, in true-equilibrium. Predicted materials can be used to access different parts of electron-hole phase diagram, including BEC-BCS crossover, enabling tantalizing applications in superfluid transport, Josephson-like tunneling, and dissipationless charge counterflow.