Evidence for equilibrium excitons and exciton condensation in monolayer WTe2

TL;DR

This study provides evidence that monolayer WTe2 hosts equilibrium excitons with large binding energy, leading to unique insulating states and topological properties, revealing strong electron-hole correlations in a two-dimensional material.

Contribution

The paper demonstrates the existence of equilibrium excitons in monolayer WTe2 with large binding energy and explores their role in its topological and insulating behaviors, a novel insight into 2D correlated systems.

Findings

Excitons form spontaneously in monolayer WTe2 at high temperatures.

The exciton binding energy exceeds 100 meV, with a radius as small as 4 nm.

A charge-ordered insulating state emerges below 100 K.

Abstract

A single monolayer of the layered semimetal WTe2 behaves as a two-dimensional topological insulator, with helical conducting edge modes surrounding a bulk state that becomes insulating at low temperatures. Here we present evidence that the bulk state has a very unusual nature, containing electrons and holes bound by Coulomb attraction (excitons) that spontaneously form in thermal equilibrium. On cooling from room temperature to 100 K the conductivity develops a V-shaped dependence on electrostatic doping, while the chemical potential develops a ~43 meV step at the neutral point. These features are much sharper than is possible in an independent-electron picture, but they can be largely accounted for by positing that some of the electrons and holes are paired in equilibrium. Our calculations from first principles show that the exciton binding energy is larger than 100 meV and the radius as small as 4 nm, explaining their formation at high temperature and doping levels. Below 100 K more strongly insulating behavior is seen, suggesting that a charge-ordered state forms. The observed absence of charge density waves in this state appears surprising within an excitonic insulator picture, but we show that it can be explained by the symmetries of the exciton wave function. Monolayer WTe2 therefore presents an exceptional combination of topological properties and strong correlations over a wide temperature range.