Evidence for equilibrium exciton condensation in monolayer WTe2

Abstract

We present evidence that the two-dimensional bulk of monolayer WTe2 contains 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 step at the neutral point. These features are much sharper than is possible in an independent-electron picture, but they can be accounted for if electrons and holes interact strongly and 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 behaviour is seen, suggesting that a charge-ordered state forms. The observed absence of charge density waves in this state is surprising within an excitonic insulator picture, but we show that it can be explained by the symmetries of the exciton wavefunction. Therefore, in addition to being a topological insulator, monolayer WTe2 exhibits strong correlations over a wide temperature range.

A scenario for high-temperature excitonic insulators

“…We hope these issues can be overcome in actual experiments, or, in other way, the method we proposed may inspire the discovery of more suitable materials for EI. Recently, references [16,17] proved that WTe 2 went through an EI transition linked to indirect exciton at around 100 K, in alignment with our expectation.…”

Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers II: Zero-field topological superconductivity

“…Our approach is based on the previous theoretical work, where it was shown that band-inverted electron-hole bilayers support an unconventional topological phase transition from trivial to the QSH insulator phase via an intermediate insulating phase with spontaneously broken time-reversal symmetry (TRS), arising from the excitonic correlations between the electrons and holes [26]. This exotic TRS broken phase is one of the most prominent candidates for the correlated phases appearing in bandinverted semiconductors due to Coulomb interactions [26][27][28][29][30][31][32][33], and it is consistent with the accumulating experimental evidence of excitonic phenomenology reported in InAs/GaSb quantum wells [34][35][36][37][38] as well as in WTe 2 [39,40]. Moreover, the properties of the TRS broken phase provide a comprehensive explanation [41] of the temperature, voltage and length dependencies of the observed conductance in InAs/GaSb bilayers [7,[42][43][44].…”

Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers I: Transport properties

“…However, the electron-electron interactions are important in these materials if the hybridization of the electron and hole bands is small compared to the exciton binding energy, as can be appreciated by noticing that the bilayer system of spatially separated electrons and holes is the wellknown paradigm system for the realization of an exciton condensate state [9,10]. Indeed, it is now theoretically understood that interactions can lead to a plethora of correlated phases in band-inverted semiconductors [11][12][13][14][15][16][17] and the recent experiments have shown evidence of excitonic phenomenology in InAs/GaSb quantum wells [18][19][20][21][22] as well as in WTe 2 [23,24].…”