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HL: Fachverband Halbleiterphysik

HL 22: 2D Materials IV (joint session HL/CPP)

HL 22.6: Talk

Wednesday, March 29, 2023, 11:00–11:15, POT 81

Evidence for equilibrium exciton condensation in monolayer WTe2 — •Massimo Rontani — CNR-NANO, Modena, Italy

We present evidence [1] 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.

This work is done in collaboration with Elisa Molinari, Daniele Varsano, Samaneh Ataei, Maurizia Palummo, Bosong Sun, David Cobden. It is partially funded by MUR PRIN2017 No. 2017BZPKSZ EXC-INS and MaX EU Center of Excellence.

[1] B. Sun et al., Nature Physics 18, 94-99 (2022).