Evidence for equilibrium exciton condensation in monolayer WTe2

• Published in: Nature physics Vol. 18; no. 1; pp. 94 - 99
• Main Authors: Sun, Bosong, Zhao, Wenjin, Palomaki, Tauno, Fei, Zaiyao, Runburg, Elliott, Malinowski, Paul, Huang, Xiong, Cenker, John, Cui, Yong-Tao, Chu, Jiun-Haw, Xu, Xiaodong, Ataei, S. Samaneh, Varsano, Daniele, Palummo, Maurizia, Molinari, Elisa, Rontani, Massimo, Cobden, David H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.01.2022; Nature Publishing Group; Nature Publishing Group (NPG)
• Subjects: 639/301/1005/1007; 639/766/119/2792/4128; 639/766/119/995; Atomic; Charge density waves; Chemical potential; Classical and Continuum Physics; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Complex Systems; Condensed Matter Physics; Doping; electronic devices; electronic materials; electronic properties; Electrons; Equilibrium; Excitons; First principles; High temperature; Mathematical and Computational Physics; Molecular; Monolayers; Optical and Plasma Physics; Physics; Physics and Astronomy; Room temperature; Theoretical; Topological insulators; Wave functions
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/s41567-021-01427-5

We present evidence that the two-dimensional bulk of monolayer WTe 2 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 WTe 2 exhibits strong correlations over a wide temperature range. Exciton condensation has been observed in various three-dimensional (3D) materials. Now, monolayer WTe 2 —a 2D topological insulator—also shows the phenomenon. Strong electronic interactions allow the excitons to form and condense at high temperature.