Evidence of high-temperature exciton condensation in a two-dimensional semimetal

Qiang Gao, Yang-hao Chan, Yuzhe Wang, Haotian Zhang, Pu Jinxu, Shengtao Cui, Yichen Yang, Zhengtai Liu, Dawei Shen, Zhe Sun, Juan Jiang, Tai C. Chiang () and Peng Chen ()
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Qiang Gao: Shanghai Jiao Tong University
Yang-hao Chan: Academia Sinica
Yuzhe Wang: University of Science and Technology of China
Haotian Zhang: Shanghai Jiao Tong University
Pu Jinxu: Shanghai Jiao Tong University
Shengtao Cui: University of Science and Technology of China
Yichen Yang: Chinese Academy of Sciences
Zhengtai Liu: Chinese Academy of Sciences
Dawei Shen: University of Science and Technology of China
Zhe Sun: University of Science and Technology of China
Juan Jiang: University of Science and Technology of China
Tai C. Chiang: University of Illinois at Urbana-Champaign
Peng Chen: Shanghai Jiao Tong University

Nature Communications, 2023, vol. 14, issue 1, 1-7

Abstract: Abstract Electrons and holes can spontaneously form excitons and condense in a semimetal or semiconductor, as predicted decades ago. This type of Bose condensation can happen at much higher temperatures in comparison with dilute atomic gases. Two-dimensional (2D) materials with reduced Coulomb screening around the Fermi level are promising for realizing such a system. Here we report a change in the band structure accompanied by a phase transition at about 180 K in single-layer ZrTe2 based on angle-resolved photoemission spectroscopy (ARPES) measurements. Below the transition temperature, gap opening and development of an ultra-flat band top around the zone center are observed. This gap and the phase transition are rapidly suppressed with extra carrier densities introduced by adding more layers or dopants on the surface. The results suggest the formation of an excitonic insulating ground state in single-layer ZrTe2, and the findings are rationalized by first-principles calculations and a self-consistent mean-field theory. Our study provides evidence for exciton condensation in a 2D semimetal and demonstrates strong dimensionality effects on the formation of intrinsic bound electron–hole pairs in solids.

Date: 2023
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DOI: 10.1038/s41467-023-36667-x

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