Critical point for Bose-Einstein condensation of excitons in graphite

Jinhua Wang; Pan Nie; Xiaokang Li; Huakun Zuo; Benoît Fauqué; Zengwei Zhu; Kamran Behnia

doi:10.1073/pnas.2012811117

Critical point for Bose-Einstein condensation of excitons in graphite

Proc Natl Acad Sci U S A. 2020 Dec 1;117(48):30215-30219. doi: 10.1073/pnas.2012811117. Epub 2020 Nov 16.

Authors

Jinhua Wang^{1

2}, Pan Nie^{1

2}, Xiaokang Li^{1

2}, Huakun Zuo^{1

2}, Benoît Fauqué³, Zengwei Zhu^{4

2}, Kamran Behnia⁵

Affiliations

¹ Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China.
² School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China.
³ Jeunes Équipes de l'Institut de Physique, Unité Mixte de Service et de Recherche 3573, CNRS, Collège de France, Paris Sciences et Lettres Research University, 75231 Paris Cedex 05, France.
⁴ Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China; zengwei.zhu@hust.edu.cn.
⁵ Laboratoire de Physique et d'Étude des Matériaux, CNRS, École Supérieure de Physique et de Chimie Industrielles Paris, Paris Sciences et Lettres Research University, 75005 Paris, France.

Abstract

An exciton is an electron-hole pair bound by attractive Coulomb interaction. Short-lived excitons have been detected by a variety of experimental probes in numerous contexts. An excitonic insulator, a collective state of such excitons, has been more elusive. Here, thanks to Nernst measurements in pulsed magnetic fields, we show that in graphite there is a critical temperature (T = 9.2 K) and a critical magnetic field (B = 47 T) for Bose-Einstein condensation of excitons. At this critical field, hole and electron Landau subbands simultaneously cross the Fermi level and allow exciton formation. By quantifying the effective mass and the spatial separation of the excitons in the basal plane, we show that the degeneracy temperature of the excitonic fluid corresponds to this critical temperature. This identification would explain why the field-induced transition observed in graphite is not a universal feature of three-dimensional electron systems pushed beyond the quantum limit.

Keywords: Bose–Einstein condensation; critical point; excitonic insulator; high-magnetic-field–induced transition.