Critical point for Bose-Einstein condensation of excitons in graphite

TL;DR

This paper demonstrates the existence of a critical temperature and magnetic field for Bose-Einstein condensation of excitons in graphite, revealing conditions for excitonic insulator formation via Nernst measurements.

Contribution

It provides experimental evidence for excitonic Bose-Einstein condensation in graphite at specific temperature and magnetic field conditions.

Findings

Critical temperature for exciton condensation is 9.2 K.

Critical magnetic field for exciton condensation is 47 T.

Identifies conditions for excitonic insulator state in graphite.

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 sub-bands 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.