Critical point for Bose–Einstein condensation of excitons 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 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.

Wide Critical Fluctuations of the Field-Induced Phase Transition in Graphite

“…The present result helps to understand the evolution of the observed anomalies in the Nernst effect [12,29] (a measure of the entropy per carrier [30]) and the ultrasound measurements [13] caused by the transition. At low temperature, one expects that ordering induces a smooth variation in entropy (see [17]) and therefore a rounded drop in the Nernst response [12,29]. As the temperature increases the mean field component of the transition strengthens and the Nernst anomaly becomes a clear kink.…”

“…Future studies of the specific heat at higher magnetic field would shed light to the BCS-BEC crossover as one approaches the maximum transition temperature around 47T, where the degeneracy temperature and critical temperature become close to each other [29]. Specific heat studies on other dilutes metals pushed to extreme quantum limit and hosting field-induced state (such as bismuth [31,32], InAs [33], TaAs [34] or ZrTe 5 [35]) could bring interesting insights.…”

Tunable Terahertz Plasmons in Graphite Thin Films

“…The sample was always at the liquid-helium temperature, so the thermal excitation is minimal. The possible excitonic effect can be ignored due to the only moderate magnetic field in our study [56]. With the increase of the field, the frequency and magnitude of the dip indicated by the orange arrow dramatically increase.…”

Antiferromagnetic ordering and excitonic pairing in the AA-stacked bilayer graphene

“…The multilayer graphene systems appear to be ideal candidates for studying the exciton formation and condensation phenomena. The excitonic pairing in bilayer graphene systems, has been reported in many studies [19][20][21][22][23][24][25][26][27]. In turn, this can provide the new possibilities to construct a new category of optoelectronics, based on the excitonic qubit-functionality [37,38].…”