Graphite in 90 T: Evidence for Strong-coupling Excitonic Pairing

Abstract

Strong magnetic field induces at least two phase transitions in graphite beyond the quantum limit where many-body effects are expected. We report on a study using a state-of-the-art non-destructive magnet allowing to attain 90.5 T at 1.4 K, which reveals a new field-induced phase and evidence that the insulating state destroyed at 75 T is an excitonic condensate of electron-hole pairs. By monitoring the angle dependence of in-plane and out-of-plane magnetoresistance, we distinguish between the role of cyclotron and Zeeman energies in driving various phase transitions. We find that, with the notable exception of the transition field separating the two insulating states, the threshold magnetic field for all other transitions display an exact cosine angular dependence. Remarkably, the threshold field for the destruction of the second insulator (phase B) is temperature-independent with no detectable Landau-level crossing nearby. We conclude that the field-induced insulator starts as a weak-coupling spin-density-wave, but ends as a strong-coupling excitonic insulator of spin-polarized electron-hole pairs.

arXiv commentMain text: 6 pages, 4 figures. Supplemental Materials included

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• Wide critical fluctuations of the field-induced phase transition in graphite
• Spin-Triplet Excitonic Insulator in the Ultra-Quantum Limit of HfTe5
• Magnetoresistance in the Extreme Quantum Limit: Field-Induced Crossover to the Unitarity Limit
• Ion-beam-milled graphite nanoribbons as mesoscopic carbon-based polarizers