Graphite in 90 T: Evidence for Strong-Coupling Excitonic Pairing GRAPHITE in 90 T: EVIDENCE for STRONG-COUPLING ... ZENGWEI ZHU et al.

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 nondestructive magnet allowing us 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 displays 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 density wave but ends as a strong-coupling excitonic insulator of spin-polarized electron-hole pairs.