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

TL;DR

This study investigates high magnetic field effects in graphite, revealing a new phase and evidence for an excitonic condensate, with detailed angular and temperature dependence of phase transitions up to 90.5 T.

Contribution

It provides experimental evidence for a transition from a weak-coupling spin-density-wave to a strong-coupling excitonic insulator in graphite under high magnetic fields.

Findings

Discovery of a new high-field phase in graphite.

Evidence that the insulating state at 75 T is an excitonic condensate.

Angular dependence of transition fields follows a cosine law.

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.