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 nondestructive 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.

Critical point for Bose–Einstein condensation of excitons in graphite

“…Above the quantum limit of 7.4 T, electrons and holes are both confined to their lowest Landau levels (23,24), which are each split to two spin-polarized subbands. A rich phase diagram consisting of distinct fieldinduced phases emerges above ∼20 T, which was documented by previous studies (25)(26)(27)(28). Our focus, here, is the identification of the peak transition temperature as the cradle of the excitonic instability.…”

A Modified Mechanical Model for the Optimization of High Field Solenoid Magnets

“…These magnets are widely employed in physics, medical science, biology and many other areas, e.g. to study the superconductivity of FeSe [34] and the phase transitions in graphite [35]. The proposed model can also be embedded in an optimization algorithm to search for better structures and explore the potential of higher magnetic field strength (>100 T) with the existing materials.…”

Failed excitonic quantum phase transition in Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$

“…The electronic-lattice dichotomy has recently come to the fore in studies of Ta 2 NiSe 5 [17][18][19] -one of the few candidate material for the excitonic insulator (EI) phase [16,[20][21][22][23][24]. EI results from a proliferation of excitons driven by Coulomb attraction between electrons and holes in a semiconductor or a semimetal [25][26][27][28][29].…”