Tuning the BCS-BEC crossover of electron-hole pairing with pressure

TL;DR

This study investigates how pressure influences the magnetic field-induced electron-hole pairing in graphite, revealing that the BCS-BEC crossover can be tuned by external parameters while the maximum critical temperature remains stable.

Contribution

It demonstrates the pressure dependence of the phase boundary and the persistence of the BCS relation in the weak-coupling regime, providing new insights into tunable electron-hole pairing in graphite.

Findings

Pressure shifts the phase boundary to higher magnetic fields.

Maximum critical temperature remains unchanged with pressure.

The BCS relation holds in the weak-coupling regime under pressure.

Abstract

In graphite, a moderate magnetic field confines electrons and holes into their lowest Landau levels. In the extreme quantum limit, two insulating states with a dome-like field dependence of the their critical temperatures are induced by the magnetic field. Here, we study the evolution of the first dome (below 60 T) under hydrostatic pressure up to 1.7 GPa. With increasing pressure, the field-temperature phase boundary shifts towards higher magnetic fields, yet the maximum critical temperature remains unchanged. According to our fermiology data, pressure amplifies the density and the effective mass of hole-like and electron-like carriers. Thanks to this information, we verify the persistent relevance of the BCS relation between the critical temperature and the density of states in the weak-coupling boundary of the dome. In contrast, the strong-coupling summit of the dome does not show any detectable change with pressure. We argue that this is because the out-of-plane BCS coherence length approaches the interplane distance that shows little change with pressure. Thus, the BCS-BEC crossover is tunable by magnetic field and pressure, but with a locked summit.