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

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Magnetism

Background:

• Graphite exhibits insulating states in high magnetic fields, with critical temperatures showing a dome-like dependence.
• The extreme quantum limit reveals two distinct insulating states influenced by magnetic field strength.

Purpose of the Study:

• Investigate the effect of hydrostatic pressure on the first insulating dome in graphite (below 60 T).
• Analyze how pressure modifies the field-temperature phase boundary and critical temperatures.
• Understand the underlying mechanisms governing the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein Condensate (BEC) crossover under pressure.

Main Methods:

• Application of hydrostatic pressure up to 1.7 GPa.
• Magnetic field-dependent measurements to probe phase transitions.
• Fermiology studies to determine carrier density and effective cyclotron mass.

Main Results:

• Increasing pressure shifts the phase boundary to higher magnetic fields, while the maximum critical temperature remains constant.
• Hydrostatic pressure increases carrier density and in-plane effective cyclotron mass.
• The BCS relation is validated in the weak-coupling regime, but the strong-coupling summit is insensitive to pressure.

Conclusions:

• The BCS-BEC crossover in graphite is tunable by both magnetic field and pressure.
• The summit of the dome is 'locked' due to the interplay between BCS coherence length and interplane distance.
• This work provides insights into the control of quantum states in layered materials.