Mott-Driven BEC-BCS Crossover in a Doped Spin Liquid Candidate κ−(BEDT−TTF)4Hg2.89Br8

• Published in: Physical review. X 2022-03, Vol.12 (1)
• Main Authors: Suzuki, Y, Wakamatsu, K, Ibuka, J, Oike, H, Fujii, T, Miyagawa, K, Taniguchi, H, Kanoda, K
• Format: Article
• Language: English
• Subjects: Absolute zero; Cold atoms; Condensates; Constraining; Electrons; Fermi liquids; Fermions; Materials selection; Organic superconductors; Spin liquid; Superconductivity; Superfluidity; Tuning
• ISSN: 2160-3308
• DOI: 10.1103/PhysRevX.12.011016

The pairing of interacting fermions leading to superfluidity has two limiting regimes: the Bardeen-Cooper-Schrieffer (BCS) scheme for weakly interacting degenerate fermions and the Bose-Einstein condensation (BEC) of bosonic pairs of strongly interacting fermions. While the superconductivity that emerges in most metallic systems is the BCS-like electron pairing, strongly correlated electrons with poor Fermi liquidity can condense into the unconventional BEC-like pairs. Quantum spin liquids harbor extraordinary spin correlation free from order, and the superconductivity that possibly emerges by carrier doping of the spin liquids is expected to have a peculiar pairing nature. The present study experimentally explores the nature of the pairing condensate in a doped spin liquid candidate material and under varying pressure, which changes the electron-electron Coulombic interactions across the Mott critical value in the system. The transport measurements reveal that the superconductivity at low pressures is a BEC-like condensate from a non-Fermi liquid and crosses over to a BCS-like condensate from a Fermi liquid at high pressures. The Nernst-effect measurements distinctively illustrate the two regimes of the pairing in terms of its robustness to the magnetic field. The present Mott tuning of the BEC-BCS crossover can be compared to the Feshbach tuning of the BEC-BCS crossover of fermionic cold atoms.