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

Abstract

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.

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Plain Language Summary

Superconductivity—the ability of a material to conduct electricity with zero resistance—arises from peculiar pairings that form among electrons. These pairings have two limiting regimes: barely overlapping small pairs and heavily overlapping large pairs, called Bose-Einstein condensation (BEC)–like and Bardeen-Cooper-Schrieffer (BCS) pairing, respectively. Most superconductors fall under the BCS regime. Here, we report that superconductivity in a material that is supposed to host a quantum spin liquid—no magnetic order even at absolute zero—is BEC-like and changes to the BCS regime by pressure application.

In our experiments, we study an organic superconductor. By gradually ratcheting the pressure on the sample up to about 1 GPa and measuring its resistivity, we identify a clear transition from BEC-like superconductivity to the BCS regime. Because increased pressure reduces the Coulomb interactions among electrons, this result suggests that strong Coulomb interactions prefer the unconventional BEC-like small pairs, and they cross over to the conventional BCS pairs as the interactions are reduced.

The realization of unconventional BEC-like superconductivity in an exotic spin-liquid candidate material widens the ground where superconductivity emerges and will stimulate the quest for novel mechanisms of pair formation. The finding of the interaction-tunable BEC-BCS crossover offers a novel type of pairing control, which adds to the more general physics of pairing instabilities of interacting fermions.