Mott-driven BEC-BCS crossover in a doped spin liquid candidate, kappa-(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. I. INTRODUCTIONStrong interactions between itinerant electrons in solids cause peculiar correlations or organizations among the electrons beyond the Fermi-liquid framework [1]. Superconductivity emerging in these circumstances can be outside the celebrated Bardeen-Cooper-Schrieffer (BCS) framework, which assumes an instability of Fermi surfaces formed by degenerate fermions with well-defined momenta [2], in that the strong correlation of electrons makes their momenta ill-defined and the electron pairing in momentum space, the central idea of BCS theory, is less pertinent. In this case, superconductivity is of the Bose-Einstein condensation (BEC) type, as the size of Cooper pairs is as small as their mean distance, or equivalently, the interaction energy is comparable to the Fermi energy. Several unconventional superconductors have been discussed in the framework of a BEC scenario [3][4][5][6][7]. More broadly, the superfluidity of cold fermionic atoms is known to be controllable between the BEC and BCS regimes by the magnetic-field tuning of the interatomic interaction through