Thermoelectric signature of quantum critical phase in a doped spin liquid candidate

TL;DR

This study uncovers quantum critical behavior in a doped spin liquid candidate through thermoelectric measurements, linking it to unconventional BEC-like superconductivity and phase emergence.

Contribution

It provides the first thermoelectric evidence of quantum criticality in a doped spin liquid, connecting it to BEC-like superconductivity and phase behavior.

Findings

Logarithmic divergence of S/T indicates quantum criticality.

Correlation between S/T enhancement and superconducting transition temperature.

Pressure-dependent changes in S/T profile support BEC-BCS crossover.

Abstract

Quantum spin liquid is a nontrivial magnetic state of longstanding interest, in which spins are strongly correlated and entangled but do not order1, 2; further intriguing is its doped version, which possibly hosts strange metal and unconventional superconductivity3. Promising and currently the only candidate of the doped spin liquid is a triangular-lattice organic conductor, kappa-(BEDT-TTF)4Hg2.89Br8, recently found to hold metallicity, spin-liquid-like magnetism and BEC-like superconductivity4-6. The nature of the metallic state with the spin-liquid behaviour is awaiting to be further clarified. Here, we report the thermoelectric signature that mobile holes in the spin liquid background is in a quantum critical state and it pertains to the BEC-like superconductivity. The Seebeck coefficient divided by temperature, S/T, is enhanced on cooling with logarithmic divergence indicative of quantum criticality. Furthermore, the logarithmic enhancement is correlated with the superconducting transition temperature under pressure variation, and the temperature and magnetic field profile of S/T upon the superconducting transition change with pressure in a consistent way with the previously suggested BEC-BCS crossover. The present results reveal that the quantum criticality in a doped spin liquid emerges in a phase, not at a point, and is involved in the unconventional BEC-like nature.