Reduced superfluid density in a doped spin liquid candidate

TL;DR

This study investigates a doped quantum spin liquid candidate, revealing an unusually low superfluid density and a high transition temperature to Fermi temperature ratio, providing insights into the nature of superconductivity in Mott insulators.

Contribution

It reports the anomalously reduced superfluid density in a doped QSL candidate, highlighting the role of incoherent spectral weight and phase stiffness in superconductivity.

Findings

Superfluid density is significantly reduced in the doped QSL candidate.

The ratio of T_c to Fermi temperature is approximately 0.1, much higher than BCS predictions.

The system shows no competing orders, isolating the effects related to the QSL state.

Abstract

A quantum spin liquid (QSL) would be an exotic stage for superconductivity. A promising candidate for a doped QSL is the organic triangular-lattice system, kappa-(BEDT-TTF)4Hg2:89Br8, which hosts a non-Fermi liquid and magnetism of a QSL nature and shows superconductivity upon cooling. Here, we report that its superfluid density is anomalously reduced, indicating the existence of substantial incoherent spectral weight and weak superconducting phase stiffness. Moreover, the ratio of the superconducting transition temperature to the nominal Fermi temperature is as large as 0.1, orders of magnitude beyond typical BCS values. These observations in a system free from competing orders that complicate the similar issue in underdoped cuprates give a clue to the enigmatic missing superfluid density in doped Mott insulators.