Different superconducting percolation regimes in the layered organic Mott system $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl -- evidence from fluctuation spectroscopy

TL;DR

This study uses fluctuation spectroscopy to reveal different superconducting percolation regimes in a layered organic Mott system, highlighting spatial inhomogeneity and the role of mesoscopic fluctuations near the transition.

Contribution

It provides new evidence for percolative superconductivity and distinguishes between different percolation regimes driven by pressure, magnetic field, and mesoscopic fluctuations.

Findings

Evidence for inhomogeneous coexistence of antiferromagnetic and superconducting states.

Identification of percolation dominated by unstable clusters near $T_c(B)$.

Observation of Lorentzian spectra indicating mesoscopic fluctuators.

Abstract

Fluctuation spectroscopy is used to investigate the organic bandwidth-controlled Mott system $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. We find evidence for percolative-type superconductivity in the spatially inhomogeneous coexistence region of antiferromagnetic insulating and superconducting states which develops under pressure at low temperatures. When the superconducting transition is driven by a magnetic field, percolation seems to be dominated by unstable superconducting clusters upon approaching $T_c (B)$ from above, before a "classical" type of percolation is resumed at low fields, dominated by the fractional change of superconducting clusters. The $1/f$ noise is resolved into Lorentzian spectra in the crossover region where the action of an individual mesoscopic fluctuator is enhanced.