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

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.