Microscopic observation of superconducting fluctuations in $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br by ${}^{13}$C NMR spectroscopy

Abstract

We performed ${}^{13}$C-NMR experiment and measured spin-lattice relaxation rate divided by temperature $1/T_{1}T$ near the superconducting (SC) transition temperature $T_c$ in $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br ($\kappa$-Br salt), and $\kappa$-(BEDT-TTF)${}_2$Cu(NCS)${}_2$ ($\kappa$-NCS salt). We observed the reduction of $1/T_{1}T$ starting at the temperature higher than $T_c$ in $\kappa$-Br salt. Microscopic observation of quasi-particle density of states in the fluctuating SC state revealed the effects of short-range Cooper pairs induced in the normal state to the quasi-particle density of states. We also performed systematic measurements in the fields both parallel and perpendicular to the conduction plane in $\kappa$-Br and $\kappa$-NCS salts, and confirmed that the reduction of $1/T_{1}T$ above $T_c$ is observed only in $\kappa$-Br salt regardless of the external field orientation.

Accepted for publication in PRB