Microscopic observation of superconducting fluctuations in $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br by $^{13}$C NMR spectroscopy

TL;DR

This study uses $^{13}$C NMR spectroscopy to observe superconducting fluctuations above $T_c$ in ${eta}$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, revealing short-range Cooper pairing effects in the normal state.

Contribution

It provides microscopic evidence of superconducting fluctuations above $T_c$ in ${eta}$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br using NMR, highlighting the effects of short-range Cooper pairs.

Findings

Reduction of $1/T_{1}T$ observed above $T_c$ in ${eta}$-Br salt

Superconducting fluctuations are field-independent in ${eta}$-Br salt

No fluctuation effects observed in ${eta}$-NCS salt

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.