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

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 $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br ($κ$-Br salt), and $κ$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ ($κ$-NCS salt). We observed the reduction of $1/T_{1}T$ starting at the temperature higher than $T_c$ in $κ$-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 $κ$-Br and $κ$-NCS salts, and confirmed that the reduction of $1/T_{1}T$ above $T_{c}$ is observed only in $κ$-Br salt regardless of the external field orientation.