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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.
The magnetic field effect on the phase diagram of the organic Mott system $kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br in which the bandwidth was tuned by the substitution of deuterated molecules was studied by means of the resistivity measurements perf
The interplane optical spectrum of the organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br was investigated in the frequency range from 40 to 40,000 cm-1. The optical conductivity was obtained by Kramers-Kronig analysis of the reflectance. The absen
Systematic investigation of the electronic phase separation on macroscopic scale is reported in the organic Mott system $kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. Real space imaging of the phase separation is obtained by means of scanning micro-regio
The effect of disorder on the electronic properties near the Mott transition is studied in an organic superconductor $kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, which is systematically irradiated by X-ray. We observe that X-ray irradiation causes Ande
Static susceptibility of kappa-[(BEDT-TTF)1-x(BEDSe-TTF)x]2Cu[N(CN)2]Br alloys with the BEDSe-TTF content near the border-line of ambient pressure superconductivity (x~0.3) has been measured as a function of temperature, magnetic field, and pressure.