$^{1}$H NMR study on X-ray irradiated $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br

The $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br ($\kappa$-Br) is a quasi-two dimensional superconductor with $T_{c}$ of 12 K. By substituting Cl for Br in the insulating layer, the system becomes a Mott insulator, k-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl ($\kappa$-Cl), with the Neel temperature of 25 K. So, $\kappa$-Br salt is situated close to the Mott transition. Recently, Sasaki {\it et al.,} have reported the transformation from a metallic (superconducting) state to an insulating state by the x-ray irradiation. The 500h-irradiated thin sample shows insulating behavior in the temperature dependence of resisitivity. We report the $^{1}$H NMR studies on the 500h-irradiated $\kappa$-Br salt. Below 150 K, where the nuclear relaxation is dominated by electron spins, 1/$T_{1}$ of the irradiated sample increases from that of the non-irradiated sample and is even larger than the value of $\kappa$-Cl. Nevertheless, there is no manifestation of magnetic ordering; that is, neither a line broadening nor a divergent peak in 1/$T_{1}$ down to 1.5 K. We will discuss the electronic state of irradiated $k$-Br salt based on experimental results.