Vibrating reed study of the flux line dynamics of κ-(ET)₂Cu[N(CN)₂]Br

Vibrating reed measurements were performed on single-crystal samples of κ-(ET)₂- Cu[N(CN)₂]Br in an applied magnetic field H oriented either parallel ("longitudinal") or perpendicular ("transverse") to the highly conducting ac-plane. Field-cooling data taken for the longitudinal orientation and H<0.7 T revealed a peak with a low-temperature shoulder in the reed dissipation 1/Q located at temperature T_Q below the superconducting transition temperature T_c(≈11.6 for H=0). The shoulder disappeared near H≈0.7 T, accompanied by an abrupt change in the slope of T_Q(H), corresponding to a similar change in the slope of the upper critical magnetic field H_c2 measured by Kwok et al. The existence of the shoulder in the dissipation peak bears on a number of current explanations for the exotic superconducting properties of κ-(ET)₂Cu[N(CN)₂]Br. T_Q data taken for the transverse orientation fell far below estimates of H_c2., indicating the existence of a substantial region of flux line (FL) mobility below H_c2(T). The location of a peak in NMR relaxation observed by De Soto et al. lies close to T_Q for the transverse orientation.