Superconducting Fluctuation of the Layered Organic Superconductor \kappa-(BEDT-TTF)

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A quasi-two-dimensional organic superconductor, \kappa-(BEDT-TTF)<inf>2</inf>Cu[N(CN)<inf>2</inf>]Br [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene], showed fluctuation behavior of superconductivity up to approximately 19 K, which was far higher than the zero-field transition temperature T_{\text{c}} (=11.6 K). We measured magnetic susceptibility under a magnetic field parallel and perpendicular to the conducting layers and determined the susceptibility difference \Delta\chi between the parallel-field and perpendicular-field susceptibilities as functions of temperature and magnetic field. From the analysis of the magnetic field evolution of \Delta\chi, we deduced that \Delta\chi was composed of anisotropy in normal-state magnetic properties and superconducting fluctuations. From the investigation of \Delta\chi in the low-field region, we derived that the fluctuation component was proportional to (T-T_{\text{c}})^{\alpha} with a constant \alpha (-2.3<\alpha<-2.0) or to T(T-T_{\text{c}})^{\beta} with a constant \beta (-2.4<\beta<-2.1).
2013-07-15