Anomalous magnetization and dimensional crossover of the vortex system in the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2.

Abstract

Magnetization measurements on high-quality single crystals of $\ensuremath{\kappa}$-${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}$Cu${(\mathrm{NCS})}_{2}$ [where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene] are performed in the magnetic field perpendicular to the conducting layers. An anomalous second peak on magnetization curves observed at ${H}_{p}$ (\ensuremath{\approx} 150 Oe) is attributed to the dimensional crossover in the vortex system. At the dimensional crossover field ${B}_{2\mathrm{D}}=\frac{{\ensuremath{\varphi}}_{0}}{({\ensuremath{\gamma}}^{2}{s}^{2})}$, the temperature dependence of the irreversibility field changes from the power-law dependence in the three-dimensional (3D) regime below ${B}_{2\mathrm{D}}$ to the exponential dependence with $\frac{1}{T}$ in the quasi-2D regime above ${B}_{2\mathrm{D}}$. It is also found that the different magnetic field dependence of the critical current density above and below ${B}_{2\mathrm{D}}$ is well described by the collective pinning theory.