Critical fields and mixed-state properties of the layered organic superconductor kappa -(BEDT-TTF)2I3.

Abstract

We report on a systematic study of the thermodynamic superconducting properties on high-quality crystals of the quasi-two-dimensional (quasi-2D) organic superconductor $\ensuremath{\kappa}$-${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}$${\mathrm{I}}_{3}$. Low-field magnetization measurements served to determine the lower critical field ${B}_{c1}$. Just below ${T}_{c}=3.5$ K, down to \ensuremath{\sim} 3 K, ${B}_{c1}$ perpendicular (\ensuremath{\perp}) to the highly conducting BEDT-TTF layers is found to be strongly suppressed below the sensitivity limit of the instrumental resolution. This might be an indication for single-vortex fluctuations expected for a strongly 2D superconductor. The critical field ${B}_{c2}^{*}$ determined by ac-susceptibility measurements is found to be related to an irreversibility line. The temperature and frequency dependences of ${B}_{c2}^{*}$ are consistent with well-established flux-creep models. The upper critical field ${B}_{c2\ensuremath{\perp}}$ extracted from magnetization measurements is consistent with specific-heat data. For both principal field orientations a crossover of the critical-field slopes $|\frac{d{B}_{c2}}{\mathrm{dT}}|$ towards a shallow tail close to ${T}_{c}$ is observed. Apparent discrepancies of the thermodynamic critical fields for parallel and perpendicular field orientation are found, indicating either a strongly reduced ${B}_{c1\ensuremath{\parallel}}$ or ${B}_{c2\ensuremath{\parallel}}$.