CRITICAL FIELDS AND MIXED-STATE PROPERTIES OF THE LAYERED ORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)(2)I-3

We report on a systematic study of the thermodynamic superconducting p roperties on high-quality crystals of the quasi-two-dimensional (quasi -2D) organic superconductor kappa-(BEDT-TTF)(2)I-3. Low-field magnetiz ation measurements served to determine the lower critical field B-c1. Just below T-c=3.5 K, down to similar to 3 K, B-c1 perpendicular (perp endicular to) to the highly conducting BEDT-TTF layers is found to be strongly suppressed below the sensitivity limit of the instrumental re solution. This might be an indication for single-vortex fluctuations e xpected for a strongly 2D superconductor. The critical field B-c2, de termined by ac-susceptibility measurements is found to be related to a n irreversibility line. The temperature and frequency dependences of B -c2 are consistent with well-established flux-creep models. The upper critical field B-c2 perpendicular to extracted from magnetization mea surements is consistent with specific-heat data. For both principal fi eld orientations a crossover of the critical-field slopes \dB(c2)/dT\ towards a shallow tail close to T-c is observed. Apparent discrepancie s of the thermodynamic critical fields for parallel and perpendicular field orientation are found, indicating either a strongly reduced B-c1 parallel to or B-c2 parallel to.