UNUSUAL BEHAVIOR IN THE UPPER CRITICAL MAGNETIC-FIELDS OF THE AMBIENT-PRESSURE ORGANIC SUPERCONDUCTOR - KAPPA-(BEDT-TTF)2CU[N(CN)2]BR [WHERE BEDT-TTF REPRESENTS BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE]

We report a determination of the temperature dependence of the upper critical magnetic fields Hc2 for -(BEDT-TTF)2Cu[N(CN)2]Br, with the use of dc-magnetization measurements that show a well-defined diamagnetic onset. We find large slopes of -20 T/K and -2.2 T/K for H ac and H b, respectively, which lead to coherence lengths of 37 and 4. There is unusual structure in the critical fields and a possible dimensional crossover at T/Tc0.97 for Hac. Neither the slopes nor the structure are observable in the superconducting transition measured by ac magnetoresistance, which exhibits strong magnetic-field broadening similar to that of the high-Tc copper oxide superconductor YBa2Cu3O7-. We find a strong depression of Tc, as large as 1.2 K, when the sample is rapidly cooled. © 1990 The American Physical Society.