Unusual behavior in the upper critical magnetic fields of the ambient-pressure organic superconductor. kappa. -(BEDT-TTF) sub 2 Cu(N(CN) sub 2 )Br (where BEDT-TTF represents bis(ethylenedithio) tetrathiofulvalene)

We report a determination of the temperature dependence of the upper critical magnetic fields {ital H}{sub {ital c}2} for {kappa}-(BEDT-TTF){sub 2}Cu(N(CN){sub 2})Br, with the use of dc-magnetization measurements that show a well-defined diamagnetic onset. We find large slopes of {minus}20 T/K and {minus}2.2 T/K for {bold H}{parallel}{ital ac} and {bold H}{parallel}{ital b}, respectively, which lead to coherence lengths of 37 and 4 A. There is unusual structure in the critical fields and a possible dimensional crossover at {ital T}/{ital T}{sub {ital c}}{approximately}0.97 for {bold H}{parallel}{ital ac}. 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 YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. We find a strong depression of {ital T}{sub {ital c}}, as large as 1.2 K, when the sample is rapidly cooled.