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

Abstract

We report a determination of the temperature dependence of the upper critical magnetic fields ${\mathit{H}}_{\mathit{c}2}$ for κ-(BEDT-TTF${)}_2$Cu[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 Å. There is unusual structure in the critical fields and a possible dimensional crossover at T/${\mathit{T}}_{\mathit{c}}$∼0.97 for H∥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-${\mathit{T}}_{\mathit{c}}$ copper oxide superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. We find a strong depression of ${\mathit{T}}_{\mathit{c}}$, as large as 1.2 K, when the sample is rapidly cooled.