Magnetic-field dependence of the T*-anomaly in quasi-2D organic superconductors

The family of quasi-2D superconductors kappa–(BEDT–TTF)₂X are model systems for strongly correlated low–dimensional metals. Recently, the unusual normal–conducting state — characterized by a line of anomalies T* (in the order of 40 K) — has attracted considerable attention: a ”pseudo-gap”behavior in analogy to the high-T_c cuprates, a crossover from an incoherent ”bad” metal to a coherent Fermi–liquid regime, and a density–wave–type phase transition have been suggested as possible scenarios. To investigate the possibility of a magnetic origin we carried out detailed transport measurements in pulsed magnetic fields up to 60 T. For two different compounds, X = Cu[N(CN)2 ]Br and Cu(NCS)2 , we observed a maximum in the relative magnetoresistance change right around T* . This indicates the significance of magnetic degrees of freedom which are coupled to the transport properties. Also, for the first time we were able to determine the magnetic–field dependence of T* showing a small negative shift with increasing field. We discuss the implications of our experimental data for possible models explaining the anomalous normal–conducting state.