Electrical and Structural Properties and Phase Diagram of a Molecular Superconductor <i>β</i>-[(CH3)4N][Pd(dmit)2]2

Abstract

Abstract β-[(CH3)4N][Pd(dmit)2]2 is isomorphic to [(CH3)4N][Ni(dmit)2]2, which is the first pure π acceptor molecular conductor exhibiting a superconducting transition. The crystal structure at low temperature, pressure dependence of the resistivity and the magnetic susceptibility of β-[(CH3)4N][Pd(dmit)2]2 were examined. The general features of the phase diagram of β-[(CH3)4N][Pd(dmit)2]2 resemble those of the organic superconductors. At ambient pressure, the resistivity increases gradually with lowering temperature. A superconducting transition was observed at 6—9 kbar. The highest Tc was 6.5 K. Between 5 and 6.3 kbar, a characteristic rapid resistivity decrease was observed at the small temperature range above the offset temperature of the superconducting transition. Although the resistivity is not zero, this temperature region can be regarded as a “pre-superconducting region.” Above 9 kbar the metallic state is stabilized down to low temperature. Above 5 kbar, most of the crystals exhibited resistivity anomalies around 60 K, which may be ascribed to the metal-insulator transition of the domains with α-type structure (α-[(CH3)4N][Pd(dmit)2]2), which became mixed in with the crystals of β-[(CH3)4N][Pd(dmit)2]2. The resistivity data of the highly conducting state were very noisy in the pressure range where the superconducting transition could be observed.