SPECIFIC-HEAT IN ZERO AND APPLIED MAGNETIC-FIELDS OF THE ORGANIC SUPERCONDUCTOR ALPHA-DI[BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE]-AMMONIUM-TETRA(THIOCYANATO)MERCURATE [ALPHA-(ET)2(NH4)HG(SCN)4]

Specific heats determined down to 0.3 K establish the existence of bulk superconductivity in a new -phase superconducting salt,- [-(ET)2(NH4)Hg(SCN)4] (Tc=1 K), a derivative of the organic radical-cation donor molecule bis(ethylenedithio)tetrathiafulvalene (ET). This salt is important because it establishes a new structural class of organic superconductors, the -phase class. The anomaly in the specific heat C is distinct but broad with a maximum in C/T at 0.8 K. The measurements yield a normal-state Sommerfield coefficient of =292 mJ/K2 mol, which is essentially identical to those of other ET-based organic superconductors, and an approximate value C/Tc=1.10.2, which is smaller than the BCS value of 1.43. A sharp but unexplained increase in C/T occurs below 0.7 K. Determinations of the temperature derivatives of the upper critical magnetic fields at Tc(-dHc2/dT), from specific heats measured in applied magnetic fields, yield an extremely large value larger than 3 T/K for ac, where ac denotes the crystallographic ac plane (which contains the tightly connected conducting layers of ET donor molecules), and a value of 0.10.02 T/K for Hb*. © 1990 The American Physical Society.