Specific-Heat In Zero And Applied Magnetic-Fields Of The Organic Superconductor Alpha-Di[Bis(Ethylenedithio)Tetrathiafulvalene]-Ammonium-Tetra(Thiocyana To)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 {alpha}-phase superconducting salt, {alpha}-di(bis (ethylenedithio) tetrathiafulvalene)- ammonium -tetra(thiocyanato)mercurate ({alpha}-(ET){sub 2}(NH{sub 4})Hg(SCN){sub 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 {alpha}-phase class. The anomaly in the specific heat {ital C} is distinct but broad with a maximum in {ital C}/{ital T} at 0.8 K. The measurements yield a normal-state Sommerfield coefficient of {gamma}=29{plus minus}2 mJ/K{sup 2} mol, which is essentially identical to those of other ET-based organic superconductors, and an approximate value {Delta}{ital C}/{gamma}{Tc}=1.1{plus minus}0.2, which is smaller than the BCS value of 1.43. A sharp but unexplained increase in {ital C}/{ital T} occurs below 0.7 K. Determinations of the temperature derivatives of the upper critical magnetic fields at {Tc}({minus}{ital dH}{sub {ital c}2}/{ital dT}), from specific heats measured in applied magnetic fields, yield an extremely large value larger than 3 T/K for {bold H}{parallel}{ital ac}, where {ital ac} denotes the crystallographic {ital ac} plane (which contains the tightly connected conducting layers of ET donor molecules), and a value of 0.1{plus minus}0.02 T/K for {bold H}{parallel}{italmore » b}{sup *}.« less