Specific heat in zero and applied magnetic fields of the organic superconductor α-di[bis(ethylenedithio)tetrathiafulvalene]-ammonium- tetra(thiocyanato)mercurate [α-(ET${)}_2$(${\mathrm{NH}}_4$)Hg(SCN${)}_4$]

Specific heats determined down to 0.3 K establish the existence of bulk superconductivity in a new α-phase superconducting salt, α- [α-(ET${)}_2$(${\mathrm{NH}}_4$)Hg(SCN${)}_4$] (${\mathit{T}}_{\mathit{c}}$=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 α-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 γ=29±2 mJ/${\mathrm{K}}^2$ mol, which is essentially identical to those of other ET-based organic superconductors, and an approximate value ΔC/γ${\mathit{T}}_{\mathit{c}}$=1.1±0.2, 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 ${\mathit{T}}_{\mathit{c}}$(-${\mathit{dH}}_{\mathit{c}2}$/dT), from specific heats measured in applied magnetic fields, yield an extremely large value larger than 3 T/K for H∥ac, where ac denotes the crystallographic ac plane (which contains the tightly connected conducting layers of ET donor molecules), and a value of 0.1±0.02 T/K for H∥${\mathit{b}}^{\mathrm{*}}$.