Heat capacity of superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ near $T_c$

Measurements of the heat capacity of single crystals of ${\mathrm{Ba}}_{0.6}$ ${\mathrm{K}}_{0.4}$ ${\mathrm{BiO}}_3$ in the range 10–40 K and in magnetic fields up to 60 kOe reveals a jump at $T_c$. The phase diagram of $H_{c2}$(T) can thus be determined calorimetrically, yielding values of $T_c$(H=0)=31.5 K and ${\mathrm{dH}}_{c2}$/dT=-5.2 kOe/K, in agreement with recent magnetization measurements. From these data, and the value of ${\mathrm{dH}}_{c1}$/dT from magnetization data, the size of the jump is consistent with at least 40% of the sample volume becoming a superconductor, indicating that superconductivity in this material is a bulk property and not due to a trace amount of a second phase. The corresponding value of the electronic specific-heat coefficient is γ=1.5 mJ/mol ${\mathrm{K}}^2$. Comparison with the band-structure density of states yields a value of the electron-phonon coupling constant λ of 0.35, consistent with the assumption that the material is a weakly coupled superconductor.