Low-temperature specific heat of polycrystalline ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$

We have measured the specific heat of a superconducting polycrystalline sample of ${\mathrm{Ba}}_{0.6}$ ${\mathrm{K}}_{0.4}$ ${\mathrm{BiO}}_3$ at temperatures from 1.7 to 91 K. The low-temperature data can be fit best by a cubic term β=0.234 mJ/mol ${\mathrm{K}}^4$ (Debye temperature ${\mathrm{FTHETA}}_D$=346 K) and a linear term γT. We find that γ may be zero; it has an upper bound of 0.30 mJ/mol ${\mathrm{K}}^2$, an order of magnitude smaller than the value found previously for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. This result supports the view that ${\mathrm{Ba}}_{0.6}$ ${\mathrm{KBi}}_{0.4}$ ${\mathrm{O}}_3$ has the properties of a BCS-like superconductor.