Oxygen dependence of the linear term in the specific heat ofYBa2Cu3O7−δ

Specific-heat data in the range 0.4–20 K for a polycrystalline specimen of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, prepared by the mixing of aqueous nitrate solutions, are reported as a function of oxygen content and compared with similar measurements made on specimens prepared by the powder ceramic route. On deoxygenation from δ=0 to 1, the linear specific-heat coefficient ${\gamma }_0$ and Debye temperature ${\mathit{FTHETA}}_{\mathit{D}}^0$ decrease by 1.7±0.7 mJ/mol ${\mathrm{K}}^2$ and 80±20 K, respectively, with ${\gamma }_0$ attaining its smallest value of 2.86 mJ/mol ${\mathrm{K}}^2$ in the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ specimen prepared by the nitrate route. It appears that ${\gamma }_0$ is largely extrinsic in origin, but a component of it is correlated to the oxygen content of the material. It is not possible to say whether this component is intrinsic to the superconducting state or is merely a property of the material, being due to some other source such as two-level systems.