Heat-capacity studies of reduced-oxygen-content ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ near the superconducting transition

We have performed high-resolution ac calorimetry near the superconducting transition of single crystals of reduced-oxygen-content ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. The heat-capacity anomaly decreases dramatically with ${\mathit{T}}_{\mathit{c}}$(x) despite the fact that the transition width and the Meissner fraction indicate that the samples are of similar quality. In addition, the sensitivity of the heat-capacity anomaly to magnetic fields is several times stronger in these samples than has been observed in previous work on fully oxygenated samples. The reduction in the amplitude of the heat-capacity anomaly corresponds to the reduction in the apparent density of states at the Fermi surface.