High-resolution heat capacity of YBa2Cu3O6.9 over the superconducting transition region

Abstract

High-resolution heat-capacity data have been determined for a single-phase sample of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ over the temperature range 77–150 K. Two anomalies were found: a λ-type transition occurring at 86.65 K and a discontinuity accompanied by a first-order transition occurring at 90.30 K. The discontinuity has a value of Δ$C_p$/T=62 mJ (mol f.u.${)}^{\mathrm{-}1}$ ${\mathrm{K}}^{\mathrm{-}2}$, where f.u. denotes formula unit. An estimation of the latent heat and entropy associated with the first-order transition is given. The anomalies in the heat capacity are related to the changes in the resistance and Meissner effect and a mixed superconducting-normal state is proposed.