Heat capacity of high-purity polycrystalline YBa2Cu3O7 from 0.4 to 400 K in applied magnetic fields of 0 and 70 kG

Abstract

The heat capacities C(T) of high-purity polycrystalline pellet and powder samples of YBa2Cu3O7 are reported for the temperature T in the range 0.4 to 400 K. In addition to the feature associated with the onset of superconductivity at T(c) congruent-to 91 K, two clear anomalies in C(T) for the pellet sample were observed near 74 and 330 K; the origins of the latter two anomalies are unknown, but the temperatures at which they occur are similar to those at which anomalies are seen in the temperature-dependent magnetization M(T). The feature in M(T) at congruent-to 70 K increases linearly with field H at a rate of 0.30 K/kG for 20 kG less-than-or-equal-to H less-than-or-equal-to 40 kG. The anomaly in C(T) at congruent-to 330 K observed for the pellet was not observed for the powder sample, which is consistent with the lack of an anomaly in M(T) near this temperature for the powder sample. For the pellet sample, the electronic entropy near T(c), the contribution of superconducting fluctuations to C(T) near T(c), and the influence of a 70-kG magnetic field on C(T) near T(c) and below 10 K are analyzed and discussed. Various features of the C(T) for the pellet sample such as the magnitude of C(T) and the derived Debye temperature and heat capacity jump at T(c), as well as the size and shape of the C(T) anomalies at congruent-to 74 and 330 K, were found to depend on the thermal- and/or magnetic-field treatment history of the sample. Differential-thermal-analysis measurements quantitatively determined the BaCuO2 impurity content in our batch of YBa2Cu3O7 to be 0.3(1) wt. %. Coupled with the analysis of the low-T C(T) measurements, we conclude that the linear C(T) coefficient gamma(0) associated with the YBa2Cu3O7 phase in the pellet sample is congruent-to 4.0 mJ/molK2. It is suggested that some fraction of gamma(0) could arise from thermal excitation of antiferromagnetic spin waves in the Cu-O chains of the structure.