The heat capacity of YBa 2Cu 3O 7 and YBa 2Cu 3O 6 in the range 0.4 to 20 K: Evidence for an intrinsic T-term
The heat capacity of a number of YBa 2Cu 3O 7 samples has been measured in the range 0.4 to 20 K to investigate the effect of paramagnetic chemical and phase impurities. Values for the T-term and Debye temperature are in the range 5.63-11.5 mJ/mol K 2 and 413-439 K, respectively. The role of various impurity phases, including BaCuO 2, is discussed. The removal of the labile oxygen from YBa 2Cu 3O 7 to form the non-superconducting YBa 2Cu 3O 6 has enabled the determination of a value of ∼1.5 mJ/mol K 2 for the coefficient of the T-term that is intrinsic to the superconducting state. The intrinsic T-term is discussed in terms of the resonating-valence-bond- model and tunnelling due to two-level-systems.