Specific heat of YBa2Cu3O7-δ

We present measurements of the specific heat of YBa2Cu3O7-δ single crystals from 2 to 10 K and from 0 to 10 T, including both twinned and untwinned crystals with various oxygen doping (7-δ=6.95, 6.97, and 6.99). The zero-field specific heat includes a linear-T term which is small compared to other YBa2Cu3O7-δ samples, but large compared to the T2 electronic specific heat predicted for a superconductor with clean lines of nodes in the gap function. This zero-field linear-T term is found to decrease with decreasing concentration of twin boundaries and oxygen vacancies δ. The specific heat increases in a magnetic field and has a strong dependence on field orientation. Several possible field-dependent contributions are compared to the data, including a Schottky anomaly, vortex-vortex interactions, a traditional mixed-state HT term associated with vortex cores, and a √(H)T term predicted by G. Volovik for the mixed state in superconductors with lines of nodes in the gap function. The field dependence of the specific heat, according to a comparison of fits based on these possibilities, supports the existence of lines of nodes and suggests the presence of quasiparticle excitations outside the vortex core. copyright 1997 The American Physical Society