SPECIFIC-HEAT OF YBA2CU3O7-DELTA

We present measurements of the specific heat of YBa2Cu3O7-delta single crystals from 2 to 10 K and from 0 to 10 T, including both twinned an d untwinned crystals with various oxygen doping (7 - delta = 6.95, 6.9 7, and 6.99). The zero-field specific heat includes a linear-T term wh ich is small compared to other YBa2Cu3O7-delta samples, but large comp ared to the T-2 electronic specific heat predicted for a superconducto r with clean lines of nodes in the gap function. This zero-field linea r-T term is found to decrease with decreasing concentration of twin bo undaries and oxygen vacancies delta. The specific heat increases in a magnetic field and has a strong dependence on field orientation. Sever al possible field-dependent contributions are compared to the data, in cluding a Schottky anomaly, vortex-vortex interactions, a traditional mixed-state HT term associated with vortex cores, and a <root(H)over b ar T> term predicted by G. Volovik for the mixed state in superconduct ors with lines of nodes in the gap function. The field dependence of t he specific heat, according to a comparison of fits based on these pos sibilities, supports the existence of lines of nodes and suggests the presence of quasiparticle excitations outside the vortex core.