SPECIFIC-HEAT OF YBA2CU3O7-DELTA SINGLE-CRYSTALS - IMPLICATIONS FOR THE VORTEX STRUCTURE

The anisotropy of the magnetic field dependence of the specific heat o f YBa2Cu3O7-delta can be used to identify different low-energy excitat ions which include phonons spin-1/2 particles and electronic contribut ions. With a magnetic field H applied perpendicular to the copper oxid e planes, we find that the specific heat includes a linear-T term prop ortional to root H. The nonlinear field dependence of the density of s tates at the Fermi level suggests that there are quasiparticle excitat ions throughout the entire vortex, not just in the vortex core. The ro ot HT term agrees quantitatively with G. Volovik's prediction for a su perconductor with lines of nodes in the gap. A similar, but much small er, effect is predicted for fields parallel to the planes and sensitiv e measurements of the in-plane anisotropic magnetic field dependence o f the specific heat could be used to map out the nodes.