Direct observation and anisotropy of the contribution of gap nodes in the low-temperature specific heat of YBa2Cu3O7 -: art. no. 094508

The specific heat due to line nodes in the superconducting gap of YBa2Cu3O7 has been blurred up to now by magnetic terms of extrinsic origin, even for high-quality crystals. We report the specific heat of a single crystal grown in a noncorrosive BaZrO3 crucible, for which paramagnetic terms are reduced to approximate to0.006% spin-1/2 per Cu atom. The contribution of line nodes shows up directly in the difference C(B,T) - C(0,T) at fixed temperatures (T < 5 K) as a function of the magnetic field parallel to the c axis (B <less than or equal to> 14 T). These data illustrate the smooth crossover from C infinityT(2) at low fields to C infinity TB1/2 at high fields, and provide new values for gap parameters that are quantitatively consistent with tunneling spectroscopy and thermal conductivity in the framework of d(x2-y2) pairing symmetry. Data for B along the nodal and antinodal directions in the ab plane are also provided. The in-plane anisotropy predicted in the clean limit is not observed.