The specific heat due to line nodes in the superconducting gap of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ 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 ${\mathrm{BaZrO}}_3$ crucible, for which paramagnetic terms are reduced to $\approx 0.006\%$ spin-$\frac{1}{2}$ per Cu atom. The contribution of line nodes shows up directly in the difference $C(B,T)\mathrm{}-C(0,T)\mathrm{}$ at fixed temperatures $(T<\mathrm{}5\mathrm{}\mathrm{K})$ as a function of the magnetic field parallel to the c axis $(B<~14\mathrm{T}).\mathrm{}$ These data illustrate the smooth crossover from $C\propto T^2$ at low fields to $C\propto {\mathrm{TB}}^{1/2}$ at high fields, and provide new values for gap parameters that are quantitatively consistent with tunneling spectroscopy and thermal conductivity in the framework of $d_{x^2-y^2}$ pairing symmetry. Data for B along the nodal and antinodal directions in the $\mathrm{ab}$ plane are also provided. The in-plane anisotropy predicted in the clean limit is not observed.

• Received 29 November 1999

DOI:https://doi.org/10.1103/PhysRevB.63.094508