Magnetic Field Dependence of the Density of States of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$ as Determined from the Specific Heat

The magnetic field dependence of the electronic density of states at the Fermi level, $N(E_F,H)$, is determined in single-crystal Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.95}$ by specific heat measurements. The total specific heat is best described by including two predictions for the electronic specific heat of $d$-wave superconductivity: a $T^2$ term in zero field and an increased linear term in a magnetic field applied perpendicular to the Cu${\mathrm{O}}_2$ planes. The additional linear term, which implies a finite $N(E_F,H)$, obeys $N(E_F,H)\propto {\left(\frac{H}{H_{c2\mathrm{}}}\right)}^{\frac{1}{2}}$ as predicted by Volovik for superconductivity with lines of nodes in the gap.