The specific heat $C$ of the heavy fermion vector superconductor U${\mathrm{Pt}}_3$ has been measured with high resolution as a function of magnetic field across the middle and upper ($H_{c2\mathrm{}}$) critical field boundaries for temperatures below the tetracritical point. For $T\ll T_c$ and $H_{c1\mathrm{}}\ll H\ll H_{c2\mathrm{}}$, $C\left(H\right)\mathrm{}$ grows at $H^{\frac{1}{2}}$, in agreement with a recent prediction for a superconductor with lines of zeros in the gap function. At higher fields, a distinct jump $\Delta C$ is found across both boundaries and in both principal crystallographic directions. The inferred susceptibility jumps imply highly anisotropic Ginzburg-Landau parameters ${\kappa }_2$. At $H_{c2\mathrm{}}$, $\frac{d{\kappa }_2}{\mathrm{dT}}$ displays a sign reversal between the two directions, unique among superconductors. This suggests a strong influence of the gap symmetry on thermodynamic quantities.

• Received 28 July 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.1218