MAGNETIC-FIELD DEPENDENT SPECIFIC-HEAT ACROSS THE MIDDLE AND UPPER CRITICAL-FIELD LINES IN UPT3

The specific heat C of the heavy fermion vector superconductor UPt3 has been measured with high resolution as a function of magnetic field across the middle and upper (Hc2) critical field boundaries for temperatures below the tetracritical point. For TTc and Hc1HHc2, C(H) grows at H12, in agreement with a recent prediction for a superconductor with lines of zeros in the gap function. At higher fields, a distinct jump ΔC is found across both boundaries and in both principal crystallographic directions. The inferred susceptibility jumps imply highly anisotropic Ginzburg-Landau parameters κ2. At Hc2, dκ2dT displays a sign reversal between the two directions, unique among superconductors. This suggests a strong influence of the gap symmetry on thermodynamic quantities. © 1995 The American Physical Society.