Thermodynamics of superconducting UPt₃ under uniaxial pressure

From the theory of Yip, Li, and Kumar, we derive convenient expressions relating the slopes of the phase boundaries near the tetracritical point (TP) in the superconducting heavy-fermion compound UPt₃ to the jumps in the specific heat across the transitions. We apply these relations to our recent measurements on the pressure dependence of the phase boundaries near the TP for magnetic field, H, parallel to both the a and c axes. Comparison with published data on the specific heat jumps at zero pressure or at zero field implies that the low-field inner transition line is first order for the field orientation H∥a for H>0, and is likely to be second order for H∥c. For H=0 the corresponding transition line in the pressure-temperature plane is likely to be second order as well.