We report on detailed measurements of the specific heat of ${\mathrm{UPt}}_3$, performed on a high-quality single crystal in a magnetic field perpendicular to the c axis, at temperatures down to 100 mK. Two distinct phase transitions at zero field are seen to converge at a critical point, near H=5 kOe, which coincides with the sharp break in the ${\mathrm{H}}_{\mathrm{c}2}$ curve. Beyond that point, there is evidence for only one phase. Combining these thermodynamic results with ultrasonic attenuation and ${\mathrm{H}}_{\mathrm{c}2}$ data, an H-T diagram is constructed which consists of a ‘‘polycritical’’ point and several superconducting phases.

• Received 24 March 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.93