Critical point in the superconducting phase diagram of UPt3

We report on detailed measurements of the specific heat of UPt3, 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 H/sub c2/ curve. Beyond that point, there is evidence for only one phase. Combining these thermodynamic results with ultrasonic attenuation and H/sub c2/ data, an H-T diagram is constructed which consists of a ''polycritical'' point and several superconducting phases