SUPERCONDUCTING PHASE-DIAGRAM OF UPT3

We present ultrasonic velocity data for the heavy fermion superconductor UPt3. UPt3 is a strong candidate for unconventional superconductivity. Conclusive evidence for this comes from the observation of multiple superconducting phases. Earlier measurements of the phase diagram relied on a variety of different measurements on different samples. The phase boundaries obtained agreed qualitatively, but the important region near the H(c2) curve remained an open question. Our velocity measurements present the first complete phase diagram of UPt3 based on a single measurement on a single sample. The sharp velocity signatures allow us to trace these signatures to within the width of the normal-superconducting transition. Our results indicate that there is a tetracritical point on the H(c2) curve.