UPt3 at very low temperatures: Specific heat in high magnetic fields

In order to determine the spectrum of low energy excitations in the heavy fermion system UPt3 in both its normal and superconducting states we have measured the specific heat of a single crystal down to temperatures below 15 mK and in magnetic fields up to 7 T. The specific heat shows a surprising upturn in c/T below 50 mK which prevails in magnetic fields far above H_c2 (after subtraction of the nuclear specific heat of ¹⁹⁵Pt). We also present data on the low field (10 Oe) d.c. susceptibility down to 1.2 mK which varies as T² in the s.c. state. This reflects the T-dependence of the London penetration depth and also proves that superconductivity persists down to this temperature.