Up 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 Upt, 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 below 50 mK which prevails in magnetic fields far above Hd (after subtraction of the nuclear specific heat of lg5F't). This upturn whose origin is undetermined can best be fitted with a T temperature dependence. We also present data on the low field (3 Oe) d.c. susceptibility down to 1.2 mK which varies as 2 in the S.C. state and lg6Pt n.m.r. spectra in fields up to 0.7 T consisting of a single narrow line with a Knight shift of for B IC.