SUPERCONDUCTIVITY AND HIGH-FIELD PHASES IN HEAVY-FERMION SYSTEMS

We present measurements of the elastic constants for some of the known heavy fermion superconductors at low temperatures and high magnetic f ields. In these compounds the superconducting phase lies embedded in a nother phase of more or less strongly magnetic nature. In contrast to the cases of UPt3, URu2Si2, and UPd2Al3 where the superconductivity oe xists with antiferromagnetic order, our results indicate that in high quality CeCu2Si2 single crystals the superconductivity does not coexis t with the embedded phase but rather expels it. The influence of the C u stoichiometry on the physical properties and on the B-T phase diagra m of CeCu2Si2, is discussed. In addition we find pronounced elastic an omalies in magnetic fields up to 23 T, which yield a detailed B-T phas e diagram for CeCu2Si2. By comparison with results obtained with other methods we gain information about the nature of various transitions. Further we show magnetoacoustic quantum oscillations (MAQO) and other magnetoacoustic effects in the heavy fermion superconductors.