UNUSUAL B-T PHASE-DIAGRAM OF THE HEAVY-FERMION SUPERCONDUCTOR CECU2SI2

Elastic constant and thermal expansion data for a high quality single crystal of CeCu2Si2 are presented. The data yield a detailed B-T phase diagram exhibiting a previously unknown high-field phase. From the un usual behavior of the elastic constants and the thermal expansion on e ntering the superconducting phase we deduce that, in contrast to other heavy-fermion superconductors, the superconducting phase of CeCU2Si2 does not coexist with the surrounding phase.