UnusualB-Tphase diagram of the heavy-fermion superconductorCeCu2Si2

Elastic constant and thermal expansion data for a high quality single crystal of ${\mathrm{CeCu}}_2$ ${\mathrm{Si}}_2$ are presented. The data yield a detailed B-T phase diagram exhibiting a previously unknown high-field phase. From the unusual behavior of the elastic constants and the thermal expansion on entering the superconducting phase we deduce that, in contrast to other heavy-fermion superconductors, the superconducting phase of ${\mathrm{CeCu}}_2$ ${\mathrm{Si}}_2$ does not coexist with the surrounding phase.