Phase diagram of CeCu 2Si 2 in the B-T plane

A combination dHvA and resistivity measurements investigations on a single crystal of the heavy fermion superconductor CeCu2Si2 has revealed a hitherto hidden complexity in the B-T phase diagram. As well as the superconducting-normal phase boundary, with T _c=0.72K and B _Cc2=1.5T, a double magnetic transition is observed which, depending on the orientation, occurs between 7.2T and 2.8T at 20mK, and which near the c axis accompanies a possible third transition. In the c-direction at least, the two transitions merge and then vanish as the temperature rises to 470mK so revealing an occlusion in the B-T phase plane. The nature of the phase transitions is not revealed by these experiments but a minor reconstruction of the Fermi surface and a change in quasiparticle mass is seen to occur across the occlusion