COMPETITION BETWEEN MAGNETISM AND SUPERCONDUCTIVITY IN CECU2SI2

The interplay between superconductivity and magnetism in CeCu2Si2 has been investigated by means of microprobe, muon spin rotation and relax ation (mu SR), and specific-heat measurements on four slightly off-sto ichiometric polycrystalline samples Ce1+xCu2+ySi2. Microprobe analysis reveals that within the errors (+/-3%) the main phases of all four sa mples exhibit the ideal stoichiometry 1:2:2 and their relative composi tion varies by less than 2%. Muon spin rotation and relaxation measure ments, however, reveal pronounced differences in their ground states. The nonsuperconducting sample Ce0.99Cu2.02Si2 exhibits a phase transit ion at T-m=0.67 K to a magnetically ordered ground state of unknown st ructure, with a lower limit on the size of the frozen moments mu appro ximate to 0.2 mu(B). For T<T-m slow residual fluctuations of these mom ents at a rate nu approximate to 3 MHz are observed. In the three supe rconducting samples comparable magnetic behavior is found in reduced v olume fractions. Paramagnetic and magnetic regions are distributed inh omogeneously in these samples, the relative volume fractions being str ongly sample and temperature dependent. In all samples considerable vo lume fractions remain magnetic down to T=60 mK. The present data provi de evidence that superconductivity sets in first in the paramagnetic r egions, and, on further cooling, reduces the magnetically ordered volu me fraction. Superconductivity and magnetic order do not appear to spa tially coexist but compete in CeCu2Si2.