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 relaxation (mu SR), and specific-heat measurements on four slightly off-stoichiometric polycrystalline samples Ce1+xCu2+ySi2. Microprobe analysis reveals that within the errors (+/-3%) the main phases of all four samples exhibit the ideal stoichiometry 1:2:2 and their relative composition varies by less than 2%. Muon spin rotation and relaxation measurements, however, reveal pronounced differences in their ground states. The nonsuperconducting sample Ce0.99Cu2.02Si2 exhibits a phase transition at T-m=0.67 K to a magnetically ordered ground state of unknown structure, with a lower limit on the size of the frozen moments mu approximate to 0.2 mu(B). For T<T-m slow residual fluctuations of these moments at a rate nu approximate to 3 MHz are observed. In the three superconducting samples comparable magnetic behavior is found in reduced volume fractions. Paramagnetic and magnetic regions are distributed inhomogeneously in these samples, the relative volume fractions being strongly sample and temperature dependent. In all samples considerable volume fractions remain magnetic down to T=60 mK. The present data provide evidence that superconductivity sets in first in the paramagnetic regions, and, on further cooling, reduces the magnetically ordered volume fraction. Superconductivity and magnetic order do not appear to spatially coexist but compete in CeCu2Si2.