Anisotropy, disorder, and superconductivity in CeCu2Si2 under high pressure

Resistivity measurements were carried out up to 8 GPa on single-crystal and polycrystalline samples of CeCu2Si2 from differing sources in the homogeneity range. The anisotropic response to current direction and small uniaxial stresses was explored, taking advantage of the quasi-hydro static environment of the Bridgman anvil cell. It was found that both the superconducting transition temperature T-c and the normal state properties are very sensitive to uniaxial stress, which leads to a shift of the valence instability pressure P-v and a small but significant change in T-c for different orientations with respect to the tetragonal c-axis. The coexistence of superconductivity and residual resistivity close to the Ioffe-Regel limit around 5 GPa provides a compelling argument for the existence of a valence fluctuation mediated pairing interaction at high pressure in CeCu2Si2.