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 CeCuSi from differing sources in the homogeneity range. The anisotropic response to current direction and small uniaxial stresses was explored, taking advantage of the quasi-hydrostatic environment of the Bridgman anvil cell. It was found that both the superconducting transition temperature T and the normal state properties are very sensitive to uniaxial stress, which leads to a shift of the valence instability pressure P and a small but significant change in T 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 CeCuSi.