Magnetism and Superconductivity on the Border of Magnetic Phase: Cu-NQR Study of Ge-substituted CeCu2Si2

We. report Cu-NQR result on Ge-doped heavy-fermion superconductor CeCu2(Si1-xGex) (0 < x <= 0.2). The main effect of the Ge doping is considered to he a negative pressure from the temperature dependence of the relaxation rate 1/T-1. We found the broadening of the Cu-NQR spectra originating from an internal field at the Cu site, and the distinct hump in 1/T-1 at TN even in the sample with x 0.02. By the Ge doping; the TN increases and the superconducting transition temperature T-c decreases from the same temperature of 0.05 K, implying that the magnetic and the superconducting phases are nearly degenerate in undoped CeCu2Si2. With increasing a., dynamical characteristics of magnetic order at x = 0 change to more static ones which are suggested to be in a localized regime above x similar to 0.25. The does not show significant reduction below T-c. This result is not consistent with a line-node-gap model, but reveals that low-energy magnetic excitations survive persistently even in the superconducting state.