Exotic magnetism and superconductivity in Ge-substituted CeCu2Si2:: A Cu NQR study -: art. no. 224502

We report Cu NQR results on Ge-doped heavy-fermion superconductors CeCu2(Si1-xGex)(2) (0<x <= 0.2) and compare with previous results on Ce0.99Cu2.02Si2 (x=0). Once only 1% Ge is substituted for Si to expand the lattice, an antiferromagnetic (AFM) order sets in at T-N similar to 0.7 K, followed by the onset of superconductivity at T-c=0.5 K. The sudden emergence of AFM order due to the slight Ge doping reinforces that an exotic magnetic phase at x=0 is in fact a marginal AFM state where slowly fluctuating AFM waves propagate over a long distance. The appearance of internal fields throughout the sample that is deduced from the NQR spectral shape below T-N, excludes the presence of phase segregation between the superconducting (SC) and the AFM phases in the coexistent state below T-c. The 1/T-1 result does not show significant reduction below T-c, followed by a T1T=const behavior. This indicates that the SC phase is in a gapless regime, dominated by magnetic excitations due to the coexistence of AFM and SC phase. As Ge content increases, T-N is progressively increased, while T-c is steeply decreased. As a result of the suppression of the slowly fluctuating AFM waves in the samples with more than x=0.06, their magnetic properties above T-N progressively change to those in a localized regime as observed in CeCu2Ge2. The exotic interplay between magnetism and superconductivity in 0 <= x<0.06 is discussed in the context of a SO(5) theory that unifies superconductivity and antiferromagnetism.