Exotic magnetism and superconductivity in Ge-substitutedCeCu2Si2:A Cu NQR study

We report Cu NQR results on Ge-doped heavy-fermion superconductors ${\mathrm{CeCu}}_2({\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x{)}_2$ $(0<x<~0.2)$ and compare with previous results on ${\mathrm{Ce}}_{0.99}{\mathrm{Cu}}_{2.02}{\mathrm{Si}}_2$ $\mathrm{}(x=0\mathrm{}).$ Once only 1% Ge is substituted for Si to expand the lattice, an antiferromagnetic (AFM) order sets in at $T_N\sim 0.7\mathrm{K},$ followed by the onset of superconductivity at $T_c=0.5\mathrm{}\mathrm{}\mathrm{K}.$ The sudden emergence of AFM order due to the slight Ge doping reinforces that an exotic magnetic phase at $x=0\mathrm{}$ 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 $T_{1}T=\mathrm{const}$ 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 ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2.$ The exotic interplay between magnetism and superconductivity in $0<~x<\mathrm{}0.06\mathrm{}$ is discussed in the context of a SO(5) theory that unifies superconductivity and antiferromagnetism.