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

Abstract

We report Cu NQR results on Ge-doped heavy-fermion superconductors ${\mathrm{CeCu}}_{2}({\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}{)}_{2}$ $(0lxl~0.2)$ and compare with previous results on ${\mathrm{Ce}}_{0.99}{\mathrm{Cu}}_{2.02}{\mathrm{Si}}_{2}$ $(x=0).$ Once only 1% Ge is substituted for Si to expand the lattice, an antiferromagnetic (AFM) order sets in at ${T}_{N}\ensuremath{\sim}0.7\mathrm{K},$ followed by the onset of superconductivity at ${T}_{c}=0.5\mathrm{}\mathrm{K}.$ 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 ${T}_{1}T=\mathrm{const}$ 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 $0l~xl0.06$ is discussed in the context of a SO(5) theory that unifies superconductivity and antiferromagnetism.