Exotic magnetism and superconductivity in Ge-substituted CeCu2Si2: A Cu NQR study

Abstract

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 TN∼0.7K, followed by the onset of superconductivity at Tc=0.5K. 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 TN, excludes the presence of phase segregation between the superconducting (SC) and the AFM phases in the coexistent state below Tc. The 1/T1 result does not show significant reduction below Tc, 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, TN is progressively increased, while Tc 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 TN 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.

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