Coexistence of superconductivity and antiferromagnetism in the heavy-fermion superconductor CeCu2(Si1-xGex)2 probed by means of Cu nuclear quadrupole resonance - a test case for the SO(5) theory

We report, on the basis of Cu nuclear quadrupole resonance measurements, that superconductivity (SC) and antiferromagnetism (AF) coexist on a microscopic level in CeCu2(Si1-xGex)2, once a tiny amount of 1% Ge (x = 0.01) is substituted for Si. This coexistence arises because Ge substitution expands the unit-cell volume in nearly homogeneous CeCu2Si2 where the SC coexists with slowly fluctuating magnetic waves. We propose that the underlying exotic phases of SC and AF in nearly homogeneous and in slightly Ge-substituted CeCu2Si2 can be accounted for on the basis of the SO(5) theory that unifies the SC and AF. We suggest that the SC and AF in CeCu2Si2 have a common mechanism.