Coexisting static magnetic order and superconductivity in CeCu2Si2 found by nuclear quadrupole resonance

63Cu nuclear quadrupole resonance (NQR) has been observed in a heavy-fermion system CeCu2Si2. In the superconducting CeCu2Si2 specimens (Tc congruent 0.6 K), NQR experiments have revealed an onset of a phase transition below ∼1 K. Zero- and longitudinal-field muon-spin-relaxation measurements suggest a static magnetic ordering below ∼0.8 K, either in the spin-glass or in incommensurate spin-density-wave states. The exotic transition observed in NQR measurements should be closely related to the static magnetic ordering in muon-spin-relaxation measurements. NQR data from a nonsuperconducting CeCu2Si2 sample are consistent with extensive disorder in Cu site occupation. Therefore, the spin-density wave may be depressed in the nonsuperconducting CeCu2Si2. The superconducting transition temperature Tc can be enhanced by the presence of spin-density waves such that, if the spin-density wave is depressed, CeCu2Si2 will lose its superconducting properties