Magnetic and superconducting properties of an S -type single-crystal CeCu2Si2 probed by Cu63 nuclear magnetic resonance and nuclear quadrupole resonance

We have performed $⁶³\mathrm{Cu}$ nuclear-magnetic-resonance/nuclear-quadrupole-resonance measurements to investigate the magnetic and superconducting (SC) properties on a ``superconductivity dominant'' ($S$-type) single crystal of ${\mathrm{CeCu}}₂{\mathrm{Si}}₂$. Although the development of antiferromagnetic (AFM) fluctuations down to 1 K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8 K, and no magnetic anomaly was observed above ${T}_{\mathrm{c}}\ensuremath{\sim}0.6$ K. These behaviors were expected in $S$-type ${\mathrm{CeCu}}₂{\mathrm{Si}}₂$. The temperature dependence of the nuclear spin-lattice relaxation rate $1/{T}₁$ at zero field was almost identical to that in the previous polycrystalline samples down to 130 mK, but the temperature dependence deviated downward below 120 mK. In fact, $1/{T}₁$ in the SC state could be fitted with the two-gap ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave model rather than the two-gap ${s}₊₊$-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below ${T}_{\mathrm{c}}$, which is indicative of the formation of spin-singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from $1/{T}₁T$, which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field ${H}_c2$, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.