Evolution from Magnetism to Unconventional Superconductivity in a Series ofCexCu2Si2Compounds Probed by Cu NQR

We report on the basis of Cu-NQR measurements that the ground state in a series of ${\mathrm{Ce}}_x{\mathrm{Cu}}_2{\mathrm{Si}}_2$ compounds evolves from a magnetically ordered phase at $x=0.975\mathrm{}$ to a heavy-electron superconducting (SC) phase at $x=1.025\mathrm{}$. We have found that the sample of $x=0.99\mathrm{}$ does not exhibit any trace of magnetic phase transition down to 0.012 K. Slow magnetic fluctuations with low frequencies comparable to the NQR frequency develop rapidly below $T_{\mathrm{m}}\sim 1.2\mathrm{K}$. This unusual “critical magnetic phase” can coexist with the SC phase. For the samples of $x=1.00\mathrm{}$ and $1.025$ such a state is expelled by the onset of the SC state.