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

Abstract

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 $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.975$ to a heavy-electron superconducting (SC) phase at $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.025$. We have found that the sample of $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.99$ 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 ${T}_{\mathrm{m}}\ensuremath{\sim}1.2\mathrm{K}$. This unusual ``critical magnetic phase'' can coexist with the SC phase. For the samples of $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.00$ and $1.025$ such a state is expelled by the onset of the SC state.