Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in Na_<i>x</i>CoO₂·<i>y</i>H₂O

Abstract

Co NQR studies were performed in recently discovered superconductor\nNa$_x$CoO$_2$$\\cdot y$H$_2$O to investigate physical properties in the\nsuperconducting (SC) and normal states. Two samples from the same Na$_x$CoO$_2$\nwere examined, SC bilayer-hydrate sample with $T_c \\sim 4.7$ K and non-SC\nmonolayer-hydrate sample. From the measurement of nuclear-spin lattice\nrelaxation rate $1/T_1$ in the SC sample, it was found that the coherence peak\nis absent just below $T_c$ and that $1/T_1$ is proportional to temperature far\nbelow $T_c$. These results, which are in qualitatively agreement with the\nprevious result by Fujimoto {\\it et al.}, suggest strongly that unconventional\nsuperconductivity is realized in this compound. In the normal state, $1/T_1T$\nof the SC sample shows gradual increase below 100K down to $T_c$, whereas\n$1/T_1T$ of the non-SC sample shows the Korringa behavior in this temperature\nrange. From the comparison between $1/T_1T$ and $\\chi_{\\rm bulk}$ in the SC\nsample, the increase of $1/T_1T$ is attributed to nearly ferromagnetic\nfluctuations. These remarkable findings suggest that the SC sample possesses\nnearly ferromagnetic fluctuations, which are possibly related with the\nunconventional superconductivity in this compound. The implication of this\nfinding is discussed.\n