Y. Tokunaga

From Ru- and Cu-NMR studies, we present evidence for coexistence of superconductivity and ferromagnetism in a cuprate superconductor ${\mathrm{RuSr}}_2{\mathrm{YCu}}_2{\mathrm{O}}_8$ (RuY1212). The observation of a large enhancement of a radio-frequency field for the Ru-NMR signal at zero field reveals the existence of a ferromagnetic (FM) component in the ordered ${\mathrm{RuO}}_2$ plane below a Curie temperature of $T_M=150\mathrm{}\mathrm{K}$. Just below the onset temperature of superconductivity $T_c^{\mathrm{onset}}=45\mathrm{}\mathrm{K}$, a remarkable decrease of the nuclear spin-lattice relaxation rate ${1/T}_1$ was observed within the ordered ${\mathrm{RuO}}_2$ plane as well as the ${\mathrm{CuO}}_2$ plane, revealing that the superconducting gap coexists with the FM component in the ${\mathrm{RuO}}_2$ plane on a microscopic scale. In addition, from the observation of a sharp peak in ${}^{101}\left({1/T}_1\right)$ at $T_c^{\mathrm{zero}}\sim 23\mathrm{K}$ where the resistivity becomes zero, we suggest that the motion of self-induced vortices originating from fluctuations of the FM component induces the resistivity between $T_c^{\mathrm{onset}}$ and $T_c^{\mathrm{zero}}$ in RuY1212.

We report Cu-NMR studies of the multilayered high-$T_c$ cuprate $\left({\mathrm{Cu}}_{0.6}{\mathrm{C}}_{0.4}\right){\mathrm{Ba}}_2{\mathrm{Ca}}_3{\mathrm{Cu}}_4{\mathrm{O}}_{12+y}$ (Cu1234) with $T_c=117\mathrm{}$ K. In the normal state, the Knight shift (K) and the nuclear spin-lattice relaxation rate ${(1/T}_1)$ of ${}^{63}\mathrm{Cu}$ give evidence that the inner ${\mathrm{CuO}}_2$ planes (IP) are underdoped, whereas the outer ones (OP) are heavily overdoped. In the superconducting (SC) state, both K and ${1/T}_1$ decrease markedly below $T_c=117\mathrm{}$ K in the IP, whereas in the OP they decrease moderately below $T_c=117\mathrm{}$ K, but markedly below $T_{c2\mathrm{}}=60\mathrm{}$ K. The unusual NMR results in the OP reveal that the SC gap does not fully develop down to $T_{c2\mathrm{}}=60\mathrm{}$ K. From comparison with a conventional d-wave model, it is shown that the SC gap in the OP increases linearly below $T_c$ and follows the mean-field type of T dependence below $T_{c2\mathrm{}}.$ We propose that these dissimilarities are caused by the large difference of doping levels between the IP and the OP. The bulk SC transition at $T_c=117\mathrm{}$ K is considered to be triggered by the underdoped IP in Cu1234.