  Pressure-induced superconductivity in a spin-ladder cuprate
Sr$_2$Ca$_{12}$Cu$_{24}$O$_{41}$ has not been studied on a microscopic level so
far although the superconductivity was already discovered in 1996. We have
improved high-pressure technique with using a large high-quality crystal, and
succeeded in studying the superconductivity using $^{63}$Cu nuclear magnetic
resonance (NMR). We found that anomalous metallic state reflecting the
spin-ladder structure is realized and the superconductivity possesses a
s-wavelike character in the meaning that a finite gap exists in the
quasi-particle excitation: At pressure of 3.5GPa we observed two excitation
modes in the normal state from the relaxation rate $T_1^{-1}$. One gives rise
to an activation-type component in $T_1^{-1}$, and the other $T$-linear
component linking directly with the superconductivity. This gapless mode likely
arises from free motion of holon-spinon bound states appearing by hole doping,
and the pairing of them likely causes the superconductivity.
