Superconductivity of the $S{r}_{2}C{a}_{12}C{u}_{24}{O}_{41}$ spin ladder system: Are the superconducting pairing and the spin-gap formation of the same origin?

Abstract

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.

to be published in Phys. Rev. Lett