Superconductivity of the $Sr_2 Ca_{12} Cu_{24} O_{41}$ spin ladder system: Are the superconducting pairing and the spin-gap formation of the same origin?

TL;DR

This study investigates the microscopic nature of pressure-induced superconductivity in Sr$_2$Ca$_{12}$Cu$_{24}$O$_{41}$, revealing a possible link between spin-gap formation and superconducting pairing through NMR analysis.

Contribution

The paper provides the first microscopic NMR evidence of superconductivity and spin dynamics in Sr$_2$Ca$_{12}$Cu$_{24}$O$_{41}$ under high pressure, suggesting a common origin for spin-gap and pairing.

Findings

Superconductivity exhibits a s-wave-like gap with finite quasiparticle excitation.

Two excitation modes observed: an activation-type and a T-linear component.

Gapless mode likely due to holon-spinon bound states facilitating pairing.

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.