Abstract
Pressure-induced superconductivity was studied for a spin-ladder cuprate ${\text{Sr}}_{2}{\text{Ca}}_{12}{\text{Cu}}_{24}{\text{O}}_{41}$ using nuclear magnetic resonance under pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a transitional change from a spin-gapped state to a Fermi-liquid state at temperatures higher than ${T}_{c}$. The relaxation rate $1/{T}_{1}$ shows activated-type behavior at an onset pressure, whereas Korringa-like behavior becomes predominant at the optimal pressure, suggesting that an increase in the density of states at the Fermi energy leads to enhancement of ${T}_{c}$. Nuclear quadrupole resonance spectra suggest that pressure application causes transfer of holes from the chain to the ladder sites. The transfer of holes increases DOS below the optimal pressure. A dome-shaped ${T}_{c}$ versus pressure curve arises from naive balance between the transfer of holes and broadening of the band width.
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