Structural and electrical properties under high pressure for the superconducting spin-ladder system Sr0.4Ca13.6Cu24O41+δ

We studied structural and electrical properties under high pressure and low temperature for a S=1/2 Heisenberg two-leg ladder compound Sr0.4Ca13.6Cu24O41+delta. It showed a superconducting transition above 3 GPa with a maximum T-c of 13 K. The Sr0.4Ca13.6Cu24O41+delta phase survives at least up to 9 GPa and down to 7 K without decomposition or phase transition, indicating that it is responsible for the observed superconductivity. The most important role of high pressure for realizing superconductivity is enhancement of the interaction between the Cu2O3 ladder and the one-dimensional CuO2 chain which accelerates redistribution of holes via locally formed Cu-ladder-O-chain hybrid orbitals. Electrical property of the normal state is affected by the carrier density, dimensionality, random potential, and spin scattering which are related to structural modification under high pressure.