Transport and NMR under pressure in the superconducting spin ladder: Sr2Ca12CU24O41+delta

Starting from the insulating compound Sr14Cu24O41+delta, the substitution o f Sr by isovalent Ca up to 12 Ca results in the transfer of carriers from t he CuO2 chains to the Cu2O3 ladders, confering 1D metallic properties to th e material along the legs of the ladders. At the same time, the amplitude o f the ladder spin gap is decreased with increasing Ca content. Then, high p ressure (>30kbar) is necessary to stabilize a superconducting state. Our me asurements of the resistivity in the plane of the ladders as a function of pressure and temperature show that the anisotropy is strongly depressed whe n the critical pressure for superconductivity is reached. compared to the a mbient pressure situation. On the same sample, the temperature dependence o f the NMR Knight shift measured on Cu-63 nuclei belonging to the ladders sh ows the sudden disappearance of the spin-gap at the critical pressure and i ts transformation into a pseudogap with low lying spin excitations.