Transport and NMR under pressure in the superconducting spin ladder: Sr2Ca12Cu24O41+δ

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