(Sr/Ca)_{14}Cu_{24}O_{41} spin ladders studied by NMR under pressure

(63)Cu-NMR measurements have been performed on two-leg hole-doped spin ladders Sr_{14-x}Ca_{x}Cu_{24}O_{41} single crystals (0-x-12) at several pressures up to the pressure domain where the stabilization of a superconducting ground state can be achieved. The data reveal marked decrease of the spin gap derived from Knight shift measurements upon Ca substitution and also under pressure and confirm the onset of low lying spin excitations around P_{c} as previously reported. The spin gap in Sr_{2}Ca_{12}Cu_{24}O_{41} is strongly reduced above 20 kbar. However, the data of an experiment performed at P=36 kbar where superconductivity has been detected at 6.7K by an inductive technique have shown that a significant amount of spin excitations remains gapped at 80K when superconductivity sets in. The standard relaxation model with two and three-magnon modes explains fairly well the activated relaxation data in the intermediate temperature regime corresponding to gapped spin excitations using the spin gap data derived from Knight shift experiments.The data of Gaussian relaxation rates of heavily doped samples support the limitation of the coherence lenght at low temperature by the average distance between doped holes. We discuss the interplay between superconductivity and the spin gap and suggest that these new results support the exciting prospect of superconductivity induced by the interladder tunnelling of preformed pairs as long as the pressure remains lower than the pressure corresponding to the maximum of the superconducting critical temperature.Comment: 15 pages Latex, 13 figures. to be published in Eur.Phys.Jour.B,200