Spin excitations in the (Sr,Ca)(14)Cu24O41 family of spin ladders: Cu-63 and O-17 NMR studies under pressure

We report the results of a NMR study of hole doped spin ladders belonging to the series Sr14-xCaxCu24O41+delta performed on Cu-63 and O-17 nuclei. The new results obtained on Ca0 and Ca12 single crystals at ambient pressure and also under 32 kbar confirm the onset of low-lying spin fluctuations modes at zero energy coexisting with spin-gapped excitations when superconductivity can be stabilized under pressure in Ca12. We found that the theoretical two and three magnons mechanisms explain fairly well the spin-lattice relaxation data using the magnitude of the pressure dependent magnon spin gap Delta(s) derived from the Knight shifts data as long as most of the spectral weight for low-frequency spin fluctuations is provided by the magnon dispersion of isolated ladders. The cross over between spin gapped and paramagnetic regimes of decoupled Heisenberg chains can be identified in heavily doped ladders via the temperature dependence of dynamical structure factors at qsimilar to(pi,pi) and qsimilar to(0,0) wave vectors. The cross over temperature scales under pressure with Delta(s)/2.