An NMR approach to the superconducting regime of the spin ladder compound Sr2Ca12Cu24O41

Cu-63-NMR experiments of Knight shift and relaxation time T-1 have been per formed on the two-leg spin ladders of a Sr2Ca12Cu24O41 single crystal at se veral pressures up to the critical pressure P-c for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at P-c observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fr action of the spin excitations remains gapped at P-c = 32 k bar. A comparison between NMR and transport data under pressure suggests th at the depression of the spin gap can be ascribed to an increase in the int erladder exchange coupling, possibly mediated by the ladder-chain interacti on along the b-direction.