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

⁶³Cu-NMR experiments of Knight shift and relaxation time T₁ have been performed on the two-leg spin ladders of a Sr₂Ca₁₂Cu₂₄O₄₁ single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction. Received 21 October 1999