Collapse of the spin gap with pressure in spin-ladder compounds

We introduce a model of weakly coupled spin ladders using a decomposition into triplet and singlet Majorana fermions. We show that a weak coupling between the ladders causes the triplet gap to vanish. Assuming that pressure enhances the interladder coupling, we find that the triplet gap vanishes with increased pressure. We suggest that this is the explanation of recent nuclear magnetic resonance experimental data in superconducting spin-ladder compounds such as Sr2Ca12Cu24O41.