Electronic screening-enhanced hole pairing in two-leg spin ladders studied by high-resolution resonant inelastic X-ray scattering at Cu M edges

We study the electronic screening mechanisms of the effective Coulomb on-site repulsion in hole-doped Sr14Cu24O41 compared to undoped La6Ca8Cu24O41 using polarization dependent high-resolution resonant inelastic x-ray scattering at Cu M edges. By measuring the energy of the effective Coulomb on-site repulsion and the spin excitations, we estimate superexchange and hopping matrix element energies along rungs and legs, respectively. Interestingly, hole doping locally screens the Coulomb on-site repulsion reducing it by as much as 25%. We suggest that the increased ratio of the electronic kinetic to the electronic correlation energy contributes to the local superexchange mediated pairing between holes.