Creation and destruction of a spin gap in weakly coupled quarter-filled ladders

We investigate weakly coupled quarter-filled ladders with model parameters relevant for NaV2O5 using density-matrix renormalization group calculations on an extended Hubbard model coupled to the lattice. NaV2O5 exhibits superantiferroelectric charge order with a zigzag pattern on each ladder. We show that this order causes a spin dimerization along the ladder and is accompanied by a spin gap of the same magnitude as that observed experimentally. The spin gap is destroyed again at large charge order due to a restructuring of the spins. An analysis of an effective spin model predicts a recreation of the gap by interladder singlets when the charge order increases further.