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

TL;DR

This paper explores how charge order in weakly coupled quarter-filled ladders influences the formation and destruction of a spin gap, using advanced computational methods to match experimental observations.

Contribution

It demonstrates the creation and destruction of a spin gap driven by charge order in a model relevant for NaV2O5, revealing a re-creation mechanism via inter-ladder singlets.

Findings

Charge order induces a spin gap consistent with experiments.

Large charge order destroys the spin gap through spin restructuring.

Inter-ladder singlets can re-establish the spin gap at higher charge order.

Abstract

We investigate weakly coupled quarter-filled ladders with model parameters relevant for NaV_2O_5 using density-matrix renormalization group calculations on an extended Hubbard model coupled to the lattice. NaV2O5 exhibits super-antiferroelectric charge order with a zigzag pattern on each ladder. We show that this order, with a periodicity of four ladders, causes a spin dimerization along the ladder and a corresponding 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 re-creation of the gap by inter-ladder singlets when the charge order increases further.