Charge ordering in quarter-filled ladder systems coupled to the lattice

TL;DR

This study explores how electron-phonon interactions influence charge ordering in quarter-filled ladder systems, revealing that lattice coupling lowers the Coulomb threshold for charge order formation and affects spin and charge excitations.

Contribution

It demonstrates the significant role of Holstein electron-phonon coupling in charge ordering and provides detailed calculations of excitations and spin interactions in ladder systems.

Findings

Lattice coupling reduces the Coulomb threshold for charge order.

Charge order is accompanied by static lattice distortion.

Calculated spin exchange J matches experimental data.

Abstract

We investigate charge ordering in the presence of electron-phonon coupling for quarter-filled ladder systems by using Exact Diagonalization. As an example we consider NaV2O5 using model parameters obtained from first-principles band-structure calculations. The relevant Holstein coupling to the lattice considerably reduces the critical value of the nearest-neighbor Coulomb repulsion at which formation of the zig-zag charge-ordered state occurs, which is then accompanied by a static lattice distortion. Energy and length of a kink-like excitation on the background of the distorted lattice are calculated. Spin and charge spectra on ladders with and without static distortion are obtained, and the charge gap and the effective spin-spin exchange parameter J are extracted. J agrees well with experimental results. Analysis of the dynamical Holstein model, restricted to a small number of phonons, shows that low frequency lattice vibrations increase the charge order, accompanied by dynamically produced zig-zag lattice distortions.