Magnetic excitations in charge- ordered NaV2O5

TL;DR

This paper investigates the spin excitation spectrum of charge ordered NaV2O5, exploring various exchange models and their implications for observed spectral features, including anisotropies, inter-chain coupling, and high-energy excitations.

Contribution

It compares multiple exchange models for NaV2O5, highlighting the significance of diagonal exchange and anisotropies, and aligns theoretical predictions with recent neutron scattering data.

Findings

Exchange anisotropies influence spectral splitting.

Strong inter-chain coupling leads to high-energy optical excitations.

The CO/MV model matches neutron scattering results well.

Abstract

An investigation of the spin excitation spectrum of charge ordered (CO) NaV2O5 is presented. We discuss several different exchange models which may be relevant for this compound, namely in- line and zig-zag chain models with weak as well as strong inter- chain coupling and also a ladder model and a CO/MV (mixed valent) model. We put special emphasis on the importance of large additional exchange across the diagonals of V- ladders and the presence of exchange anisotropies on the excitation spectrum. It is shown that the observed splitting of transverse dispersion branches may both be interpreted as anisotropy effect as well as acoustic- optic mode splitting in the weakly coupled chain models. In addition we calculate the field dependence of excitation modes in these models. Furthermore we show that for strong inter- chain coupling, as suggested by recent LDA+U results, an additional high energy optical excitation appears and the spin gap is determined by anisotropies. The most promising CO/MV model predicts a spin wave dispersion perpendicular to the chains which agrees very well with recent results obtained by inelastic neutron scattering.