Magnon Spectra of Cuprates beyond Spin Wave Theory

TL;DR

This paper uses matrix product states to accurately compute magnon spectra in cuprates, revealing limitations of traditional spin wave theory and providing refined exchange parameters that better match experimental data.

Contribution

It introduces a MPS-based method to go beyond spin wave theory for magnon spectra, improving parameter estimates and understanding of high-energy continua in cuprates.

Findings

Standard model accurately describes low-energy magnon dispersion in La2CuO4.

Corrected exchange parameters reduce overestimation from spin wave theory.

High-energy continuum features are not fully captured by the model.

Abstract

The usual starting point for understanding magnons in cuprate antiferromagnets such as La$_2$CuO$_4$ is a spin model incorporating cyclic exchange, which descends from a one-band Hubbard model, and has parameters taken from fits based on non-interacting spin wave theory. Here we explore whether this provides a reliable description of experiment, using matrix product states (MPS) to calculate magnon spectra beyond spin wave theory. We find that analysis based on low orders of spin wave theory leads to systematic overestimates of exchange parameters, with corresponding errors in estimates of Hubbard $t/U$. Once these are corrected, the ''standard'' model provides a good account of magnon dispersion and lineshape in La$_2$CuO$_4$, but fails to fully capture the continuum observed at high energies. The extension of this analysis to CaCuO$_2$ and Sr$_2$IrO$_4$ is also discussed.