Theory of Phonon-Assisted Multimagnon Optical Absorption and Bimagnon States in Quantum Antiferromagnets

TL;DR

This paper develops a theoretical framework for understanding phonon-assisted multimagnon optical absorption in quantum antiferromagnets, explaining experimental spectra and predicting bimagnon states and line shapes.

Contribution

It introduces a comprehensive spin-wave theory approach to calculate bimagnon states and spectra, aligning with experimental data and proposing new experimental observations.

Findings

The primary absorption peak matches experimental spectra in cuprates.

Bimagnon states are well-defined quasiparticles with specific energy and momentum.

Predicted line shapes for La$_2$NiO$_4$ provide new insights into spin systems.

Abstract

We calculate the effective charge for multimagnon infrared (IR) absorption assisted by phonons in a perovskite like antiferromagnet and we compute the spectra for two magnon absorption using interacting spin-wave theory. The full set of equations for the interacting two magnon problem is presented in the random phase approximation for arbitrary total momentum of the magnon pair. The spin wave theory results fit very well the primary peak of recent measured bands in the parent insulating compounds of cuprate superconductors. The line shape is explained as being due to the absorption of one phonon plus a new quasiparticle excitation of the Heisenberg Hamiltonian that consists off a long lived virtual bound state of two magnons (bimagnon). The bimagnon states have well defined energy and momentum in a substantial portion of the Brillouin zone. The higher energy bands are explained as one phonon plus higher multimagnon absorption processes. Other possible experiments for observing bimagnons are proposed. In addition we predict the line shape for the spin one system La$_2$NiO$_4$.