Electromagnon dispersion probed by inelastic X-ray scattering in LiCrO2.

Sándor Tóth,Hiroshi Takatsu,Kenta Kimura,Tsuyoshi Kimura,Katharina Rolfs,Christian Rüegg,Henrik M Rønnow,Turan Birol,Björn Wehinger,Uwe Stuhr

doi:10.1038/ncomms13547

Abstract

Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross-section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system contains not only the phonon dispersion but also the so far undetected magnetic correlation function. Here we report the observation of strong magnon–phonon coupling in LiCrO2 that enables the measurement of magnetic correlations throughout the Brillouin zone via IXS. We find electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and heavily damped electromagnons in the paramagnetic phase of LiCrO2. We predict that several (frustrated) magnets with dominant direct exchange and non-collinear magnetism show surprisingly large IXS cross-section for magnons and multi-magnon processes.

Highlights

Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids
We report the spin and lattice excitation spectrum of LiCrO2 measured via inelastic X-ray scattering with meV energy resolution (IXS) and inelastic neutron scattering (INS), and a direct observation of an electromagnon
For a full interpretation of the IXS spectrum of LiCrO2, we start with the pure phonon spectrum in the paramagnetic phase determined from ab initio calculation

Summary

Introduction

Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. We show that LiCrO2 is an exceptional material, where the magnon–phonon coupling is strong enough to make the transferred spectral weight from phonons to magnons visible for inelastic X-ray scattering and enables the direct measurement of the electromagnon dispersion. We report the spin and lattice excitation spectrum of LiCrO2 measured via inelastic X-ray scattering with meV energy resolution (IXS) and inelastic neutron scattering (INS), and a direct observation of an electromagnon. We show that the observed electromagnon is the result of the strong coupling between the phason spin wave mode and the longitudinal acoustic (LA) phonon in the magnetically ordered phase of LiCrO2. We present a model that describes both the measured quasiparticle dispersion and the cross-sections for IXS and INS at low temperature This model shows that non-collinear magnetic order and exchange striction (ES) can induce linear coupling between magnons and phonons. We report the observation of two-magnon (2M) excitations in the ordered phase and strongly damped excitations above the Neel temperature from our IXS data

Methods

Results

Conclusion