Strong spin-phonon coupling unveiled by coherent phonon oscillations in Ca2RuO4

TL;DR

This study uses femtosecond laser pulses to reveal strong spin-phonon coupling in Ca2RuO4, showing how magnetic order influences phonon behavior and lattice distortions.

Contribution

It demonstrates the direct observation of spin-phonon coupling through coherent phonon oscillations and theoretical analysis linking lattice distortions to magnetic ordering.

Findings

Abrupt changes in phonon amplitude and phase with magnetic order

Redshift of phonon mode in the magnetically ordered state

Theoretical confirmation of lattice distortions caused by spin-phonon coupling

Abstract

We utilize near-infrared femtosecond pulses to investigate coherent phonon oscillations of Ca2RuO4. The coherent Ag phonon mode of the lowest frequency changes abruptly not only its amplitude but also the oscillation-phase as the spin order develops. In addition, the phonon mode shows a redshift entering the magnetically ordered state, which indicates a spin-phonon coupling in the system. Density functional theory calculations reveal that the Ag oscillations result in octahedral tilting distortions, which are exactly in sync with the lattice deformation driven by the magnetic ordering. We suggest that the structural distortions by the spin-phonon coupling can induce the unusual oscillation-phase shift between impulsive and displacive type oscillations.