Tea in the Atrium Room: 307

Colloquium Room: 307 Title: Hybrid Magnon–Phonon Excitations in Quantum Magnets Abstract: Magnons and phonons are collective excitations in crystals that often behave as independent quasiparticles when their mutual coupling is weak. When magnetoelastic coupling becomes strong, interactions between spin and lattice degrees of freedom can open gaps at band crossings and give rise to hybridized excitations known as magnon-polarons. Because such hybridized modes appear only when energy matching, symmetry compatibility, and microscopic coupling all align, their presence and momentum dependence provide a particularly sensitive probe of the underlying spin–lattice interactions. In this talk, I will use the olivine-type silicate Co₂SiO₄ to illustrate how this physics plays out in a real material. I will begin by presenting inelastic neutron-scattering measurements that reveal a rich set of hybridized excitations involving magnons, phonons, and spin–orbit excitons. To understand these observations, I will introduce an effective spin Hamiltonian that captures the main features of the magnetic excitations and discuss density functional theory calculations that reproduce the phonon spectrum. Building on these microscopic descriptions, a symmetry analysis of the magnon and phonon modes reveals which excitations can hybridize and where in momentum space such coupling is allowed. This framework naturally leads to a minimal hybridization model that explains the observed avoided crossings and mode mixing, providing a unified picture of the hybridized excitation spectrum in Co₂SiO₄. More broadly, this work establishes olivine-type oxides as a clean setting for studying magnetoelastic coupling and illustrates how combining dynamical measurements with symmetry-based modeling can reveal microscopic interactions that are difficult to access otherwise.