Spin-phonon interaction in a symmetry-enforced spin-polarized state

Abstract

Symmetry-governed magnetic materials have emerged as a promising platform for spintronic functionalities without net magnetization or stray magnetic fields, motivating the exploration of how lattice dynamics couple to symmetry-derived spin-polarized electronic states. Understanding spin-phonon coupling in these systems is therefore essential for uncovering the microscopic origin of spin-lattice interactions and for enabling their control in quantum materials. However, this mechanism remains poorly understood because spin polarization originates from crystal symmetry rather than conventional magnetic order. Here, we address this issue in the g-type altermagnet CoNb4Se8 using temperature- and polarization-resolved Raman spectroscopy, complemented by measurements on a structurally analogous Co-deficient compound lacking well-defined long-range magnetic order. We observe pronounced symmetry-selective phonon renormalization across the magnetic transition in CoNb4Se8, while related phonon anomalies persist in the Co-deficient system, demonstrating that the lattice response cannot be explained solely by conventional exchange-striction associated with coherent magnetic ordering. First-principles calculations reveal that spin-orbit coupling establishes a symmetry-dependent interaction channel between lattice vibrations and symmetry-governed electronic states. Our results identify an alternative mechanism for spin-phonon coupling in symmetry-governed magnetic materials and demonstrate that phonons provide a sensitive probe of symmetry-driven spin polarization even without robust magnetic order. More broadly, this work provides a framework for understanding and engineering spin-lattice functionality in symmetry-driven quantum materials, offering design principles for coupling lattice dynamics to spin-polarized electronic states.