Spin-Phonon Coupling in Altermagnetic CoNb_₄Se_₈

Altermagnetic states combine the spin-polarized electronic bands typical of ferromagnetism with antiferromagnetic spin order, which creates exciting opportunities for spintronic devices that are more efficient, faster, and smaller than the existing ferromagnetic devices. The intercalated transition metal dichalcogenide CoNb₄Se₈, which consists of 2H-NbSe₂ layers separated by Co monolayers, has recently been confirmed to exhibit altermagnetism. This behavior derives from antiferromagnetic coupling between the ferromagnetic Co monolayers along the c-axis. While transport, neutron scattering, and magnetization measurements have all provided evidence for altermagnetism in CoNb₄Se₈, the role of spin-phonon coupling in this material remains unknown.

Here, we explore spin-phonon coupling in CoNb₄Se₈ via temperature-dependent Raman spectroscopy. We find that all Raman-active modes soften with increasing temperature, a behavior that can be captured with anharmonic models based on phonon decay. The in-plane Co Raman mode is unique, red-shifting rapidly yet insensitive to the altermagnetic transition at 160 K. Our results compare favorably with DFT calculations of CoNb₄Se₈, providing insights regarding the coupling of altermagnetic spin order to atomic displacements.