Interlayer magnetophononic coupling in MnBi₂Te₄

The emergence of magnetism in quantum materials creates a platform to realize spin-based applications in spintronics, magnetic memory, and quantum information science. A key to unlocking new functionalities in these materials is the discovery of tunable coupling between spins and other microscopic degrees of freedom. We present evidence for magnetophononic coupling in the layered magnetic topological insulator MnBi₂Te₄. Employing magneto-Raman spectroscopy, we observe phonon spectral weight anomalies across magnetic field-driven phase transitions, despite the absence of discernible static structural changes. This behavior is a consequence of a magnetophononic wave-mixing process that allows for the excitation of zone-boundary phonons that are otherwise ‘forbidden’ by momentum conservation. A microscopic description of this phenomenon using density functional theory highlights the critical role played by phonons modulating the interlayer exchange coupling.  Moreover, signatures of magnetophononic coupling are also observed in the time domain at sub-picosecond timescales through the ultrafast excitation and detection of coherent phonons across magnetic transitions. In light of the intimate connection between magnetism and topology in MnBi₂Te₄, the magnetophononic coupling represents an important step towards coherent on demand manipulation of magnetic topological phases.