"Collective Excitations in Quantum Materials: From Ferrons to Chiral Phonons"
Collective excitations in quantum materials offer a powerful toute to understanding and controlling emergent phases of matter. In this seminar, I will present two recent advances that highlight how bosonic modes, traditionally exemplified by magnons and phonons, can acquire new roles and functionalities. In van der Waals ferroelectric NbOI2, we demonstrate the excitation and transport of polarization waves, or ferrons, the bosonic quanta of ferroelectric order [1,2]. These coherent ferrons propagate uniaxially at hypersonic velocities, emit narrow-band terahertz radiation, and directly modulate the ferroelectric order parameter, opening pathways, for ferronic information processing and coherent electric control. In a separate line of work, I will discuss the interplay of spin-orbit excitons (SOEs) and lattice vibrations in the quantum magnet CoTiO3 (CTO) [3]. Here strong spin-orbit coupling, crystal field effects, and trigonal distortion in CTO enable hybridization between SOEs and phonons, endowing phonon modes with chirality and large magnetic moments, as observed in magneto-Raman spectra. This magneto-photonic effect reveals a new mechanism for engineering chiral phonons through coupling with topological bosons. Together, these studies underscore how coupling between different degrees of freedom can give rise to novel quasiparticles and functionalities, with implications for both fundamental physics and future quantum technologies.
[1] J. Choe*, T. Handa* et al. “Observation of coherent ferrons.” arXiv:2505.22559 (2025)
[2] P. Tang et al. “Excitations of the ferroelectric order.” Phys. Rev. B 106, 8 L081105 (2022)
[3] D. Lujan*, J. Choe* et al. “Spin-orbit exciton-induced phonon chirality in a quantum magent.” PNAS 121, 11 e2304360121 (2024)
