Xiaoqin Elaine Li
The interplay between charge, spin, lattice, and orbital degrees of freedom in correlated materials often gives rise to intriguing and exotic properties. Recent studies have shed new light on bosonic collective excitations in these materials. In particular, inelastic neutron scattering experiments have uncovered non-trivial band topology in magnons and spin-orbit excitons within the quantum magnet CoTiO3 [1,2].
Here, we report previously unrecognized phonon characteristics driven by the interplay of strong spin-orbit coupling, substantial crystal field splitting, and trigonal distortion in CoTiO3. Notably, the coupling between spin-orbit excitons and phonons imparts chirality to two Eg phonon modes, resulting in pronounced phonon magnetic moments observed through magneto-Raman spectroscopy [3]. This striking magneto-phonon effect arises from the hybridization of spin-orbit excitons and phonons, facilitated by their close energy alignment.
Through polarization-resolved Raman spectroscopy, we further identify multiple magnon peaks by conducting temperature- and magnetic-field-dependent measurements. The presence of saddle points at the high-symmetry M points along the Brillouin zone boundary results in an exceptionally high density of states, which dominates two-magnon scattering processes. Notably, these exchange magnons, characterized by nanometer-scale wavelengths, exhibit remarkably sharp linewidths—limited only by instrument resolution—compared to the G-point magnon at the zone center. This unusual momentum dependence of magnon lifetimes in CoTiO3 arises from a combination of factors, including antiferromagnetic ordering, particle number non-conserving interactions, a low joint two-magnon density of states, exchange anisotropy, and specific single-magnon band features.
Reference:
[1] Yuan, B. et al. Phys. Rev. X 10, 011062, 2020.
[2] Elliot, M. et al. Nat. Commun. 12, 3936, 2021.
[3] Lujan, D. et al., , PNAS, 121, e2304360121, 2024.