A Quasiparticle Soup—Phonons, Magnons and Spin-Orbit Excitons in CoTiO3

CoTiO₃ has recently been in the spotlight due to its promise of exotic magnetic behavior especially topological magnons. It is a semiconductor with a band gap of ~2.25 eV with the Co²⁺ in edge-sharing octahedra form a stacked honeycomb lattice along the c-axis, with an easy-plane, A-type, antiferromagnetic ordering below 38 K. The spins in each honeycomb plane, which is perpendicular to the c-axis, are ferromagnetically aligned, while the planes are antiferromagnetically aligned with each other. There is significant bond-dependent exchange coupling, which is quite unique for 3d ions. Our magneto-Raman spectroscopic capabilities enable diffraction-limited, spatially resolved optical measurements while simultaneously varying the temperature, laser wavelength and magnetic field to study the photo-physics of layered quantum materials like CoTiO₃. Additionally, coupling to a triple grating spectrometer provides access to low-frequency (down to 6 cm^-1, or 0.75 meV) phonon and magnon modes, which are sensitive to magnetic order and interactions. Two magnons are clearly identified and studied as a function of temperature and magnetic field, both parallel and perpendicular to the c-axis. Wild phonon behavior is seen which is corroborated by recent neutron scattering measurements and will be discussed.