Revisiting longitudinal optical modes in materials with plasmon and plasmon-like absorptions – SrTiO3 and β-Ga2O3

We investigate exemplary the longitudinal optical (LO) mode order in compounds with a plasmon or plasmon-like phonon mode and additional phonon modes. When the oscillator strength of the plasmon or plasmon-like mode is gradually increased, a reordering of the modes takes place. Since it is not possible in crystals with orthorhombic or higher symmetry that a LO mode crosses a transverse optical (TO) mode’s position, this reordering takes place via mode hybridization. During this mode hybridization, the plasmon or plasmon-like LO mode gradually becomes the originally higher situated LO mode while the latter morphs into the former. As a consequence, inner (LO-TO) and an outer (TO-LO) mode pairs are formed. These insights can be readily transferred to other semiconductors or many mode materials with reststrahlen bands, as long as their dielectric tensor can be (partly) diagonalized, and allow easily correct mode assignments.