Dielectric response of (Ba, Sr)TiO$_3$ thin films in a terahertz and IR ranges

Dependences of the dielectric response spectra on the thickness of the films based on the (Ba, Sr)TiO$_3$ (BST) solid solutions, which were deposited on single-crystal MgO substrates, have been analyzed. Using the Lorentz and Lyddane-Sachs-Teller models, the mutual correspondence of longitudinal optical (LO) and transverse optical (TO) phonon frequencies has established. The giant LO–TO splitting of the lowest-frequency vibration and inversion of LO and TO frequencies of other phonons in the IR spectrum of BST films has been found for the first time.