Bulg. J. Phys. vol.34 no.s1 (2007), pp. 250-258

Hardening of Soft Phonon Modes in SrTiO3 Ferroelectric Thin Films

St. Kovachev, J.M. Wesselinowa
University of Sofia, Department of Physics, Blvd. J. Bouchier 5, 1164 Sofia, Bulgaria
Abstract. The renormalized phonon spectrum beyond the RPA is obtained based on the transverse Ising model and Green's function technique. The dependence of the soft-phonon mode energy ω on temperature and film thickness for positive and negative values of the anharmonic spin-phonon interaction constant R is discussed. For the case of smaller surface constant compared to the bulk, Rs < Rb, we obtain that ω of the thin film is larger in comparison to that of the bulk, i.e. we obtain hardening of the soft mode. This result could be used to explain the experimentally observed hardening of the soft mode in SrTiO3 ferroelectric thin films.

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