Vibrational and Electronic Properties of Fullerene and Carbon-Based Clustors. Final Reports for period July 1, 1997 - June 30, 2001

Lattice dynamics is of central importance for the mechanism of ferroelectricity. In particular, the soft mode behaviors are directly related to many of their ferroelectric and dielectric properties. In this project, we have carried out experimental studies of the vibrational spectra of SrTiO3 films grown by pulsed laser deposition using a metal-oxide bilayer structure. Raman scattering, with and without bias electric field, and Fourier-transform far-infrared ellipsometry were utilized. These results are compared with the low-frequency dielectric properties. We found that in the films the soft mode is harder compared to that in bulk crystals, in agreement with the Lyddane-Sachs-Teller (LST) formalism. We have studied electric field-induced Raman scattering in SrTiO3 thin films using an indium-tin oxide/SrTiO3/SrRuO3 structure. The soft mode polarized along the field becomes Raman active. Experimental data for electric field-induced hardening of the soft modes and the tuning of the static dielectric constant are in agreement described by the LST formalism. The markedly different behavior of the soft modes in thin films from that in the bulk is explained by the existence of local polar regions. The study was extended to BaxSr1-xTiO3 films with Ba contents x = 0.05, 0.1, 0.2 and 0.5. The temperature dependence of the soft mode frequency shows evidence of the ferroelectric phase transition in the films. Relative Raman intensity of hard phonon modes shows the ferroelectric phase transition occurs over a broad range of temperatures in thin films, which is different from bulk behavior. Comparison of temperature evolution of Raman spectra for films grown on SrTiO3 and LaAlO3 substrates shows the influence of strain on the temperature of ferroelectric phase transition