Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 59: Oxide and Insulator Surfaces: Structure, Epitaxy and Growth

O 59.1: Vortrag

Mittwoch, 18. März 2015, 15:00–15:15, MA 042

Surface phonons of BaTiO₃(001) and BaTiO₃ ultrathin films on Pt(001) and Au(001) — •Florian Schumann¹, Klaus Meinel¹, Andreas Trützschler¹, and Wolf Widdra^1,2 — ¹Institute of Physics, Martin-Luther-Universität Halle-Wittenberg — ²Max-Planck- Institut für Mikrostrukturphysik, Halle

Phonons and their softening are key elements for the understanding of the long-range coupling in ferroelectric and multiferroic materials, which causes, e.g., the paraelectric to ferroelectric phase transition. In thin films, these ferroelectric properties are often controlled by strain from the underlying substrate or heterostructures. Here we study the surface phonons of BaTiO₃(001) ultrathin films in comparison with a BaTiO₃ single-crystal with high-resolution electron energy loss spectroscopy (HREELS). For all BaTiO₃ structures, three well-developed phonon peaks at about 210, 460, and 670 cm⁻¹ dominate the HREELS spectra. Calculations of the energy loss spectra within a semi-classical approach identify the three phonons as the A(TO) surface phonon-polaritons (Fuchs-Kliewer modes) of BaTiO₃(001). In comparison to the single crystal, the surface phonons of the thin films on Pt(001) and on Au(001) are shifted due to the lattice misfit of +2% and -2%, respectively. Furthermore, the surface phonons show a thickness dependent red shift in the thin films. Calculations identify this shift as phonon-polariton coupling to the metallic substrate.