Low-energy excitations and electron-phonon coupling in oxide thin films

Tutorial

  • Date: Mar 27, 2019
  • Time: 16:00
  • Speaker: Prof. Dr. Wolf Widdra
  • Martin-Luther-Universität Halle-Wittenberg
  • Location: Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle (Saale)
  • Room: Lecture Hall, B.1.11
  • Host: IMPRS-STNS
Low-energy excitations and electron-phonon coupling in oxide thin films

Phonons and their softening are key elements for the basic understanding of many solid-state phenomena. In polar media like oxides, optical phonons are important since they carry substantial dynamical dipole moments and couple efficiently to electromagnetic low-energy excitations.  In ferroelectric and multiferroic complex oxides, the softening of optical phonons might indicate imminent phase transitions. The complex dielectric function known also as complex permittivity is a concept used to describe such low-energy excitations.  The presence of charge carriers as in metallic or doped oxides will add a plasmonic response.

In this talk, I will discuss the surface and thin film dielectric function for the perovskite oxides BaTiO3, SrRuO3, SrTiO3, and KTaO3 and its quantitative determination by surface vibrational spectroscopy. It will be shown that phonons and plasmons form specific phonon-polaritons and plasmon-polaritons at the interface.  The influence of different doping levels on the full dielectric response will be discussed and used to follow in-situ doping level changes quantitatively. The electron-phonon coupling will be discussed with respect to the formation of a two-dimensional electron gas at the surface of SrTiO3 and the phonon line shape for SrRuO3.

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