Surface vibrations, polaritons and their coupling to a 2D electron gas: interface-bound vibrations at the SrTiO3(001) surface

Abstract

The low-energy excitations of SrTiO₃, a large-bandgap oxide perovskite, are dominated by phonons and phonon polaritons. At the surface they couple to dipole-active surface phonon polaritons that are bound to the SrTiO₃–vacuum interface. These excitations can be addressed by surface vibrational spectroscopy techniques such as, e.g., high resolution electron energy loss spectroscopy (HREELS). The HREELS experiments identify all SrTiO₃(001) dipole-active surface excitations, including their specific line shape and their electron–phonon coupling to a two-dimensional electron gas (2DEG). The latter is prepared by a surface reduction via annealing under ultrahigh-vacuum conditions. With formation of the 2DEG, the discrete surface phonon polaritons couple to the electron–hole pair continuum as is witnessed by a substantial line broadening and asymmetric Fano-like line shape.