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Register a new userControlling polarization at insulating surfaces: quasiparticle calculations for molecules adsorbed on insulator films
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Added by
Christoph Freysoldt
Publication date
2009
fields
Physics
and research's language is
English
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By means of quasiparticle-energy calculations in the G0W0 approach, we show for the prototypical insulator/semiconductor system NaCl/Ge(001) that polarization effects at the interfaces noticeably affect the excitation spectrum of molecules adsorbed on the surface of the NaCl films. The magnitude of the effect can be controlled by varying the thickness of the film, offering new opportunities for tuning electronic excitations in e.g. molecular electronics or quantum transport. Polarization effects are visible even for the excitation spectrum of the NaCl films themselves, which has important implications for the interpretation of surface science experiments for the characterization of insulator surfaces.
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We report the first observation of coherent surface states on cubic perovskite oxide SrVO3(001) thin films through spectroscopic imaging scanning tunneling microscopy. A direct link between the observed atomic-scale interference patterns and the formation of a dxy-derived surface state is supported by first-principles calculations. Furthermore, we show that the apical oxygens on the topmost VO2 plane play a critical role in controlling the spectral weight of the observed coherent surface state.
Metal-insulator transition is observed in the La0.8Sr0.2MnO3 thin films with thickness larger than 5 unit cells. Insulating phase at lower temperature appeared in the ultrathin films with thickness ranging from 6 unit cells to 10 unit cells and it is found that the Mott variable range hopping conduction dominates in this insulating phase at low temperature with a decrease of localization length in thinner films. A deficiency of oxygen content and a resulted decrease of the Mn valence have been observed in the ultrathin films with thickness smaller than or equal to 10 unit cells by studying the aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy of the films. These results suggest that the existence of the oxygen vacancies in thinner films suppresses the double-exchange mechanism and contributes to the enhancement of disorder, leading to a decrease of the Curie temperature and the low temperature insulating phase in the ultrathin films. In addition, the suppression of the magnetic properties in thinner films indicates stronger disorder of magnetic moments, which is considered to be the reason for this decrease of the localization length.
The structure and dynamics of atomic oxygen adsorbed on Ag(410) and Ag(210) surfaces have been investigated using density functional theory. Our results show that the adsorption configuration in which O adatoms decorate the upper side of the (110) steps forming O--Ag--O rows is particularly stable for both surfaces. On Ag(210), this arrangement is more stable than other configurations at all the investigated coverages. On Ag(410), adsorption on the terrace and at the step edge are almost degenerate, the former being slightly preferred at low coverage while the latter is stabilized by increasing the coverage. These findings are substantiated by a comparison between the vibrational modes, calculated within density-functional perturbation theory, and the HREEL spectrum which has been recently measured in these systems.
A microscopic theory of the transport in a scanning tunnelling microscope (STM) set-up is introduced for pi-conjugated molecules on insulating films, based on the density matrix formalism. A key role is played in the theory by the energy dependent tunnelling rates which account for the coupling of the molecule to the tip and to the substrate. In particular, we analyze how the geometrical differences between the localized tip and extended substrate are encoded in the tunnelling rate and influence the transport characteristics. Finally, using benzene as an example of a planar, rotationally symmetric molecule, we calculate the STM current voltage characteristics and current maps and analyze them in terms of few relevant angular momentum channels.
In the context of photoelectron spectroscopy, the $GW$ approach has developed into the method of choice for computing excitation spectra of weakly correlated bulk systems and their surfaces. To employ the established computational schemes that have been developed for three-dimensional crystals, two-dimensional systems are typically treated in the repeated-slab approach. In this work we critically examine this approach and identify three important aspects for which the treatment of long-range screening in two dimensions differs from the bulk: (1) anisotropy of the macroscopic screening (2) $mathbf k$-point sampling parallel to the surface (3) periodic repetition and slab-slab interaction. For prototypical semiconductor (silicon) and ionic (NaCl) thin films we quantify the individual contributions of points (1) to (3) and develop robust and efficient correction schemes derived from the classic theory of dielectric screening.
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