294 research outputs found
The local structure of OH species on the V2O3(0 0 0 1) surface: a scanned-energy mode photoelectron diffraction study
Scanned-energy mode photoelectron diffraction (PhD), using O 1s photoemission, together with multiple-scattering simulations, have been used to investigate the structure of the hydroxyl species, OH, adsorbed on a V2O3(0 0 0 1) surface. Surface OH species were obtained by two alternative methods; reaction with molecular water and exposure to atomic H resulted in closely similar PhD spectra. Both qualitative assessment and the results of multiple-scattering calculations are consistent with a model in which only the O atoms of outermost layer of the oxide surface are hydroxylated. These results specifically exclude significant coverage of OH species atop the outermost V atoms, i.e. in vanadyl O atom sites. Ab initio density-functional theory cluster calculations provide partial rationalisation of this result, which is discussed the context of the general understanding of this system
Structural and vibrational properties of two-dimensional nanolayers on Pd(100)
Using different experimental techniques combined with density functional
based theoretical methods we have explored the formation of
interface-stabilized manganese oxide structures grown on Pd(100) at
(sub)monolayer coverage. Amongst the multitude of phases experimentally
observed we focus our attention on four structures which can be classified into
two distinct regimes, characterized by different building blocks. Two
oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers,
whereas the other two have a lower oxygen content and are based on a
MnO(100)-like monolayer structure. The excellent agreement between calculated
and experimental scanning tunneling microscopy images and vibrational electron
energy loss spectra allows for a detailed atomic description of the explored
models.Comment: 14 pages, 11 figure
Equilibrium crystal shapes in the Potts model
The three-dimensional -state Potts model, forced into coexistence by
fixing the density of one state, is studied for , 3, 4, and 6. As a
function of temperature and number of states, we studied the resulting
equilibrium droplet shapes. A theoretical discussion is given of the interface
properties at large values of . We found a roughening transition for each of
the numbers of states we studied, at temperatures that decrease with increasing
, but increase when measured as a fraction of the melting temperature. We
also found equilibrium shapes closely approaching a sphere near the melting
point, even though the three-dimensional Potts model with three or more states
does not have a phase transition with a diverging length scale at the melting
point.Comment: 6 pages, 3 figures, submitted to PR
Interplay between magnetic, electronic and vibrational effects in monolayer Mn_3O_4 grown on Pd(100)
The surface stabilized MnO(100)-like monolayer, characterised by a regular
c(4x2) distribution of Mn vacancies, is studied by hybrid functionals and
discussed in the light of available scanning tunneling microscopy and
high-resolution electron energy loss spectroscopy data. We show that the use of
hybrid functionals is crucial to account for the intermingled nature of
magnetic ineractions, electron localization, structural distortions and surface
phonons. The proposed Pd(100) supported Mn3O4 structure is excellently
compatible with the experiments previously reported in literature.Comment: 16 pages, 5 figure
Fluctuations of an Atomic Ledge Bordering a Crystalline Facet
When a high symmetry facet joins the rounded part of a crystal, the step line
density vanishes as sqrt(r) with r denoting the distance from the facet edge.
This means that the ledge bordering the facet has a lot of space to meander as
caused by thermal activation. We investigate the statistical properties of the
border ledge fluctuations. In the scaling regime they turn out to be
non-Gaussian and related to the edge statistics of GUE multi-matrix models.Comment: Version with major revisions -- RevTeX, 4 pages, 2 figure
Towards a first-principles theory of surface thermodynamics and kinetics
Understanding of the complex behavior of particles at surfaces requires
detailed knowledge of both macroscopic and microscopic processes that take
place; also certain processes depend critically on temperature and gas
pressure. To link these processes we combine state-of-the-art microscopic, and
macroscopic phenomenological, theories. We apply our theory to the O/Ru(0001)
system and calculate thermal desorption spectra, heat of adsorption, and the
surface phase diagram. The agreement with experiment provides validity for our
approach which thus identifies the way for a predictive simulation of surface
thermodynamics and kinetics.Comment: 4 pages including 3 figures. Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Theoretical study of O adlayers on Ru(0001)
Recent experiments performed at high pressures indicate that ruthenium can
support unusually high concentrations of oxygen at the surface. To investigate
the structure and stability of high coverage oxygen structures, we performed
density functional theory calculations, within the generalized gradient
approximation, for O adlayers on Ru(0001) from low coverage up to a full
monolayer. We achieve quantitative agreement with previous low energy electron
diffraction intensity analyses for the (2x2) and (2x1) phases and predict that
an O adlayer with a (1x1) periodicity and coverage of 1 monolayer can form on
Ru(0001), where the O adatoms occupy hcp-hollow sites.Comment: RevTeX, 6 pages, 4 figure
Automatic kelvin probe compatible with ultrahigh vacuum
This article describes a new type of in situ ultrahighâvacuum compatible kelvin probe based on a voiceâcoil driving mechanism. This design exhibits several advantages over conventional mechanical feedâthrough and (in situ) piezoelectric devices in regard to the possibility of multiple probe geometry, flexibility of probe geometry, amplitude of oscillation, and pure parallel vibration. Automatic setup and constant spacing features are achieved using a digitalâtoâanalog converter (DAC) steered offset potential. The combination of very low driver noise pickâup and dataâacquisition system (DAS) signal processing techniques results in a work function (wfâ ) resolution, under optimal conditions, of <0.1 meV. Due to its high surface sensitivity and compatibility with standard sample cleaning and analysis techniques this design has numerous applications in surface studies, e.g., adsorption kinetics, sample topography and homogeneity, sputter profiles, etc. For semiconductor specimens the high wf resolution makes it eminently suitable for surface photovoltage (SPV) spectroscopy
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