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P Varga

Publications and source records attributed to P Varga.

At least 19 recordsLinked to original sources
PubMed2006

Unusual process of water formation on RuO2(110) by hydrogen exposure at room temperature.

The reduction mechanism of the RuO(2)(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO(2)(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (O(br)-H) species and subsequently part of these O(br)-H groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.

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PubMed2006

Oxygen-deficient line defects in an ultrathin aluminum oxide film.

A model for the straight antiphase domain boundary of the ultrathin aluminum oxide film on the NiAl(110) substrate is derived from scanning tunneling microscopy measurements and density-functional theory calculations. Although the local bonding environment of the perfect film is maintained, the structure is oxygen deficient and possesses a favorable adsorption site. The domain boundary exhibits a downwards band bending and three characteristic unoccupied electronic states, in excellent agreement with scanning tunneling spectroscopy measurements.

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PubMed2006

Kinetics of the reduction of the Rh(111) surface oxide: linking spectroscopy and atomic-scale information.

The reduction of the surface oxide on Rh(111) by H(2) was observed in situ by scanning tunneling microscopy (STM) and high-resolution core level spectroscopy (HRCLS). At room temperature, H(2) does not adsorb on the oxide, only in reduced areas. Reduction starts in very few sites, almost exclusively in stepped areas. One can also initiate the reduction process by deliberately creating defects with the STM tip allowing us to examine the reduction kinetics in detail. Depending on the size of the reduced area and the hydrogen pressure, two growth regimes were found. At low H(2) pressures or small reduced areas, the reduction rate is limited by hydrogen adsorption on the reduced area. For large reduced areas, the reduction rate is limited by the processes at the border of the reduced area. Since a near-random distribution of the reduction nuclei was found and the reduction process at defects starts at a random time, one can use Johnson-Mehl-Avrami-Kolmogoroff (JMAK) theory to describe the process of reduction. The microscopic data from STM agree well with spatially averaged data from HRCLS measurements.

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PubMed2006

Structure of Ag(111)-p(4 x 4)-O: no silver oxide.

The structure of the oxygen-induced p(4 x 4) reconstruction of Ag(111) is determined by a combination of scanning tunneling microscopy, surface x-ray diffraction, core level spectroscopy, and density functional theory. We demonstrate that all previous models of this surface structure are incorrect and propose a new model which is able to explain all our experimental findings but has no resemblance to bulk silver oxide. We also shed some light on the limitations of current density functional theories and the potential role of van der Waals interactions in the stabilization of oxygen-induced surface reconstructions of noble metals.

Journal Article↗
PubMed2005

Progesterone-dependent immunomodulation.

The biological effects of progesterone are mediated by a 34-kDa protein named the progesterone-induced blocking factor (PIBF). PIBF, synthesized by lymphocytes of healthy pregnant women in the presence of progesterone, inhibits arachidonic acid release as well as NK activity, and modifies the cytokine balance. Within the cell the full-length PIBF is associated with the centrosome, while secretion of shorter forms is induced by activation of the cell. PIBF induces nuclear translocation of STAT6 as well as PKC phosphorylation and exerts a negative effect on STAT4 phosphorylation. The concentration of PIBF in pregnancy urine is related to the positive or negative outcome of pregnancy; furthermore, premature pregnancy termination is predictable by lower than normal pregnancy PIBF values. In vivo data suggest the biological importance of the above findings. Treatment of pregnant Balb/c mice with the antiprogesterone RU 486 results in an increased resorption rate, which is associated with the inability of spleen cells to produce PIBF. High resorption rates induced by progesterone receptor block as well as those due to high NK activity are corrected by simultaneous PIBF treatment.

Abortifacient Agents, Steroidal↗
PubMed2004

High-coverage oxygen structures on Rh111: adsorbate repulsion and site preference is not enough.

A new O induced structure on Rh(111) displaying a (2 sqaureroot[3] x 2sqaureroot[3])R30 degrees periodicity with an oxygen coverage of 2/3 has been studied by high resolution core level spectroscopy, scanning tunneling microscopy, and density functional theory. Although O favors fcc hollow sites in all other known phases, it occupies both fcc and hcp sites in this structure, which cannot be explained by pairwise adsorbate repulsion only. Both the (2sqaureroot[3] x 2sqaureroot[3])R30 degrees and (2 x 2)-3O structures also exemplify that density-of-states contrast can lead to oxygen adatoms appearing as protrusions in scanning tunneling microscopy images.

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PubMed2004

Self-limited growth of a thin oxide layer on Rh(111).

The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.

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PubMed2004

Visualization of atomic processes on ruthenium dioxide using scanning tunneling microscopy.

The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO2(110) and RuO2(100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO2. CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO2 at the oxide surface. In contrast to the RuO2(110) surface, the RuO2(100) surface stabilizes also a catalytically inactive c(2 x 2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO2(100)-c(2 x 2) phase may play an important role in the deactivation of RuO2 catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions.

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PubMed2003

When scanning tunneling microscopy gets the wrong adsorption site: H on Rh(100).

At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques.

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PubMed2003

Progesterone regulates IL12 expression in pregnancy lymphocytes by inhibiting phospholipase A2.

PROBLEM: Progesterone-induced blocking factor (PIBF) is one of the pathways that mediate the immunological effects of progesterone. PIBF inhibits natural killer (NK) cytotoxic activity. Recently we showed that neutralization of PIBF results in an increased interleukin (IL)-12 expression, which is corrected by cyclooxygenase inhibitors. As exogenous arachidonic acid (AA) voids the NK blocking effect of PIBF, it is likely that PIBF acts before the level of the cyclooxygenase enzyme. Therefore in this study we investigated the effect of PIBF neutralizing antibody and simultaneous phospholipase A2 inhibitor quinacrine (Q) treatment on IL-12 production. METHODS: Pregnancy lymphocytes were treated with anti-PIBF antibody or lipopolysaccharide (LPS) as a positive control, in the presence or absence of Q. IL-12 expression by PBMC was detected by immunocytochemistry. RESULTS: Neutralization of PIBF as well as LPS treatment resulted in an increased IL-12 expression, which was corrected by simultaneous Q treatment. Pre-treatment of lymphocytes with progesterone prevented the stimulating effect of LPS on IL-12 production. CONCLUSION: Progesterone binding of the lymphocytes is followed by the release of PIBF that inhibits AA release. The subsequent block of prostaglandin synthesis reduces IL-12 production and results in a lowered cytotoxic NK activity, which may contribute to a normal pregnancy outcome.

Antibodies↗
PubMed2002

Two-dimensional oxide on Pd(111).

The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.

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PubMed2002

Geometry of the valence transition induced surface reconstruction of Sm(0001).

We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species.

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PubMed2001

Crystallographic structure of ultrathin Fe films on Cu(100).

We report the observation of bcc-like crystal structures in 2-4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.

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PubMed2001

Adsorption sites and ligand effect for CO on an alloy surface: a direct view.

CO adsorption on a PtCo(111) surface was studied by scanning tunneling microscopy. Comparison of images with chemical contrast of Pt and Co and images showing the CO molecules indicates that CO resides exclusively on top of Pt sites and never on Co. CO bonding is highly sensitive to the chemical environment. The probability to find CO on a Pt atom increases drastically with the number of its Co nearest neighbors. Ab initio calculations show that this ligand effect is due to different positions of the center of the Pt d band.

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PubMed2001

Kinetically assisted potential sputtering of insulators by highly charged ions.

A new form of potential sputtering has been found for impact of slow ( < or = 1500 eV) multiply charged Xe ions (charge states up to q = 25) on MgO(x). In contrast to alkali-halide or SiO2 surfaces this mechanism requires the simultaneous presence of electronic excitation of the target material and of a kinetically formed collision cascade within the target in order to initiate the sputtering process. This kinetically assisted potential sputtering mechanism has been identified to be present for other insulating surfaces as well.

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PubMed2001

Influence of impurities on localized transition metal surface states: scanning tunneling spectroscopy on V(001).

The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to approximately 0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.

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PubMed2001

Nucleation of bcc iron in ultrathin fcc films.

Needle-shaped bcc nucleation centers in fcc films of Fe on Cu(100) are observed by scanning tunneling microscopy. They form virtually without mass transfer and nearly under conservation of volume, which causes a large strain within the nascent bcc grain. The corresponding strain energy almost equals the gain in structural energy, rendering the bcc nucleation very sensitive to any effect influencing this subtle balance. We suggest that modifying the film by straining, alloying, or surface adsorption may inhibit the bcc nucleation and lead to thick metastable fcc films.

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