Pulsed laser deposition have been adapted for precursor fabrication of Hg-Ba-Ca-Cu-O thin films. The preparation involves a sequential deposition of HgO and Ba2CaCu2Ox thin layers at 200 degree(s)C on MgO and SrTiO3 substrates. Processing... more
Pulsed laser deposition have been adapted for precursor fabrication of Hg-Ba-Ca-Cu-O thin films. The preparation involves a sequential deposition of HgO and Ba2CaCu2Ox thin layers at 200 degree(s)C on MgO and SrTiO3 substrates. Processing parameters were optimized by studying surface morphology of the individual layers and by strict Hg content control. Ex-situ thermal postannealing and oxygen content have not yet been optimized. The obtained films show almost single phase HgBa2CaCu2O6+(delta ) with a c-axis of 12.65 angstrom and Tc (onset) of 110 K as determined from magnetic measurements.
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ABSTRACT The surface morphology and nucleation of molecular beam epitaxy-grown ‘infinite-layer’ structure, (Sr,Ca)CuO2 films on atomically flat SrTiO3 are studied by reflection high-energy electron diffraction (RHEED) and atomic force... more
ABSTRACT The surface morphology and nucleation of molecular beam epitaxy-grown ‘infinite-layer’ structure, (Sr,Ca)CuO2 films on atomically flat SrTiO3 are studied by reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The SrTiO3 (100) surfaces were prepared by an ultra-high vacuum (UHV) annealing and displayed regular arrays of 0.4 nm steps spaced by atomically flat terraces of about 400 nm. The terraces exhibited smooth edges which were parallel to the [110] substrate direction. AFM images of only a two unit cell thick (0.66 nm) (Sr,Ca)CuO2 layers were recorded. Edge roughness of the substrate terraces was found to be affected by the growth process. Atomically flat (Sr,Ca)CuO2 islands with the vertical steps of multiple heights of one unit cell were identified. The obtained AFM and RHEED data strongly suggested the island to step-growth mode of ‘infinite-layer’, (Sr,Ca)CuO2 films grown on atomically smooth (100) SrTiO3 surfaces. Different surface morphology of thicker (Sr,Ca)CuO2 films grown with other nominal compositions were also investigated by AFM.
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Superconducting HgBa2CaCu2O6+δ (Hg-1212) thin films have been prepared by laser ablation followed by post-annealing at high temperatures. Two synthesis methods were investigated, using (1) direct reaction of Hg–Ba–Ca–Cu–O precursor films... more
Superconducting HgBa2CaCu2O6+δ (Hg-1212) thin films have been prepared by laser ablation followed by post-annealing at high temperatures. Two synthesis methods were investigated, using (1) direct reaction of Hg–Ba–Ca–Cu–O precursor films in a Hg-controlled ambience, and (2) thermal diffusion of Hg into Ba–Ca–Cu–O precursor films in a controlled atmosphere containing both Hg- and Tl-bearing species. Effects of the annealing temperature, time, and bulk material composition on the Hg-1212 film and residual impurities are presented. Surface morphologies, growth defects, and transport properties of Hg-1212 on SrTiO3 substrates are discussed. Formation of Hg-1212 films on MgO and LaAlO3 substrates is briefly described.
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The adsorption of water on two silicon surfaces [reconstructed planar (100)2×1 and single domain stepped (100)2×1 cut 5° towards (011)] was studied using low-energy electron diffraction and digital imaging electron stimulated desorption... more
The adsorption of water on two silicon surfaces [reconstructed planar (100)2×1 and single domain stepped (100)2×1 cut 5° towards (011)] was studied using low-energy electron diffraction and digital imaging electron stimulated desorption ion angular distributions (ESDIAD) as a function of temperature (145–700 K) and coverage. Water has been shown previously to chemisorb dissociatively to form surface OH groups. At 300 K the H+ ESDIAD pattern for the planar surface is a nearly symmetric halo, indicating that OH is oriented with its bond vector inclined away from the surface normal, while at low temperatures (≤200 K) a four-lobed pattern that preserves substrate symmetry is observed. This reversible temperature dependence is related to librations and rotations of the OH complexes. ESDIAD from the stepped surface exhibits a two-lobed pattern, with enhanced emission towards the steps, consistent with bonding of OH to single-domain terrace sites. An interpretation is presented based on the dimer model of the Si(100) reconstruction in which the OH bond axis azimuths are oriented nearly perpendicular to the dimer azimuths.
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High resolution core-level spectroscopy was utilized to study the clean and NH3 exposed Si (100) 2× 1 surface. The clean surface exhibits two approximately equal intensity surface core-level components at− 0.48 and 0.28 eV binding energy... more
High resolution core-level spectroscopy was utilized to study the clean and NH3 exposed Si (100) 2× 1 surface. The clean surface exhibits two approximately equal intensity surface core-level components at− 0.48 and 0.28 eV binding energy referred to the bulk ...
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Research Interests: Chemistry and Adsorption
After the discovery that core level binding energies are sensitive to the charge distribution of the valence electrons, core level photoelectron spectroscopy has become one of the main analytical techniques in physics and chemistry. For... more
After the discovery that core level binding energies are sensitive to the charge distribution of the valence electrons, core level photoelectron spectroscopy has become one of the main analytical techniques in physics and chemistry. For example, an increase in oxidation number is accompanied by stepwise changes in the core level binding energy. The core level chemical shifts observed for atoms in different charge states usually range from 0.5 to 5 eV. Under the acronym ESCA (electron spectroscopy for chemical analysis), Siegbahn and co-workers developed the technique with extensive studies of chemical shifts in solid compounds,1 and later in molecules.2 This work was performed with fixed photon energy sources, MgKa and AlKa primarily. The main objective driving the field was to develop excitation sources and kinetic-energy electron analyzers with improved resolution to detect smaller chemical shifts with increased precision.
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Recently we have investigated the 4d → 4f giant resonances in electron and H+-ion yield spectra of oxidized Sm, Eu,and Yb [1]. Here we report on a continuation of this experiment, i.e., PSD from a metal overlayer system which consists of... more
Recently we have investigated the 4d → 4f giant resonances in electron and H+-ion yield spectra of oxidized Sm, Eu,and Yb [1]. Here we report on a continuation of this experiment, i.e., PSD from a metal overlayer system which consists of oxidized Yb on bulk Sm metal. Experiments were carried out at the Hamburger Synchrotronstrahlungslabor HASYLAB at the storage ring DORIS with the FLIPPER-monochromator. Photoelectron spectra were recorded with a commercial double-pass cylindrical mirror analyzer (CMA). A time-of-flight (TOF) mass analyzer utilizing the time structure of the storage ring was used for the detection of the photon-stimulated desorbed ions.
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ABSTRACTSwitching from superconducting (S) to normal (N) state was investigated using short rise time (∼ 10−10 s) electric pulses in MBE grown Bi-Sr-Ca-Cu-O and rf magnetron sputtered Y-Ba-Cu-O submicron thickness microstrips. It was... more
ABSTRACTSwitching from superconducting (S) to normal (N) state was investigated using short rise time (∼ 10−10 s) electric pulses in MBE grown Bi-Sr-Ca-Cu-O and rf magnetron sputtered Y-Ba-Cu-O submicron thickness microstrips. It was found that two reversible processes: fast electronic (τ∼10∼−12 s) and slow thermal (τ∼10∼−8s) one take place during switching. At high current densities (∼107 A/cm2) irreversible changes in strip material induced by thermal runaway occurred. The use of a superconducting element as fast fault-current limiter or pulse sharpening device is pointed out.
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Synchrotron radiation excited photoelectron spectra of the 4f emission region for all lanthanide metals Ce58 to Lu71 (except Pm61) have been recorded. Photon energies ranging from 30 eV to 200 eV have been used to excite the 4f electrons... more
Synchrotron radiation excited photoelectron spectra of the 4f emission region for all lanthanide metals Ce58 to Lu71 (except Pm61) have been recorded. Photon energies ranging from 30 eV to 200 eV have been used to excite the 4f electrons with maximum surface sensitivity. The 4f emission spectra of all metals studied, with the exception of Ce, show 4f emission from the surface layer atoms and the bulk atoms as clearly separated structures with different binding energies. For Ce no unambiguous separation of the 4f structure into bulk and surface emission could be made. We fitted the experimental spectra with calculated 4f emission spectra obtained by adding a bulk and a shifted surface multiplet for each metal. The intensity ratios within the multiplets were obtained from an atomic calculation utilizing the intermediate coupling scheme. The intensity ratio between bulk and surface emission (IB/IS) and the magnitude of the surface core level shift (SCS) were used as fit parameters. The deduced SCSs are all positive (towards higher binding energy), 0.40 eV-0.77 eV, and in fair agreement with calculated SCSs using the Johansson-Mårtensson-Rosengren model. For the trivalent lanthanides we found a systematic variation of the SCSs through the series, which can be attributed to differences in the actual electron distribution between s, p and d-like valence states. For the trivalent Sm we found the surface to be completely divalent and deduced a surface shift of > 0.46 eV, being the energy necessary to change the surface valency.
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ABSTRACT The influence of sulfur dioxide on the reduction of nitric oxide over copper, in the presence of isobutene and oxygen, has been examined under reducing conditions in the 720 to 820 K temperature range. The catalytic activity of... more
ABSTRACT The influence of sulfur dioxide on the reduction of nitric oxide over copper, in the presence of isobutene and oxygen, has been examined under reducing conditions in the 720 to 820 K temperature range. The catalytic activity of NO conversion was drastically decreased by the presence of a few ppm of SO2 in the gas phase, and the toxicity of sulfur was reduced when the temperature was increased. The Cu sample was characterized by X-ray photoelectron spectroscopy, at different stages of the reaction. It was demonstrated that sulfur dioxide poisoned NO conversion by hindering the partial oxidation of the hydrocarbon and by occupying the active sites, both factors contributing to a decrease in the surface concentration of the active intermediate (oxygenated product). Two kinds of adsorbed sulfided species, SO2−4 and S2−, were identified on the Cu surface. At the end of the reaction, the catalyst surface was poisoned both by carbon and sulfur species.
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ABSTRACT The mechanism of NO reduction on copper in the presence of oxygen and isobutene was studied at T = 770 K, under reducing conditions. Mass spectroscopy was used to identify gas-phase intermediates and reaction products. One... more
ABSTRACT The mechanism of NO reduction on copper in the presence of oxygen and isobutene was studied at T = 770 K, under reducing conditions. Mass spectroscopy was used to identify gas-phase intermediates and reaction products. One intermediate species resulting from the partial oxidation of isobutene was shown to be active in the NO conversion. X-ray photoelectron spectroscopy was used to analyze the copper surface at different stages of the reaction. Our conclusion is that oxygen first activates the surface, by forming coexisting phases of Cu2O and CuO. The copper surface is then reduced by the hydrocarbon, leading to Cu+ in majority, and to the formation of an active intermediate. At last, the reduction of NO proceeds rapidly on a zero-valent copper surface together with consumption of the previously formed intermediate. At the end of the reaction, the surface is poisoned by carbon.
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ABSTRACT A review of synchrotron radiation excited photoemission experiments on semiconductor surfaces is given. We stress the importance of angle resolved photoemission experiments using synchrotron radiation to study the surface and... more
ABSTRACT A review of synchrotron radiation excited photoemission experiments on semiconductor surfaces is given. We stress the importance of angle resolved photoemission experiments using synchrotron radiation to study the surface and bulk electronic structure of semiconductors. Recent progress to use excitations to energy regions, where calculated non free-electron like final bands are important, is reported. We briefly discuss photoemission core level spectroscopy results from semiconductor surfaces and their impact on deciding between different reconstruction models.
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ABSTRACTVarious compositions Bi-Sr-Ca-Cu-O thin films having 2212–1 ike structure have been synthesized on MgO substrates using Molecular Beam Epitaxy. Four Knudsen metal sources were utilized to prepare alloy films which were... more
ABSTRACTVarious compositions Bi-Sr-Ca-Cu-O thin films having 2212–1 ike structure have been synthesized on MgO substrates using Molecular Beam Epitaxy. Four Knudsen metal sources were utilized to prepare alloy films which were subsequently post annealed in air atmosphere. Quenching of the films from the annealing temperature, 800°C-820°C, increased the superconducting transition temperature Tc. The highest Tcs, 90 K - 95 K, were attained for compositions with slightly lower Ca and Sr content if compared to the nominal 2212 and 2223 ones. Plasma frequency of free carriers, ωp, determined from optical reflectivity measurements have been found to increase with the average density of CuO2 units. However, no substantial shift ofωp was displayed by the highest Tc films.
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Superconducting Bi2Sr3-xCaxCu2Oy films with x ranging from 1.05 to 2.20 have been prepared using molecular beam epitaxy (MBE) with four metal sources and post-annealing of the evaporated metal mixture films in oxygen and air atmospheres.... more
Superconducting Bi2Sr3-xCaxCu2Oy films with x ranging from 1.05 to 2.20 have been prepared using molecular beam epitaxy (MBE) with four metal sources and post-annealing of the evaporated metal mixture films in oxygen and air atmospheres. Annealing in air at 800 degrees C for 1-3 h yields a single-phase state with 2212-like structure for films with Ca content up to x=1.5. A slight decrease of the zero resistance superconducting transition temperature from 85 to 80 K and a lattice constant c from 30.8 to 30.6 AA were measured upon substituting Sr with Ca. Annealing times up to 20 h showed, for Ca-rich films, changes in the X-ray diffraction spectra as well as in superconducting transition curves. The changes are interpreted in terms of 2202- and 2223-like interlayers and intergrowth formation due to a structural instability of the 2212-like structure with Sr partially substituted by Ca.
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The performance of a scanning photoelectron microscope at the MAX I storage ring in Lund is presented. The microscope utilizes undulator radiation in the energy range 15–150 eV and is comprised of a plane-grating monochromator with a... more
The performance of a scanning photoelectron microscope at the MAX I storage ring in Lund is presented. The microscope utilizes undulator radiation in the energy range 15–150 eV and is comprised of a plane-grating monochromator with a Kirkpatrick–Baez objective and a gracing incidence ellipsoidal focusing mirror. The instrument with its high photon flux, 109–1010 photons/s, and narrow bandwidth, better than 0.2 eV, is excellently suited for high-resolution core-level spectroscopy and imaging of samples with lateral inhomogeneties in the micrometer range.
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The design of a photoemission microprobe beamline at the MAX laboratory is presented. The beamline will utilize the radiation from a 26 period undulator on the 550-MeV MAX storage ring. The beamline will be capable of providing about 1010... more
The design of a photoemission microprobe beamline at the MAX laboratory is presented. The beamline will utilize the radiation from a 26 period undulator on the 550-MeV MAX storage ring. The beamline will be capable of providing about 1010 photons/s of monochromatized radiation (bandwidth better than 0.25 eV) in the energy range of 20 to 150 eV focused into a submicrometer focal spot. It will be used for scanning photoemission experiments with a lateral resolution better than 1 μm. The optical system is comprised of a plane-grating monochromator with a Kirkpatrick–Baez objective and a specially designed ellipsoidal focusing mirror.
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Thin films of superconducting BiCaSrCuO have been grown by molecular beam epitaxy depositions of Bi, Cu, and CaF2/SrF2 on MgO and SrTiO3 followed by a high-temperature oxygen anneal. The films showed a fairly wide transition of 20 K at a... more
Thin films of superconducting BiCaSrCuO have been grown by molecular beam epitaxy depositions of Bi, Cu, and CaF2/SrF2 on MgO and SrTiO3 followed by a high-temperature oxygen anneal. The films showed a fairly wide transition of 20 K at a Tc of 80 K. Residual amounts of fluorine, detected with secondary-ion mass spectrometry, are believed to cause the broad superconducting transition. X-ray diffraction analysis of the superconducting films revealed strongly c-axis oriented films consisting of Bi2CaSr2Cu2O8 and Bi2Sr2Cu1O6 phase. Surface structure studies, with scanning electron microscopy and scanning tunneling microscopy, show oriented 1 μm sized very smooth flat grains of superconducting phase in the plane of the film with islands of CuOx scattered on top of these.
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ABSTRACT Valence‐band photoemission spectra have been studied on single‐crystal aluminum faces using polarized sychrotron radiation. New intrinsic and oxygen‐related extrinsic surface resonance features are observed within 5 eV of the... more
ABSTRACT Valence‐band photoemission spectra have been studied on single‐crystal aluminum faces using polarized sychrotron radiation. New intrinsic and oxygen‐related extrinsic surface resonance features are observed within 5 eV of the Fermi level. These surface resonances are correlated with oxygen uptake on the (100), (110), and (111) faces as observed in both valence and core‐level spectroscopy. Considerable face dependence to the oxidation is shown. Although aluminum is a free‐electron‐like metal, its surface behavior on single‐crystal faces shows effects due to localized charge.