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127 results on '"Schumann, F. O."'

1. Electron pair emission from surfaces: Some general experimental considerations

Author

Kamrla, R., Widdra, W., Chiang, C. -T., and Schumann, F. O.

Subjects
Condensed Matter - Materials Science
Abstract

We discuss some experimental facets of electron pair emission from surfaces using two different experimental approaches. In the first case the instrument consists of a pair of hemispherical analyzers which are operated with continuous primary beams of electrons or photons. The second instrument employs a pair of time-of-flight spectrometers which require a pulsed excitation source. A key experimental quantity is the ratio of `true' to `random' coincidences which can be determined in different ways. Regardless of the type of instrument the primary flux has to adopt a much smaller value than in single electron spectroscopy. We describe different approaches to obtain the relevant count rates, in particular the concept of operating with a delayed coincidence circuit. We also address the question on how to compare the two types of spectrometer in terms of their performance.

Published
2021

2. The CoESCA station at BESSY: Auger electron-Photoelectron coincidences from surfaces demonstrated for Ag MNN

Author

Leitner, T., Born, A., Bidermane, I., Ovsyannikov, R., Johansson, F. O. L., Sassa, Y., Föhlisch, A., Lindblad, A., Schumann, F. O., Svensson, S., and Mårtensson, N.

Subjects
Physics - Instrumentation and Detectors
Abstract

In this work, we present the CoESCA station for electron-electron coincidence spectroscopy from surfaces, built in a close collaboration between Uppsala University and Helmholtz-Zentrum Berlin at the BESSY II synchrotron facility in Berlin, Germany. We start with a detailed overview of previous work in the field of electron-electron coincidences, before we describe the CoESCA setup and its design parameters. The system is capable of recording shot-to-shot resolved 6D coincidence datasets, i.e. the kinetic energy and the two take off angles for both coincident electrons. The mathematics behind extracting and analysing these multi-dimensional coincidence datasets is introduced, with a focus on coincidence statistics, resulting in fundamental limits of the signal-to-noise ratio and its implications for acquisition times and the size of the raw data stream. The functionality of the CoESCA station is demonstrated for the example of Auger electron - photoelectron coincidences from silver surfaces for photoelectrons from the Ag 3d core levels and their corresponding MNN Auger electrons. The Auger spectra originating from the different core levels, 3d$_{3/2}$ and 3d$_{5/2}$ could be separated and further, the two-hole state energy distributions were determined for these Auger decay channels., Comment: 14 pages, 6 figures

Published
2020

3. Surface phonons of NiO(001) ultrathin films grown pseudomorphically on Ag(001)

Author

Kostov, K. L., Polzin, S., Schumann, F. O., and Widdra, W.

Subjects
Condensed Matter - Materials Science, Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

For a ultrathin NiO(001) film of 4 monolayers (ML) thickness grown on Ag(001), the vibrational properties have been determined by high-resolution electron energy loss spectroscopy (HREELS). For the well-ordered pseudomorphically grown film, nine phonon modes have been identified and their dispersions have been revealed along the Gamma-X high-symmetry direction. The comparison with phonon data for a 25 ML thick NiO(001) film shows that the NiO (001) phonon properties are already fully developed at 4 ML. Significant differences are found for the surface-localized phonon S6 which has an increased dispersion for the ultrathin film. The dipole-active Fuchs-Kliewer phonon-polariton exhibits a narrower lineshape than the mode found for a single-crystal surface, which might hint to a reduced antiferromagnetic coupling in the ultrathin film., Comment: submitted to Surface Science

Published
2015
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6. The CoESCA station at BESSY : Auger electron-photoelectron coincidences from surfaces demonstrated for Ag MNN

Author

Leitner, Torsten, Born, A., Bidermane, Ieva, Ovsyannikov, R., Johansson, Fredrik O. L., Sassa, Y., Föhlisch, A., Lindblad, Andreas, Schumann, F. O., Svensson, Svante, Mårtensson, Nils, Leitner, Torsten, Born, A., Bidermane, Ieva, Ovsyannikov, R., Johansson, Fredrik O. L., Sassa, Y., Föhlisch, A., Lindblad, Andreas, Schumann, F. O., Svensson, Svante, and Mårtensson, Nils

Abstract

In this work, we present the CoESCA station for electron-electron coincidence spectroscopy from surfaces, built in a close collaboration between Uppsala University and Helmholtz-Zentrum Berlin at the BESSY II synchrotron facility in Berlin, Germany. We start with a detailed overview of previous work in the field of electron-electron coincidences, before we describe the CoESCA setup and its design parameters. The system is capable of recording shot-to-shot resolved 6D coincidence datasets, i.e. the kinetic energy and the two take off angles for both coincident electrons. The mathematics behind extracting and analysing these multi-dimensional coincidence datasets is introduced, with a focus on coincidence statistics, resulting in fundamental limits of the signal-to-noise ratio and its implications for acquisition times and the size of the raw data stream. The functionality of the CoESCA station is demonstrated for the example of Auger electron-photoelectron coincidences from silver surfaces for photoelectrons from the Ag 3d core levels and their corresponding MNN Auger electrons. The Auger spectra originating from the different core levels, 3d3/2 and 3d5/2 could be separated and further, the two-hole state energy distributions were determined for these Auger decay channels.

Published
2021
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13. Magnetic switching, relaxation, and domain structure of a Co/Si(111) film.

Author

Baird, M. J., Bland, J. A. C., Gu, E., Ives, A. J. R., Schumann, F. O., and Hughes, H. P.

Subjects
MAGNETIZATION, SCANNING electron microscopy, POLYCRYSTALS, SILICON
Abstract

Presents a study which used scanning magneto-optic Kerr effect microscopy to investigate the magnetic relaxation of a polycrystalline hcp cobalt film grown on silicon (111). Mechanism of magnetization reversal; Details on the orientation-dependent magnetic switching behavior for the cobalt/silicon(111) film.

Published
1993
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15. Observation and Structure Determination of an Oxide Quasicrystal Approximant

Author

Förster, S., primary, Trautmann, M., additional, Roy, S., additional, Adeagbo, W. A., additional, Zollner, E. M., additional, Hammer, R., additional, Schumann, F. O., additional, Meinel, K., additional, Nayak, S. K., additional, Mohseni, K., additional, Hergert, W., additional, Meyerheim, H. L., additional, and Widdra, W., additional

Published
2016
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16. Magnetic properties of Fe-based alloys.

Author

Schumann, F. O. and Tobin, J. G.

Subjects
*IRON-nickel alloys, *IRON-cobalt alloys, *LINEAR dichroism
Abstract

The magnetic properties of fcc Fe[sub x]Ni[sub 1-x] and Fe[sub x]Co[sub 1-x] alloys grown on Cu(100) were investigated in an element-specific fashion. The technique employed was linear dichroism in photoemission, which by varying the chirality can also determine the magnetization axis. We observed a different behavior for the two alloys at Fe concentrations above 60%. At this concentration the Fe[sub x]Ni[sub 1-x] alloy shows a strong reduction of the Fe dichroism associated with the invar instability. This is in contrast to the Fe[sub x]Co[sub 1-x] alloy, where the Fe dichroism stays essentially constant across the concentration. Despite these differences both systems show a change of the easy axis at roughly the same electron count. For small Fe concentrations the easy axis is in-plane along the [011] direction. This changes into the [001] direction at Fe[sub 63]Ni[sub 37], which is at 0.7 excess electrons per atom when compared with Fe. This is different to the bulk, where a change occurs at Fe[sub 25]Ni[sub 75]. We find the easy axis change for Fe[sub x]Co[sub 1-x] to occur at 49% Fe. This would be equivalent to 0.5 excess electrons when compared with Fe. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Published
2000
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20. Growth and magnetic properties of FexNi1-x ultrathin films on Cu(100).

Author

Schumann, F. O., Wu, S. Z., Mankey, G. J., and Willis, R. F.

Subjects
*IRON-nickel alloys, *THIN films, *COPPER, *CURIE temperature
Abstract

Presents a study that investigated the growth and magnetic properties of iron-nickel alloy ultrathin films on copper(100). Methodology; Analysis of the thickness dependence of the Curie temperature for different alloy concentrations; Comparison of the Curie temperature with the Curie temperature of bulk iron-nickel alloys.

Published
1996
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22. Magnetic properties of ultrathin Co/Cu(001) films during growth.

Author

Schumann, F. O. and Bland, J. A. C.

Subjects
*METALLIC films, *HYSTERESIS, *KERR electro-optical effect, *COBALT alloys
Abstract

Presents a study which investigated the evolution of the in-plane magnetic hysteresis behavior of cobalt/copper (001) films by means of the transverse magneto-optical Kerr effect. Analysis of the thickness dependence of the coercivity of ultrathin cobalt/copper (001) films; Methods; Results and discussion.

Published
1993
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39. Dynamics of two-electron photoemission from Cu(111)

Author

Hattass, M., primary, Jahnke, T., additional, Schössler, S., additional, Czasch, A., additional, Schöffler, M., additional, Schmidt, L. Ph. H., additional, Ulrich, B., additional, Jagutzki, O., additional, Schumann, F. O., additional, Winkler, C., additional, Kirschner, J., additional, Dörner, R., additional, and Schmidt-Böcking, H., additional

Published
2008
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47. Correlation spectroscopy of condensed matter systems.

Author

Schumann, F. O., Kirschner, J., Kouzakov, K. A., and Berakdar, J.

Subjects
*ELECTRON-electron interactions, *ELECTRON spectroscopy, *SPECTRUM analysis, *ATOMIC spectroscopy, *ELECTRONS, *ATOMS, *IONS, *PHYSICS, *PHYSICAL sciences
Abstract

We present experimental and theoretical evidence for the potential of the two-particle spectroscopy to explore the electron-electron interaction in condensed matter. The experiment consists of a single electron impinging onto a clean surface. Two electrons are then emitted simultaneously and their momentum vectors are resolved. The measured energy and angular pair correlations within the pair carry direct information akin to the electron-electron interaction in the sample. We also point out that the presence of the Fermi sea leads to a damping, and a suppression of the range of the electron-electron interaction. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Published
2006
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