| 1. |
- Abdo, A. A., et al.
(författare)
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OBSERVATIONS OF MILKY WAY DWARF SPHEROIDAL GALAXIES WITH THE FERMI-LARGE AREA TELESCOPE DETECTOR AND CONSTRAINTS ON DARK MATTER MODELS
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 712:1, s. 147-158
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Tidskriftsartikel (refereegranskat)abstract
- We report on the observations of 14 dwarf spheroidal galaxies (dSphs) with the Fermi Gamma-Ray Space Telescope taken during the first 11 months of survey mode operations. The Fermi telescope, which is conducting an all-sky gamma-ray survey in the 20 MeV to > 300 GeV energy range, provides a new opportunity to test particle dark matter models through the expected gamma-ray emission produced by pair annihilation of weakly interacting massive particles (WIMPs). Local Group dSphs, the largest galactic substructures predicted by the cold dark matter scenario, are attractive targets for such indirect searches for dark matter because they are nearby and among the most extreme dark matter dominated environments. No significant gamma-ray emission was detected above 100 MeV from the candidate dwarf galaxies. We determine upper limits to the gamma-ray flux assuming both power-law spectra and representative spectra from WIMP annihilation. The resulting integral flux above 100 MeV is constrained to be at a level below around 10(-9) photons cm(-2) s(-1). Using recent stellar kinematic data, the gamma-ray flux limits are combined with improved determinations of the dark matter density profile in eight of the 14 candidate dwarfs to place limits on the pair-annihilation cross section of WIMPs in several widely studied extensions of the standard model, including its supersymmetric extension and other models that received recent attention. With the present data, we are able to rule out large parts of the parameter space where the thermal relic density is below the observed cosmological dark matter density and WIMPs (neutralinos here) are dominantly produced non-thermally, e. g., in models where supersymmetry breaking occurs via anomaly mediation. The gamma-ray limits presented here also constrain some WIMP models proposed to explain the Fermi and PAMELA e(+)e(-) data, including low-mass wino-like neutralinos and models with TeV masses pair annihilating into muon-antimuon pairs.
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| 2. |
- Gustafsson, Alexander, 1982-, et al.
(författare)
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Analysis of STM images with pure and CO-functionalized tips : A first-principles and experimental study
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:8
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Tidskriftsartikel (refereegranskat)abstract
- We describe a first-principles method to calculate scanning tunneling microscopy (STM) images, and compare the results to well-characterized experiments combining STM with atomic force microscopy (AFM). The theory is based on density functional theory with a localized basis set, where the wave functions in the vacuum gap are computed by propagating the localized-basis wave functions into the gap using a real-space grid. Constant-height STM images are computed using Bardeen's approximation method, including averaging over the reciprocal space. We consider copper adatoms and single CO molecules adsorbed on Cu(111), scanned with a single-atom copper tip with and without CO functionalization. The calculated images agree with state-of-the-art experiments, where the atomic structure of the tip apex is determined by AFM. The comparison further allows for detailed interpretation of the STM images.
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| 3. |
- Gustafsson, Alexander, 1982-
(författare)
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Modeling of non-equilibrium scanning probe microscopy
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work in this thesis is basically divided into two related but separate investigations.The first part treats simple chemical reactions of adsorbate molecules on metallic surfaces, induced by means of a scanning tunneling probe (STM). The investigation serves as a parameter free extension to existing theories. The theoretical framework is based on a combination of density functional theory (DFT) and non-equilibrium Green's functions (NEGF). Tunneling electrons that pass the adsorbate molecule are assumed to heat up the molecule, and excite vibrations that directly correspond to the reaction coordinate. The theory is demonstrated for an OD molecule adsorbed on a bridge site on a Cu(110) surface, and critically compared to the corresponding experimental results. Both reaction rates and pathways are deduced, opening up the understanding of energy transfer between different configurational geometries, and suggests a deeper insight, and ultimately a higher control of the behaviour of adsorbate molecules on surfaces.The second part describes a method to calculate STM images in the low bias regime in order to overcome the limitations of localized orbital DFT in the weak coupling limit, i.e., for large vacuum gaps between a tip and the adsorbate molecule. The theory is based on Bardeen's approach to tunneling, where the orbitals computed by DFT are used together with the single-particle Green's function formalism, to accurately describe the orbitals far away from the surface/tip. In particular, the theory successfully reproduces the experimentally well-observed characteristic dip in the tunneling current for a carbon monoxide (CO) molecule adsorbed on a Cu(111) surface. Constant height/current STM images provide direct comparisons to experiments, and from the developed method further insights into elastic tunneling are gained.
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| 4. |
- Gustafsson, Alexander, 1982-, et al.
(författare)
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STM contrast of a CO dimer on a Cu(111) surface: a wave-function analysis
- 2017
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 29:505301
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Tidskriftsartikel (refereegranskat)abstract
- We present a method used to intuitively interpret the scanning tunneling microscopy (STM) contrast by investigating individual wave functions originating from the substrate and tip side. We use localized basis orbital density functional theory, and propagate the wave functions into the vacuum region at a real-space grid, including averaging over the lateral reciprocal space. Optimization by means of the method of Lagrange multipliers is implemented to perform a unitary transformation of the wave functions in the middle of the vacuum region. The method enables (i) reduction of the number of contributing tip-substrate wave function combinations used in the corresponding transmission matrix, and (ii) to bundle up wave functions with similar symmetry in the lateral plane, so that (iii) an intuitive understanding of the STM contrast can be achieved. The theory is applied to a CO dimer adsorbed on a Cu(1 1 1) surface scanned by a single-atom Cu tip, whose STM image is discussed in detail by the outlined method.
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| 5. |
- Gustafsson, Alexander, 1982-
(författare)
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Theoretical modeling of scanning tunneling microscopy
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main body of this thesis describes how to calculate scanning tunneling microscopy (STM) images from first-principles methods. The theory is based on localized orbital density functional theory (DFT), whose limitations for large-vacuum STM models are resolved by propagating localized-basis wave functions close to the surface into the vacuum region in real space. A finite difference approximation is used to define the vacuum Hamiltonian, from which accurate vacuum wave functions are calculated using equations based on standard single-particle Green’s function techniques, and ultimately used to compute the conductance. By averaging over the lateral reciprocal space, the theory is compared to a series of high-quality experiments in the low- bias limit, concerning copper surfaces with adsorbed carbon monoxide (CO) species and adsorbate atoms, scanned by pure and CO-functionalized copper tips. The theory compares well to the experiments, and allows for further insights into the elastic tunneling regime.A second significant project in this thesis concerns first-principles calculations of a simple chemical reaction of a hydroxyl (oxygen-deuterium) monomer adsorbed on a copper surface. The reaction mechanism is provided by tunneling electrons that, via a finite electron-vibration coupling, trigger the deuterium atom to flip between two nearly identical configurational states along a frustrated rotational motion. The theory suggests that the reaction primarily occurs via nuclear tunneling for the deuterium atom through the estimated reaction barrier, and that over-barrier ladder climbing processes are unlikely.
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| 6. |
- Jandrić, Petar, et al.
(författare)
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Teaching in the Age of Covid-19
- 2020
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Ingår i: Postdigital Science and Education. - : Springer Science and Business Media LLC. - 2524-4868 .- 2524-485X. ; 2:3, s. 1069-1230
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A collection of 84 author's testimonies and workspace photographs between 18 March and 5 May 2020.
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| 7. |
- Jandrić, Petar, et al.
(författare)
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Teaching in the Age of Covid-19 : The New Normal
- 2022
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Ingår i: Postdigital Science and Education. - : Springer Science and Business Media LLC. - 2524-485X .- 2524-4868. ; 4:3, s. 877-1015
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Tidskriftsartikel (refereegranskat)abstract
- On 17 March 2020, Postdigital Science and Education launched a call for testimonies about teaching and learning during very frst Covid-19 lockdowns. The resulting article, ‘Teaching in the Age of Covid-19’ (attached), presents 81 written testimonies and 80 workspace photographs submitted by 84 authors from 19 countries. On 17 March 2021, Postdigital Science and Education launched a call for a sequel article of testimonies about teaching and learning during very first Covid-19 lockdowns. The resulting article, ‘Teaching in the Age of Covid-19—1 Year Later’(attached), consists of 74 textual testimonies and 76 workspace photographs submitted by 77 authors from 20 countries.These two articles have been downloaded almost 100,000 times and have been cited more than 100 times. This shows their value as historical documents. Recent analyses, such as ‘Teaching in the Age of Covid-19—A Longitudinal Study ’(attached), also indicate their strong potential for educational research. As the Covid-19 pandemic seems to wind down, pandemic experiences have entered the mainstream. They shape all educational research of today and arguably do not require special treatment. Yet, our unique series of pandemic testimonies provides a unique opportunity to longitudinally trace what happens to the same people over the years—and this opportunity should not be missed.Today, we launch a call for fnal sequel: Teaching in the Age of Covid-19—The New Normal. In this sequel, we would like to hear about ways in which you—contributors to the previous articles—have established your own new normal. We hope that this will be the last iteration in this series of testimony articles. Unless the world faces another strong pandemic outburst, we would like to end the series with this last article.
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| 8. |
- Jandrić, Petar, et al.
(författare)
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Teaching in the Age of Covid-19—1 Year Later
- 2021
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Ingår i: Postdigital Science and Education. - : Springer. - 2524-485X .- 2524-4868. ; 3:3, s. 1073-1223
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Tidskriftsartikel (refereegranskat)
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| 9. |
- Okabayashi, Norio, et al.
(författare)
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Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy, and density functional theory
- 2016
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 93:16
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Tidskriftsartikel (refereegranskat)abstract
- Achieving a high intensity in inelastic scanning tunneling spectroscopy (IETS) is important for precise measurements. The intensity of the IETS signal can vary by up to a factor of 3 for various tips without an apparent reason accessible by scanning tunneling microscopy (STM) alone. Here, we show that combining STM and IETS with atomic force microscopy enables carbon monoxide front-atom identification, revealing that high IETS intensities for CO/Cu(111) are obtained for single-atom tips, while the intensity drops sharply for multiatom tips. Adsorption of the CO molecule on a Cu adatom [CO/Cu/Cu(111)] such that the molecule is elevated over the substrate strongly diminishes the tip dependence of IETS intensity, showing that an elevated position channels most of the tunneling current through the CO molecule even for multiatom tips, while a large fraction of the tunneling current bypasses the CO molecule in the case of CO/Cu(111).
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