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Berlin Brandenburg

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  • 1
    Description: We study the spin magnetic moment of a single impurity embedded in a finite-size non-magnetic host exhibiting a band gap. The calculations were performed using a tight-binding model Hamiltonian. The simple criterion for the magnetic to non-magnetic transition as given in the Anderson impurity model breaks down in these cases. We show how the spin magnetic moment of the impurity that normally would be quenched can be restored upon introducing a gap at the Fermi level in the host density of states. The magnitude of the impurity spin magnetic moment scales monotonically with the size of the band gap. This observation even holds for a host material featuring a strongly discretized density of states. Thus, it should be possible to tune the magnetic moment of doped nano-particles by varying their size and thereby their band gap.
    Keywords: Condensed Matter - Strongly Correlated Electrons
    Source: Cornell University
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  • 2
    Language: English
    In: Angewandte Chemie International Edition, 07 April 2015, Vol.54(15), pp.4498-4501
    Description: X‐ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr. Our results indicate that the removal of a single electron from the 4sσ bonding orbital of Cr drastically changes the preferred coupling of the 3d electronic spins. While the neutral molecule has a zero‐spin ground state with a very short bond length, the molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of S=11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling between the 3d electrons of the two atoms that is mediated by the single 4s electron through a strong intraatomic 3d‐4s exchange interaction. Our finding allows an estimate of the relative energies of two states that are often discussed as ground‐state candidates, the ferromagnetically coupled Σ and the low‐spin Σ state. from the 4sσ bonding orbital of Cr fully localizes all the 3d electrons and drastically changes the preferred coupling of their spins. The molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of =11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling.
    Keywords: Chromium ; Exchange Interactions ; Ion Trap ; Magnetic Properties ; X‐Ray Absorption Spectroscopy
    ISSN: 1433-7851
    E-ISSN: 1521-3773
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  • 3
    Language: English
    In: Chemistry – A European Journal, 28 May 2018, Vol.24(30), pp.7631-7636
    Description: Preservation of protein conformation upon transfer into the gas phase is key for structure determination of free single molecules, for example using X‐ray free‐electron lasers. In the gas phase, the helicity of melittin decreases strongly as the protein's protonation state increases. We demonstrate the sensitivity of soft X‐ray spectroscopy to the gas‐phase structure of melittin cations ([melittin+H], =2–4) in a cryogenic linear radiofrequency ion trap. With increasing helicity, we observe a decrease of the dominating carbon 1 s–π* transition in the amide C=O bonds for non‐dissociative single ionization and an increase for non‐dissociative double ionization. As the underlying mechanism we identify inelastic electron scattering. Using an independent atom model, we show that the more compact nature of the helical protein conformation substantially increases the probability for off‐site intramolecular ionization by inelastic Auger electron scattering. : Soft X‐ray absorption in gas‐phase protein cations leads to Auger‐electron emission. Emitted Auger electrons can induce electron‐impact ionization elsewhere in the same molecule. The cross‐section of this process is sensitive to protein secondary structure.
    Keywords: Auger Electrons ; Gas-Phase Biomolecules ; Mass Spectrometry ; Protein Conformation ; Soft X-Ray Spectroscopy
    ISSN: 0947-6539
    E-ISSN: 1521-3765
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  • 4
    Language: English
    In: The Journal of Physical Chemistry C, 05/21/2015, Vol.119(20), pp.11148-11152
    ISSN: 1932-7447
    E-ISSN: 1932-7455
    Source: American Chemical Society (via CrossRef)
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  • 5
    Language: German
    In: Angewandte Chemie, 07 April 2015, Vol.127(15), pp.4581-4584
    Description: Mithilfe des magnetischen zirkularen Röntgendichroismus wurden die elektronische Struktur und das magnetische Moment von Cr bestimmt. Unsere Ergebnisse zeigen, dass sich die bevorzugte Kopplung der 3d‐Spins durch das Entfernen eines einzelnen Elektrons aus dem bindenden 4sσ‐Orbital des Cr dramatisch umkehrt. Im Unterschied zum neutralen Molekül, das einen Grundzustand mit verschwindendem Spin bei kurzer Bindungslänge besitzt, zeigt das Molekülkation einen ferromagnetisch gekoppelten Grundzustand mit dem höchstmöglichen Spin von S=11/2 bei nahezu der doppelten Bindungslänge des neutralen Moleküls. Diese besondere Spinkonfiguration ist das Resultat einer indirekten Austauschkopplung der an den Ionenrümpfen lokalisierten 3d‐Elektronen durch das delokalisierte 4sσ‐Elektron über den starken intraatomaren 3d‐4s‐Austausch. Unsere Ergebnisse ermöglichen erstmals eine Abschätzung der relativen Energien des ferromagnetisch gekoppelten Σ‐Grundzustands und des angeregten Σ‐Zustands. aus dem bindenden 4sσ‐Orbital des Cr führt zu vollständiger Lokalisierung sämtlicher 3d‐Elektronen und ändert deren Spinkopplung drastisch. Das Molekülkation hat einen ferromagnetisch gekoppelten Grundzustand mit dem höchstmöglichen Spin von =11/2 bei nahezu der doppelten Bindungslänge des neutralen Moleküls. Diese Spinkonfiguration kann als das Resultat indirekter Austauschkopplung interpretiert werden.
    Keywords: Austauschwechselwirkung ; Chrom ; Ionenfallen ; Magnetische Eigenschaften ; Röntgenabsorptionsspektroskopie
    ISSN: 0044-8249
    E-ISSN: 1521-3757
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  • 6
    Language: English
    In: Review of Scientific Instruments, March 2016, Vol.87(3)
    Description: X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L 3,2 -edges. The high brightness of the X-ray free electron laser combined with high accuracy detection of incident and transmitted X-rays enables ultrafast X-ray magnetic circular dichroism studies of unprecedented sensitivity. This new capability is applied to a study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature.
    Keywords: Articles
    ISSN: 0034-6748
    E-ISSN: 1089-7623
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  • 7
    In: J. Chem. Phys. 142, 234301 (2015)
    Description: The electronic structure and magnetic moments of free Mn$_2^+$ and Mn$_3^+$ are characterized by $2p$ x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap that is coupled to a synchrotron radiation beamline. Our results show directly that localized magnetic moments of 5 $\mu_B$ are created by $3d^5 (^6\mathrm{S})$ states at each ionic core, which are coupled in parallel to form molecular high-spin states via indirect exchange that is mediated in both cases by a delocalized valence electron in a singly-occupied $4s$ derived orbital with an unpaired spin. This leads to total magnetic moments of 11 $\mu_B$ for Mn$_2^+$ and 16 $\mu_B$ for Mn$_3^+$, with no contribution of orbital angular momentum.
    Keywords: Physics - Chemical Physics ; Physics - Atomic And Molecular Clusters
    ISSN: 00219606
    E-ISSN: 10897690
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  • 8
    Language: English
    In: Journal of Physical Chemistry Letters, 01 November 2016, Vol.7(22)
    Description: Cobalt-benzene cluster ions of the form Co(bz) (n = 0-3) were produced in the gas phase, mass-selected, and cooled in a cryogenic ion trap held at 3-4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co has a spin magnetic moment of μ = 6 μ and an orbital magnetic moment of μ = 3 μ. Co(bz) and Co(bz) complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co. However, coordination of the third benzene to form Co(bz) completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co(bz) species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for n = 0 → 3.
    Keywords: Catalysis (Heterogeneous), Materials and Chemistry By Design, Synthesis (Novel Materials) ; Chemistry
    ISSN: 1948-7185
    E-ISSN: 1948-7185
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  • 9
    Language: English
    In: Journal of Physics: Conference Series, 2015, Vol.635(11), p.112083 (1pp)
    Description: For gas-phase experiments on complex biomolecular ions, the target density is a most critical issue. We developed a novel high-current electrospray ionization source, delivering 10 times higher currents than conventional systems.
    Keywords: Weiche Röntgenstrahlung ; Photoionisation ; Ionenquelle ; Biomolekül ; Protein ; Gasphase ; Starkstrom ; Umwelteinfluss ; Flüssige Phase ; Physics;
    ISSN: 1742-6588
    E-ISSN: 1742-6596
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  • 10
    In: Physical Chemistry Chemical Physics, 2019, Vol.21(12), pp.6651-6661
    Description: We present 2p core-level spectra of size-selected aluminum and silicon cluster cations from soft X-ray photoionization efficiency curves and density functional theory. The experimental and theoretical results are in very good quantitative agreement and allow for geometric structure determination. New ground state geometries for Al 12 + , Si 15 + , Si 16 + , and Si 19 + are proposed on this basis. The chemical shifts of the 2p electron binding energies reveal a substantial difference for aluminum and silicon clusters: while in aluminum the 2p electron binding energy decreases with increasing coordination number, no such correlation was observed for silicon. The 2p binding energy shifts in clusters of both elements differ strongly from those of the corresponding bulk matter. For aluminum clusters, the core-level shifts between outer shell atoms and the encapsulated atom are of opposite sign and one order of magnitude larger than the corresponding core-level shift between surface and bulk atoms in the solid. For silicon clusters, the core-level shifts are of the same order of magnitude in clusters and in bulk silicon but no obvious correlation of chemical shift and bond length, as present for reconstructed silicon surfaces, are observed.
    Keywords: Aluminium ; Cations ; Electrons ; Chemical-Shift ; Binding-Energy ; Earthquake-Intensity ; Soft-X-Ray-Radiation ; Photoionization ; Structural-Analysis ; Density-Functional-Theory ; Coordination-Number ; Ground-State ; Bond:Chemical ; Reconstruction ; Aluminium ; Kation ; Elektron ; Chemische Verschiebung ; Bindungsenergie ; Erdbebenmagnitude ; Weiche Röntgenstrahlung ; Photoionisation ; Strukturbestimmung ; Dichtefunktionaltheorie ; Koordinationszahl ; Grundzustand ; Chemische Bindung ; Rekonstruktion ; Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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