75 research outputs found
Mapping the magnetic exchange interactions from first principles: Anisotropy anomaly and application to Fe, Ni, and Co
Mapping the magnetic exchange interactions from model Hamiltonian to density
functional theory is a crucial step in multi-scale modeling calculations.
Considering the usual magnetic force theorem but with arbitrary rotational
angles of the spin moments, a spurious anisotropy in the standard mapping
procedure is shown to occur provided by bilinear-like contributions of high
order spin interactions. The evaluation of this anisotropy gives a hint on the
strength of non-bilinear terms characterizing the system under investigation.Comment: 11 pages, 1 figur
The combined effect of temperature and disorder on interlayer exchange coupling in magnetic multilayers
We study the combined effect of temperature and disorder in the spacer on the
interlayer exchange coupling. The temperature dependence is treated on ab
initio level. We employ the spin-polarized surface Green function technique
within the tight-binding linear muffin-tin orbital method and the Lloyd
formulation of the IEC. The integrals involving the Fermi-Dirac distribution
are calculated using an efficient method based on representation of integrands
by a sum of complex exponentials. Application is made to
Co/Cu_{100-x}M_x/Co(001) trilayers (M=Zn, Au, and Ni) with varying thicknesses
of the spacer.Comment: 5 pages, LaTeX, 1 figure. Submitted to Phil. Mag.
Ballistic Spin Injection and Detection in Fe/Semiconductor/Fe Junctions
We present {\it ab initio} calculations of the spin-dependent electronic
transport in Fe/GaAs/Fe and Fe/ZnSe/Fe (001) junctions simulating the situation
of a spin-injection experiment. We follow a ballistic Landauer-B\"uttiker
approach for the calculation of the spin-dependent dc conductance in the
linear-responce regime, in the limit of zero temperature. We show that the bulk
band structure of the leads and of the semiconductor, and even more the
electronic structure of a clean and abrupt interface, are responsible for a
current polarisation and a magnetoresistance ratio of almost the ideal 100%, if
the transport is ballistic. In particular we study the significance of the
transmission resonances caused by the presence of two interfaces.Comment: 13 pages, 9 figure
Surface state scattering by adatoms on noble metals
When surface state electrons scatter at perturbations, such as magnetic or
nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized
at the adatom site, can develop below the bottom of the surface state band for
both spin channels. In the case of adatoms, these states have been found very
recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for
the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried
out a systematic theoretical investigation of the electronic structure of these
surface states in the presence of magnetic and non-magnetic atoms on Cu(111).
We found that Ca and all 3 adatoms lead to a split-off state at the bottom
of the surface band which is, however, not seen for the elements Ga and
Ge. The situation is completely reversed if the impurities are embedded in the
surface: Ga and Ge are able to produce a split-off state whereas the 3
impurities do not. The resonance arises from the s-state of the impurities and
is explained in terms of strength and interaction nature (attraction or
repulsion) of the perturbing potential.Comment: 6 pages, 5 figure
RKKY-like contributions to the magnetic anisotropy energy: 3d adatoms on Pt(111) surface
The magnetic anisotropy energy defines the energy barrier that stabilizes a
magnetic moment. Utilizing density functional theory based simulations and
analytical formulations, we establish that this barrier is strongly modified by
long-range contributions very similar to Frieden oscillations and
Rudermann-Kittel-Kasuya-Yosida interactions. Thus, oscillations are expected
and observed, with different decaying factors and highly anisotropic in
realistic materials, which can switch non-trivially the sign of the magnetic
anisotropy energy. This behavior is general and for illustration we address
transition metals adatoms, Cr, Mn, Fe and Co deposited on Pt(111) surface. We
explain in particular the mechanisms leading to the strong site-dependence of
the magnetic anisotropy energy observed for Fe adatoms on Pt(111) surface as
revealed previously via first-principles based simulations and inelastic
scanning tunneling spectroscopy (A. A. Khajetoorians et al. Phys. Rev. Lett.
111, 157204 (2013)). The same mechanisms are probably active for the
site-dependence of the magnetic anisotropy energy obtained for Fe adatoms on Pd
or Rh(111) surfaces and for Co adatoms on Rh(111) surface (P. Blonski et al.
Phys. Rev. B 81, 104426 (2010)).Comment: published manuscript with additional figures and comment
Quantum well states and amplified spin-dependent Friedel oscillations in thin films
Electrons mediate many of the interactions between atoms in a solid. Their
propagation in a material determines its thermal, electrical, optical, magnetic
and transport properties. Therefore, the constant energy contours
characterizing the electrons, in particular the Fermi surface, have a prime
impact on the behavior of materials. If anisotropic, the contours induce strong
directional dependence at the nanoscale in the Friedel oscillations surrounding
impurities. Here we report on giant anisotropic charge density oscillations
focused along specific directions with strong spin-filtering after scattering
at an oxygen impurity embedded in the surface of a ferromagnetic thin film of
Fe grown on W(001). Utilizing density functional theory, we demonstrate that by
changing the thickness of the Fe films, we control quantum well states confined
to two dimensions that manifest as multiple flat energy contours, impinging and
tuning the strength of the induced charge oscillations which allow to detect
the oxygen impurity at large distances ( 50nm).Comment: This paper has an explanatory supplemen
Räumliche Organisationsstrukturen und Standortanforderungen im deutschen Online-Lebensmitteleinzelhandel: Beispiele aus ergänzendem, reinen und kombinierten Onlinehandel
Nicht erst seit der Covid-19-Pandemie nimmt der Online-Lebensmitteleinzelhandel in Deutschland zu und bringt neue, teilweise hybride, Betriebsformen und Vertriebsmodelle hervor. Hiermit gehen bisher kaum untersuchte räumliche Veränderungen der einzelnen Wertschöpfungsschritte einher, beispielsweise in den Bereichen vorgelagerte Logistik, Filialstruktur und Warenübergabe. Anhand von drei ausgewählten Fallbeispielen (Picnic, Wochenmarkt24 und Rewe) wurden neuere Betriebsformen und deren räumliche Logistik- und Vertriebsstrukturen identifiziert und unterschiedliche Standortfaktoren aufgeführt. Diese beinhalten neben den typischen Faktoren der Standortwahl für Distributionslager (Nähe zu Kunden, Arbeitskräften und Lieferanten) auch spezifische betriebsformen- und vertriebsmodellabhängige Faktoren, wie eine stärkere Verkürzung der "letzten Meile", eine Mindest- oder Maximalverdichtung von Haushalten im Einzugsgebiet oder die Nähe zu einer (landwirtschaftlichen) Erzeugerstruktur.Even before the current covid-19-pandemic, online grocery retailing in Germany is achieving a high level of momentum, giving rise to new and increasingly hybrid forms of operation and distribution models in the retail sector. These new operational forms of complementary, pure and combined e-commerce are accompanied by spatial changes of the individual value-added steps, e.g. in the areas of logistics, store structure and transfer of goods, which have hardly been investigated so far. Using three selected cases studies (Picnic, Wochenmarkt24 and Rewe) newer types of spatial logistics and distribution structures were identified and different location factors of these were listed. Thus, the paper illustrates that, in addition to the typical factors of location selection for distribution warehouses (proximity to customers, employees. and suppliers) in food retailing, other aspects, such as a shortening of the last mile, a minimum or maximum density of households in the surrounding area or the proximity to an (agricultural) producer structure, can also be of importance for the new forms of operation
Theory of real space imaging of Fermi surfaces
A scanning tunneling microscope can be used to visualize in real space Fermi
surfaces with buried impurities far below substrates acting as local probes. A
theory describing this feature is developed based on the stationary phase
approximation. It is demonstrated how a Fermi surface of a material acts as a
mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
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