170 research outputs found
Electrical resistivity, magnetism and electronic structure of the intermetallic 3d/4f Laves phase compounds ErNi2Mnx
The non-stoichiometric intermetallic compounds RENi2Mnx (RE = rare earth) with
the cubic MgCu2-type structure display a large variety of magnetic properties which
is due to a complex interplay between the degrees of freedom of the 3d and
4f electrons and their interactions. We performed a comprehensive study of the
electrical resistivity, magnetic properties and the electronic structure of ErNi2Mnx
(x =0, 0.25, 0.5, 0.75, 1, 1.25) compounds by employing a suitable set of complementary
experimental approaches. We find an increase in electrical resistance compared
to ErNi2 upon Mn doping, the residual resistivity ratio decreases with increasing
manganese content. The Curie temperature exhibits a sharp increase to around 50 K
for Mn concentrations x 0.5, whereas the saturation magnetization decreases
with growing Mn content x 0.5. Valence band X-ray photoelectron spectroscopy
reveals an increasing intensity of Mn 3d states near Fermi energy in dependence
of Mn concentration and Curie temperature. Resonant photoelectron spectroscopy
of ErNi2Mn0.75 reveals that the photoemission decay channels dominate the valence
band spectra across the Er N5 and Mn L3 X-ray absorption maxima, whereas the
L3VV Auger dictates the resonant valence band spectra close to and at the Ni L3
X-ray absorption edge
From Fe3O4/NiO bilayers to NiFe2O4-like thin films through Ni interdiffusion
Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni ions out of NiO into FeO ultrathin films, resulting in off-stoichiometric nickel ferrite–like thin layers. We synthesized epitaxial FeO bilayers on Nb-doped SrTiO(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400C, 600C, and 800C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFeO. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni and Fe ions and the Fe ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films
An update on the Hirsch conjecture
The Hirsch conjecture was posed in 1957 in a letter from Warren M. Hirsch to
George Dantzig. It states that the graph of a d-dimensional polytope with n
facets cannot have diameter greater than n - d.
Despite being one of the most fundamental, basic and old problems in polytope
theory, what we know is quite scarce. Most notably, no polynomial upper bound
is known for the diameters that are conjectured to be linear. In contrast, very
few polytopes are known where the bound is attained. This paper collects
known results and remarks both on the positive and on the negative side of the
conjecture. Some proofs are included, but only those that we hope are
accessible to a general mathematical audience without introducing too many
technicalities.Comment: 28 pages, 6 figures. Many proofs have been taken out from version 2
and put into the appendix arXiv:0912.423
Behavioral Economics and the Public Sector
This thesis consists of four essays dealing with topics that are relevant for the public sector. The essays cover diverse issues of economics partly overlapping with political science. The topics reach from the taxation of labor over monetary policy to preferences over voting institutions. Throughout this thesis it is, in contrast to classical economics, not assumed that humans are necessarily fully rational. Once full rationality is no longer assumed, experiments become an important tool to learn about human behavior. Consequently, most of the work in this thesis makes use of economic experiments
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