2,906 research outputs found
Electronic ground states of Fe and Co as determined by x-ray absorption and x-ray magnetic circular dichroism spectroscopy
The electronic ground state of the Co diatomic molecular cation
has been assigned experimentally by x-ray absorption and x-ray magnetic
circular dichroism spectroscopy in a cryogenic ion trap. Three candidates,
, , and , for the electronic ground state of Fe
have been identified. These states carry sizable orbital angular momenta that
disagree with theoretical predictions from multireference configuration
interaction and density functional theory. Our results show that the ground
states of neutral and cationic diatomic molecules of transition elements
cannot generally be assumed to be connected by a one-electron process
Direct Observation of High-Spin States in Manganese Dimer and Trimer Cations by X-ray Magnetic Circular Dichroism Spectroscopy in an Ion Trap
The electronic structure and magnetic moments of free Mn and Mn
are characterized by 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 are created by 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 derived orbital with an unpaired spin. This leads to total
magnetic moments of 11 for Mn and 16 for Mn, with
no contribution of orbital angular momentum
Reconstruction of Defect Geometries in Ultrasonic NDT
The international activities in developing new flaw characterization methods with special emphasis on acoustic imaging have been increased. To reduce the dependency upon amplitude information and due to the fact that flaw information is buried in the shape and fine structure of wave fronts, considerable attention has been given to the development of methods using time-of-flight information from different probe positions. For this reason, with mechanical scanners and specially build data acquisition and evaluation systems, a vareity of ways to produce images has been developed. These include echotomography, linear or two dimensional mono- or multi-frequency holography, tip echo interference methods, ALOK (amplitude-,time-of-flight-locus curves), Phased Array, SAFT or Rayleigh-Sommerfeld Holography. These methods use mathematical algorithms which seem to be independent or which have been derived heuristically. Based upon the concept of elastodynamic diffraction theory together with that of tomography a concept can be derived which reveals the inner connection of these algorithms. Differences in the reconstructions arise due to limitations like limited aperture, limited bandwidth, use of mode converted signals or due to complex surface shapes. An attempt is made to cover the theoretical background, to give an overview on existing data acquisition systems and to describe the strength, weaknesses, and difficulties in producing acoustic images
Coordination-driven magnetic-to-nonmagnetic transition in manganese doped silicon clusters
The interaction of a single manganese impurity with silicon is analyzed in a
combined experimental and theoretical study of the electronic, magnetic, and
structural properties of manganese-doped silicon clusters. The structural
transition from exohedral to endohedral doping coincides with a quenching of
high-spin states. For all geometric structures investigated, we find a similar
dependence of the magnetic moment on the manganese coordination number and
nearest neighbor distance. This observation can be generalized to manganese
point defects in bulk silicon, whose magnetic moments fall within the observed
magnetic-to-nonmagnetic transition, and which therefore react very sensitively
to changes in the local geometry. The results indicate that high spin states in
manganese-doped silicon could be stabilized by an appropriate lattice
expansion
Modelling survival and connectivity of Mnemiopsis leidyi in the south-western North Sea and Scheldt estuaries
Three different models were applied to study the reproduction, survival and dispersal of Mnemiopsis leidyi in the Scheldt estuaries and the southern North Sea: a high-resolution particle tracking model with passive particles, a low-resolution particle tracking model with a reproduction model coupled to a biogeochemical model, and a dynamic energy budget (DEB) model. The results of the models, each with its strengths and weaknesses, suggest the following conceptual situation: (i) the estuaries possess enough retention capability to keep an overwintering population, and enough exchange with coastal waters of the North Sea to seed offshore populations; (ii) M. leidyi can survive in the North Sea, and be transported over considerable distances, thus facilitating connectivity between coastal embayments; (iii) under current climatic conditions, M. leidyi may not be able to reproduce in large numbers in coastal and offshore waters of the North Sea, but this may change with global warming; however, this result is subject to substantial uncertainty. Further quantitative observational work is needed on the effects of temperature, salinity and food availability on reproduction and on mortality at different life stages to improve models such as used here
Phase Modulated Thermal Conductance of Josephson Weak Links
We present a theory for quasiparticle heat transport through superconducting
weak links. The thermal conductance depends on the phase difference () of
the superconducting leads. Branch conversion processes, low-energy Andreev
bound states near the contact and the suppression of the local density of
states near the gap edge are related to phase-sensitive transport processes.
Theoretical results for the influence of junction transparency, temperature and
disorder, on the phase modulation of the conductance are reported. For
high-transmission weak links, , the formation of an Andreev bound state
at leads to suppression of the
density of states for the continuum excitations that transport heat, and thus,
to a reduction in the conductance for . For low-transmission
() barriers resonant scattering at energies
leads to an increase in the thermal conductance
as drops below (for phase differences near ).Comment: 4 pages, 3 figures Expanded discussion of boundary conditions for
Ricatti amplitude
Spin-phonon coupling in epitaxial Sr0.6Ba0.4MnO3 thin films
Spin-phonon coupling is investigated in epitaxially strained Sr1-xBaxMnO3 thin films with perovskite structure by means of microwave (MW) and infrared (IR) spectroscopy. In this work we focus on the Sr0.6Ba0.4MnO3 composition grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate. The MW complex electromagnetic response shows a decrease in the real part and a clear anomaly in the imaginary part around 150 K. Moreover, it coincides with a 17% hardening of the lowest-frequency polar phonon seen in IR reflectance spectra. In order to further elucidate this phenomenon, low-energy muon-spin spectroscopy was carried out, signaling the emergence of antiferromagnetic order with Néel temperature (TN) around 150 K. Thus, our results confirm that epitaxial Sr0.6Ba0.4MnO3 thin films display strong spin-phonon coupling below TN, which may stimulate further research on tuning the magnetoelectric coupling by controlling the epitaxial strain and chemical pressure in the Sr1-xBaxMnO3 system
Spin and orbital magnetic moments of size-selected iron, cobalt, and nickel clusters and their link to the bulk phase diagrams
Spin and orbital magnetic moments of cationic iron, cobalt, and nickel
clusters have been determined from x-ray magnetic circular dichroism
spectroscopy. In the size regime of atoms, these clusters show
strong ferromagnetism with maximized spin magnetic moments of 1~ per
empty state because of completely filled majority spin bands. The
only exception is where an unusually low average spin
magnetic moment of ~ per unoccupied state is
detected; an effect, which is neither observed for nor
.\@ This distinct behavior can be linked to the existence
and accessibility of antiferromagnetic, paramagnetic, or nonmagnetic phases in
the respective bulk phase diagrams of iron, cobalt, and nickel. Compared to the
experimental data, available density functional theory calculations generally
seem to underestimate the spin magnetic moments significantly. In all clusters
investigated, the orbital magnetic moment is quenched to \,\% of the
atomic value by the reduced symmetry of the crystal field. The magnetic
anisotropy energy is well below 65 eV per atom
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