651 research outputs found
Microscopic origin of Heisenberg and non-Heisenberg exchange interactions in ferromagnetic bcc Fe
By means of first principles calculations we investigate the nature of
exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the
basic electronic structure reveals a drastic difference between the
orbitals of and symmetries. The latter ones define the shape of
the Fermi surface, while the former ones form weakly-interacting impurity
levels. We demonstrate that, as a result of this, in Fe the orbitals
participate in exchange interactions, which are only weakly dependent on the
configuration of the spin moments and thus can be classified as
Heisenberg-like. These couplings are shown to be driven by Fermi surface
nesting. In contrast, for the states the Heisenberg picture breaks down,
since the corresponding contribution to the exchange interactions is shown to
strongly depend on the reference state they are extracted from. Our analysis of
the nearest-neighbour coupling indicates that the interactions among
states are mainly proportional to the corresponding hopping integral and thus
can be attributed to be of double-exchange origin.Comment: 5 pages, 4 figure
The Fermi Surface Effect on Magnetic Interlayer Coupling
The oscillating magnetic interlayer coupling of Fe over spacer layers
consisting of CuPd alloys is investigated by first principles
density functional theory. The amplitude, period and phase of the coupling, as
well as the disorder-induced decay, are analyzed in detail and the consistency
to the Ruderman-Kittel-Kasuya-Yoshida (RKKY) theory is discussed. For the first
time an effect of the Fermi surface nesting strength on the amplitude is
established from first principles calculations. An unexpected variation of the
phase and disorder-induced decay is obtained and the results are discussed in
terms of asymptotics
Role of innate signalling pathways in the immunogenicity of alphaviral replicon-based vaccines
<p>Abstract</p> <p>Background</p> <p>Alphaviral replicon-based vectors induce potent immune responses both when given as viral particles (VREP) or as DNA (DREP). It has been suggested that the strong immune stimulatory effect induced by these types of vectors is mediated by induction of danger signals and activation of innate signalling pathways due to the replicase activity. To investigate the innate signalling pathways involved, mice deficient in either toll-like receptors or downstream innate signalling molecules were immunized with DREP or VREP.</p> <p>Results</p> <p>We show that the induction of a CD8<sup>+ </sup>T cell response did not require functional TLR3 or MyD88 signalling. However, IRF3, converging several innate signalling pathways and important for generation of pro-inflammatory cytokines and type I IFNs, was needed for obtaining a robust primary immune response. Interestingly, type I interferon (IFN), induced by most innate signalling pathways, had a suppressing effect on both the primary and memory T cell responses after DREP and VREP immunization.</p> <p>Conclusions</p> <p>We show that alphaviral replicon-based vectors activate multiple innate signalling pathways, which both activate and restrict the induced immune response. These results further show that there is a delicate balance in the strength of innate signalling and induction of adaptive immune responses that should be taken into consideration when innate signalling molecules, such as type I IFNs, are used as vaccine adjuvant.</p
Lattice dynamics and reduced thermal conductivity of filled skutterudites
The great reduction in thermal conductivity of skutterudites upon filling the
``void'' sites with Rare Earth (RE) ions is key to their favorable
thermoelectric properties but remains to be understood. Using lattice dynamic
models based on first principles calculations, we address the most popular
microscopic mechanism, reduction via rattling ions. The model withstands
inelastic neutron scattering and specific heat measurements, and refutes
hypotheses of an anharmonic RE potential and of two distinct localized RE
vibrations of disparate frequencies. It does indicate a strong hybridization
between bare La vibrations and certain Sb-like phonon branches, suggesting
anharmonic scattering by harmonic RE motions as an important mechanism for
suppression of heat conductivity.Comment: modified version resubmitted to PRB. Results unchanged, text changed
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Quantum Corrections to the Reissner-Nordstrom and Kerr-Newman Metrics: Spin 1
A previous evaluation of one-photon loop corrections to the energy-momentum
tensor has been extended to particles with unit spin and speculations are
presented concerning general properties of such forms.Comment: 21 pages, 1 Figur
XANES study of rare-earth valency in LRu4P12 (L = Ce and Pr)
Valency of Ce and Pr in LRu4P12 (L = Ce and Pr) was studied by L2,3-edge
x-ray absorption near-edge structure (XANES) spectroscopy. The Ce-L3 XANES
spectrum suggests that Ce is mainly trivalent, but the 4f state strongly
hybridizes with ligand orbitals. The band gap of CeRu4P12 seems to be formed by
strong hybridization of 4f electrons. Pr-L2 XANES spectra indicate that Pr
exists in trivalent state over a wide range in temperature, 20 < T < 300 K. We
find that the metal-insulator (MI) transition at TMI = 60 K in PrRu4P12 does
not originate from Pr valence fluctuation.Comment: 4 page
Tachyonic perturbations in AdS orbifolds
We show that scalar as well as vector and tensor metric perturbations in the
Randall-Sundrum II braneworld allow normalizable tachyonic modes, i.e.,
possible instabilities. These instabilities require nonvanishing initial
anisotropic stresses on the brane. We show with a specific example that within
the Randall-Sundrum II model, even though the tachyonic modes are excited, no
instability develops. We argue, however, that in the cosmological context
instabilities might in principle be present. We conjecture that the tachyonic
modes are due to the singularity of the orbifold construction. We illustrate
this with a simple but explicit toy model.Comment: 11 pages, matches published versio
Conformally flat spacetimes and Weyl frames
We discuss the concepts of Weyl and Riemann frames in the context of metric
theories of gravity and state the fact that they are completely equivalent as
far as geodesic motion is concerned. We apply this result to conformally flat
spacetimes and show that a new picture arises when a Riemannian spacetime is
taken by means of geometrical gauge transformations into a Minkowskian flat
spacetime. We find out that in the Weyl frame gravity is described by a scalar
field. We give some examples of how conformally flat spacetime configurations
look when viewed from the standpoint of a Weyl frame. We show that in the
non-relativistic and weak field regime the Weyl scalar field may be identified
with the Newtonian gravitational potential. We suggest an equation for the
scalar field by varying the Einstein-Hilbert action restricted to the class of
conformally-flat spacetimes. We revisit Einstein and Fokker's interpretation of
Nordstr\"om scalar gravity theory and draw an analogy between this approach and
the Weyl gauge formalism. We briefly take a look at two-dimensional gravity as
viewed in the Weyl frame and address the question of quantizing a conformally
flat spacetime by going to the Weyl frame.Comment: LATEX - 18 page
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