175 research outputs found
Model of graviton-dusty universe
Primary features of a new cosmological model, which is based on conjectures
about an existence of the graviton background and superstrong gravitational
quantum interaction, are considered. An expansion of the universe is impossible
in such the model because of deceleration of massive objects by the graviton
background, which is similar to the one for the NASA deep space probes Pioneer
10, 11. Redshifts of remote objects are caused in the model by interaction of
photons with the graviton background, and the Hubble constant depends on an
intensity of interaction and an equivalent temperature of the graviton
background. Virtual massive gravitons would be dark matter particles. They
transfer energy, lost by luminous matter radiation, which in a final stage may
be collected with black holes and other massive objects.Comment: Contribution to the 15th SIGRAV Conference on General Relativity and
Gravitational Physics, September 9-12, 2002, Rome, Ital
Structural dichroism in the antiferromagnetic insulating phase of V_2O_3
We performed near-edge x-ray absorption spectroscopy (XANES) at V K edge in
the antiferromagnetic insulating (AFI) phase of a 2.8% Cr-doped V_2O_3 single
crystal. Linear dichroism of several percent is measured in the hexagonal plane
and found to be in good agreement with ab-initio calculations based on multiple
scattering theory. This experiment definitively proves the structural origin of
the signal and therefore solves a controversy raised by previous
interpretations of the same dichroism as non-reciprocal. It also calls for a
further investigation of the role of the magnetoelectric annealing procedure in
cooling to the AFI phase.Comment: 4 pages 3 figures. To be published in Phys. Rev. B (2005
Orbital currents, anapoles, and magnetic quadrupoles in CuO
We show that orbital currents in a CuO2 plane, if present, should be
described by two independent parity and time-reversal odd order parameters, a
toroidal dipole (anapole) and a magnetic quadrupole. Based on this, we derive
the resonant X-ray diffraction cross-section for monoclinic CuO at the
antiferromagnetic wavevector and show that the two order parameters can be
disentangled. From our analysis, we examine a recent claim of detecting
anapoles in CuO.Comment: 7 pages, 5 figure
Nearby quasar remnants and ultra-high energy cosmic rays
As recently suggested, nearby quasar remnants are plausible sites of
black-hole based compact dynamos that could be capable of accelerating
ultra-high energy cosmic rays (UHECRs). In such a model, UHECRs would originate
at the nuclei of nearby dead quasars, those in which the putative underlying
supermassive black holes are suitably spun-up. Based on galactic optical
luminosity, morphological type, and redshift, we have compiled a small sample
of nearby objects selected to be highly luminous, bulge-dominated galaxies,
likely quasar remnants. The sky coordinates of these galaxies were then
correlated with the arrival directions of cosmic rays detected at energies EeV. An apparently significant correlation appears in our data. This
correlation appears at closer angular scales than those expected when taking
into account the deflection caused by typically assumed IGM or galactic
magnetic fields over a charged particle trajectory. Possible scenarios
producing this effect are discussed, as is the astrophysics of the quasar
remnant candidates. We suggest that quasar remnants be also taken into account
in the forthcoming detailed search for correlations using data from the Auger
Observatory.Comment: 2 figures, 4 tables, 11 pages. Final version to appear in Physical
Review
On the nature of the magnetic ground-state wave function of V_2O_3
After a brief historical introduction, we dwell on two recent experiments in
the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray
scattering and non-reciprocal linear dichroism, whose interpretations are in
conflict, as they require incompatible magnetic space groups. Such a conflict
is critically reviewed, in the light of the present literature, and new
experimental tests are suggested, in order to determine unambiguously the
magnetic group. We then focus on the correlated, non-local nature of the
ground-state wave function, that is at the basis of some drawbacks of the LDA+U
approach: we singled out the physical mechanism that makes LDA+U unreliable,
and indicate the way out for a possible remedy. Finally we explain, by means of
a symmetry argument related to the molecular wave function, why the magnetic
moment lies in the glide plane, even in the absence of any local symmetry at
vanadium sites.Comment: 7 pages, 1 figur
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