621 research outputs found
Search of low-dimensional magnetics on the basis of structural data: spin-1/2 antiferromagnetic zigzag chain compounds In2VO5, beta-Sr(VOAsO4)2,(NH4,K)2VOF4 and alpha-ZnV3O8
A new technique for searching low-dimensional compounds on the basis of
structural data is presented. The sign and strength of all magnetic couplings
at distances up to 12 A in five predicted new antiferromagnetic zigzag spin-1/2
chain compounds In2VO5, beta-Sr(VOAsO4)2, (NH4)2VOF4, K2VOF4 and alpha-ZnV3O8
were calculated. It was stated that in the compound In2VO5 zigzag spin chains
are frustrated, since the ratio (J2/J1) of competing antiferromagnetic (AF)
nearest- (J1) and AF next-to-nearest-neighbour (J2) couplings is equal to 1.68
that exceeds the Majumdar-Ghosh point by 1/2. In other compounds the zigzag
spin chains are AF magnetically ordered single chains as value of ratios J2/J1
is close to zero. The interchain couplings were analyzed in detail.Comment: 14 pages, 6 figure, 1 table, minor change
Radiography of the Earth's Core and Mantle with Atmospheric Neutrinos
A measurement of the absorption of neutrinos with energies in excess of 10
TeV when traversing the Earth is capable of revealing its density distribution.
Unfortunately, the existence of beams with sufficient luminosity for the task
has been ruled out by the AMANDA South Pole neutrino telescope. In this letter
we point out that, with the advent of second-generation kilometer-scale
neutrino detectors, the idea of studying the internal structure of the Earth
may be revived using atmospheric neutrinos instead.Comment: 4 pages, LaTeX file using RevTEX4, 2 figures and 1 table included.
Matches published versio
Factors Affecting the Improvement of District Heating. Case Studies of Estonia and Serbia
© 2020 Pavel Rušeljuk et al., published by Sciendo 2020. Factors affecting both the Estonian and Serbian district heating improvement are analysed, including geographical and climate factors, as well as economic and legal factors. This analysis is added by evaluation of main technical and economic parameters related to the district heating networks from the case studies (Estonian Narva city and Serbian Kragujevac district heating). This analysis uncovered the weakest points of Kragujevac district heating and explain why district heating is not considered as sustainable and environmentally friendly heating option
Quasiparticle dynamics and phonon softening in FeSe superconductors
Quasiparticle dynamics of FeSe single crystals revealed by dual-color
transient reflectivity measurements ({\Delta}R/R) provides unprecedented
information on Fe-based superconductors. The amplitude of fast component in
{\Delta}R/R clearly tells a competing scenario between spin fluctuations and
superconductivity. Together with the transport measurements, the relaxation
time analysis further exhibits anomalous changes at 90 K and 230 K. The former
manifests a structure phase transition as well as the associated phonon
softening. The latter suggests a previously overlooked phase transition or
crossover in FeSe. The electron-phonon coupling constant {\lambda} is found to
be 0.16, identical to the value of theoretical calculations. Such a small
{\lambda} demonstrates an unconventional origin of superconductivity in FeSe.Comment: Final published version; 5 pages; 4 figure
Prompt neutrino fluxes from atmospheric charm
We calculate the prompt neutrino flux from atmospheric charm production by
cosmic rays, using the dipole picture in a perturbative QCD framework, which
incorporates the parton saturation effects present at high energies. We compare
our results with the next-to-leading order perturbative QCD result and find
that saturation effects are large for neutrino energies above 10^6 GeV, leading
to a substantial suppression of the prompt neutrino flux. We comment on the
range of prompt neutrino fluxes due to theoretical uncertainties.Comment: 13 pages with 11 figures; expanded discussion, added references,
version to be published in Phys. Rev.
Granular Rheology in Zero Gravity
We present an experimental investigation on the rheological behavior of model
granular media made of nearly elastic spherical particles. The experiments are
performed in a cylindrical Couette geometry and the experimental device is
placed inside an airplane undergoing parabolic flights to cancel the effect of
gravity. The corresponding curves, shear stress versus shear rate, are
presented and a comparison with existing theories is proposed. The quadratic
dependence on the shear rate is clearly shown and the behavior as a function of
the solid volume fraction of particles exhibits a power law function. It is
shown that theoretical predictions overestimate the experiments. We observe, at
intermediate volume fractions, the formation of rings of particles regularly
spaced along the height of the cell. The differences observed between
experimental results and theoretical predictions are discussed and related to
the structures formed in the granular medium submitted to the external shear.Comment: 10 pages, 6 figures to be published in Journal of Physics : Condensed
Matte
Lepton Fluxes from Atmospheric Charm
We reexamine the charm contribution to atmospheric lepton fluxes in the
context of perturbative QCD. We include next-to-leading order corrections and
discuss theoretical uncertainties due to the extrapolations of the gluon
distributions at small-x. We show that the charm contribution to the
atmospheric muon flux becomes dominant over the conventional contribution from
pion and kaon decays at energies of about 10^5 GeV. We compare our fluxes with
previous calculations.Comment: 19 pages, latex, revtex, psfi
Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry
In this paper we examine the role of crystal chemistry factors in creating
conditions for formation of magnetoelectric ordering in BiFeO3. It is generally
accepted that the main reason of the ferroelectric distortion in BiFeO3 is
concerned with a stereochemical activity of the Bi lone pair. However, the lone
pair is stereochemically active in the paraelectric orthorhombic beta-phase as
well. We demonstrate that a crucial role in emerging of phase transitions of
the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order
types belongs to the change of the degree of the lone pair stereochemical
activity - its consecutive increase with the temperature decrease. Using the
structural data, we calculated the sign and strength of magnetic couplings in
BiFeO3 in the range from 945 C down to 25 C and found the couplings, which
undergo the antiferromagnetic-ferromagnetic transition with the temperature
decrease and give rise to the antiferromagnetic ordering and its delay in
regard to temperature, as compared to the ferroelectric ordering. We discuss
the reasons of emerging of the spatially modulated spin structure and its
suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table
Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn2O5 and BiMn2O5
The origin of magnetic frustration was stated and the ions whose shift is
accompanied by emerging magnetic ordering and ferroelectricity in TbMn2O5 and
BiMn2O5 were determined on the basis of calculation of magnetic coupling
parameters by using the structural data. The displacements accompanying the
magnetic ordering are not polar, they just induce changes of bond valence
(charge disordering) of Mn1 and Mn2, thus creating instability of the crystal
structure. To approximate again the bond valence to the initial value (charge
ordering) under magnetic ordering conditions is possible only due to polar
displacement of Mn2 (or O1) and O4 ions along the b axis that is the cause of
ferroelectric transition.Comment: 17 pages, 3 figures, 5 table
NEUTRINOS FROM PRIMORDIAL BLACK HOLES
The emission of particles from black holes created in the early Universe has
detectable astrophysical consequences. The most stringent bound on their
abundance has been obtained from the absence of a detectable diffuse flux of
100 MeV photons. Further scrutiny of these bounds is of interest as they, for
instance, rule out primordial black holes as a dark matter candidate. We here
point out that these bounds can, in principle, be improved by studying the
diffuse cosmic neutrino flux. Measurements of near-vertical atmospheric
neutrino fluxes in a region of low geomagnetic latitude can provide a
competitive bound. The most favorable energy to detect a possible diffuse flux
of primordial black hole origin is found to be a few MeV. We also show that
measurements of the diffuse flux is the most promising to improve
the existing bounds deduced from gamma-ray measurements. Neutrinos from
individual black hole explosions can be detected in the GeV-TeV energy region.
We find that the kilometer-scale detectors, recently proposed, are able to
establish competitive bounds.Comment: 19 pages plus 9 uuencoded and compressed postscript figure
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