195 research outputs found
Structural disorder, magnetism, and electrical and thermoelectric properties of pyrochlore Nd2Ru2O7
Polycrystalline Nd2Ru2O7 samples have been prepared and examined using a
combination of structural, magnetic, and electrical and thermal transport
studies. Analysis of synchrotron X-ray and neutron diffraction patterns
suggests some site disorder on the A-site in the pyrochlore sublattice: Ru
substitutes on the Nd-site up to 7.0(3)%, regardless of the different
preparative conditions explored. Intrinsic magnetic and electrical transport
properties have been measured. Ru 4d spins order antiferromagnetically at 143 K
as seen both in susceptibility and specific heat, and there is a corresponding
change in the electrical resistivity behaviour. A second antiferromagnetic
ordering transition seen below 10 K is attributed to ordering of Nd 4f spins.
Nd2Ru2O7 is an electrical insulator, and this behaviour is believed to be
independent of the Ru-antisite disorder on the Nd site. The electrical
properties of Nd2Ru2O7 are presented in the light of data published on all
A2Ru2O7 pyrochlores, and we emphasize the special structural role that Bi3+
ions on the A-site play in driving metallic behaviour. High-temperature
thermoelectric properties have also been measured. When considered in the
context of known thermoelectric materials with useful figures-of-merit, it is
clear that Nd2Ru2O7 has excessively high electrical resistivity which prevents
it from being an effective thermoelectric. A method for screening candidate
thermoelectrics is suggested.Comment: 19 pages, 10 figure
On The Injection Spectrum of Ultrahigh Energy Cosmic Rays in the Top-Down Scenario
We analyze the uncertainties involved in obtaining the injection spectra of
UHECR particles in the top-down scenario of their origin. We show that the
DGLAP evolution of fragmentation functions (FF) to (mass of the X
particle) from their initial values at low is subject to considerable
uncertainties. We therefore argue that, for x\lsim 0.1 (the region of
interest for most large values of interest, being the
scaled energy variable), the FF obtained from DGLAP evolution is no more
reliable than that provided, for example, by a simple Gaussian form (in the
variable ) obtained under the Modified Leading Log Approximation
(MLLA). Additionally, we find that for x\gsim0.1, the evolution in of
the singlet FF, which determines the injection spectrum, is ``minimal'' -- the
singlet FF changes by barely a factor of 2 after evolving it over 14
orders of magnitude in . We, therefore, argue that as long as the
measurement of the UHECR spectrum above \sim10^{20}\ev is going to remain
uncertain by a factor of 2 or larger, it is good enough for most practical
purposes to directly use any one of the available initial parametrisations of
the FFs in the region x\gsim0.1 based on low energy data even without
evolving them to the requisite value.Comment: Minor changes, added a reference, version to appear in Phys. Rev.
Unstable superheavy relic particles as a source of neutrinos responsible for the ultrahigh-energy cosmic rays
Decays of superheavy relic particles may produce extremely energetic
neutrinos. Their annihilations on the relic neutrinos can be the origin of the
cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin cutoff. The red
shift acts as a cosmological filter selecting the sources at some particular
value z_e, for which the present neutrino energy is close to the Z pole of the
annihilation cross section. We predict no directional correlation of the
ultrahigh-energy cosmic rays with the galactic halo. At the same time, there
can be some directional correlations in the data, reflecting the distribution
of matter at red shift z=z_e. Both of these features are manifest in the
existing data. Our scenario is consistent with the neutrino mass reported by
Super-Kamiokande and requires no lepton asymmetry or clustering of the
background neutrinos.Comment: 3 pages, revtex; references adde
Neutrino cross sections at high energies and the future observations of ultrahigh-energy cosmic rays
We show that future detectors of ultrahigh-energy cosmic-ray neutrinos will
be able to measure neutrino-nucleon cross section at energies as high as
10^{11}GeV or higher. We find that the flux of up-going charged leptons per
unit surface area produced by neutrino interactions below the surface is
inversely proportional to the cross section. This contrasts with the rate of
horizontal air showers (HAS) due to neutrino interactions in the atmosphere,
which is proportional to the cross section. Thus, by comparing the HAS and
up-going air shower (UAS) rates, the neutrino-nucleon cross section can be
inferred. Taken together, up-going and horizontal rates ensure a healthy total
event rate, regardless of the value of the cross section.Comment: 4 pages, 2 figures, revtex; final draf
Superheavy Dark Matter and Thermal Inflation
The thermal inflation is the most plausible mechanism that solves the
cosmological moduli problem naturally. We discuss relic abundance of superheavy
particle in the presence of the thermal inflation assuming that its
lifetime is longer than the age of the universe, and show that the long-lived
particle of mass -- GeV may form a part of the dark
matter in the present universe in a wide region of parameter space of the
thermal inflation model. The superheavy dark matter of mass GeV
may be interesting in particular, since its decay may account for the observed
ultra high-energy cosmic rays if the lifetime of the particle is
sufficiently long.Comment: 13 pages (RevTex file) including 8 figures, revised version to be
published in Physical Review
Lorentz invariance violation in top-down scenarios of ultrahigh energy cosmic ray creation
The violation of Lorentz invariance (LI) has been invoked in a number of ways
to explain issues dealing with ultrahigh energy cosmic ray (UHECR) production
and propagation. These treatments, however, have mostly been limited to
examples in the proton-neutron system and photon-electron system. In this paper
we show how a broader violation of Lorentz invariance would allow for a series
of previously forbidden decays to occur, and how that could lead to UHECR
primaries being heavy baryonic states or Higgs bosons.Comment: Replaced with heavily revised (see new Abstract) version accepted by
Phys. Rev. D. 6 page
Quintessence Restrictions on Negative Power and Condensate Potentials
We study the cosmological evolution of scalar fields that arise from a phase
transition at some energy scale \Lm_c. We focus on negative power potentials
given by V=c\Lm_c^{4+n}\phi^{-n} and restrict the cosmological viable values
of \Lm_c and . We make a complete analysis of and impose
conditions on the different cosmological parameters. The cosmological
observations ruled out models where the scalar field has reached its attractor
solution. For models where this is not the case, the analytic approximated
solutions are not good enough to determine whether a specific model is
phenomenologically viable or not and the full differential equations must be
numerically solved. The results are not fine tuned since a change of 45% on the
initial conditions does not spoil the final results. We also determine the
values of that give a condensation scale \Lm_c consistent with
gauge coupling unification, leaving only four models that satisfy unification
and SN1a constraints.Comment: 15 pages, LaTeX, 8 Figures. Minor changes in text, a discussion on
initial conditions added (accepted in Phys.Rev.D
Air fluorescence measurements in the spectral range 300-420 nm using a 28.5 GeV electron beam
Measurements are reported of the yield and spectrum of fluorescence, excited
by a 28.5 GeV electron beam, in air at a range of pressures of interest to
ultra-high energy cosmic ray detectors. The wavelength range was 300 - 420 nm.
System calibration has been performed using Rayleigh scattering of a nitrogen
laser beam. In atmospheric pressure dry air at 304 K the yield is 20.8 +/- 1.6
photons per MeV.Comment: 29 pages, 10 figures. Submitted to Astroparticle Physic
Natural Quintessence with Gauge Coupling Unification
We show that a positive accelerating universe can be obtained simply by the
dynamics of a non-abelian gauge group. It is the condensates of the chiral
fields that obtain a negative power potential, below the condensation scale,
and allow for a quintessence interpretation of these fields. The only free
parameters in this model are and and the number of dynamically
gauge singlet bilinear fields generated below the condensation scale. We
show that it is possible to have unification of all coupling constants,
including the standard and non standard model couplings, while having an
acceptable phenomenology of as the cosmological constant. This is done
without any fine tuning of the initial conditions. The problem of coincidence
(why the universe has only recently started an accelerating period) is not
solved but it is put at the same level as what the particle content of the
standard model is.Comment: minor changes(discussion on field normalization included), reference
added, accepted in Phy.Rev.Lett., 5 pages,LateX,2 Figure
Long-Lived Superheavy Particles in Dynamical Supersymmetry-Breaking Models in Supergravity
Superheavy particles of masses with lifetimes
are very interesting, since their decays may
account for the ultra-high energy (UHE) cosmic rays discovered beyond the
Greisen-Zatsepin-Kuzmin cut-off energy . We show
that the presence of such long-lived superheavy particles is a generic
prediction of QCD-like SU(N_c) gauge theories with N_f flavors of quarks and
antiquarks and the large number of colors N_c. We construct explicit models
based on supersymmetric SU(N_c) gauge theories and show that if the dynamical
scale and N_c = 6-10 the lightest
composite baryons have the desired masses and lifetimes to explain the UHE
cosmic rays. Interesting is that in these models the gaugino condensation
necessarily occurs and hence these models may play a role of so-called hidden
sector for supersymmetry breaking in supergravity.Comment: 13 pages, Late
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