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
Centennial-scale variability of the Southern Hemisphere westerly wind belt in the eastern Pacific over the past two millennia
We present the first high-resolution (sub-annual) dust particle data set from West Antarctica, developed from the West Antarctic Ice Sheet (WAIS) Divide deep ice core (79.468° S, 112.086° W), and use it to reconstruct changes in atmospheric circulation over the past 2400 years. We find a background dust flux of ~4 mg mâ2 yearâ1 and a mode particle size of 5â8 ÎŒm diameter. Through comparing the WAIS Divide record with other Antarctic ice core particle records, we observe that coastal and lower-elevation sites have higher dust fluxes and coarser particle size distributions (PSDs) than sites on the East Antarctic plateau, suggesting input from local dust sources at these lower-elevation sites. In order to explore the use of the WAIS Divide dust PSD as a proxy for past atmospheric circulation, we make quantitative comparisons between both mid-latitude zonal wind speed and West Antarctic meridional wind speed and the dust size record, finding significant positive interannual relationships. We find that the dust PSD is related to mid-latitude zonal wind speed via cyclonic activity in the Amundsen Sea region. Using our PSD record, and through comparison with spatially distributed climate reconstructions from the Southern Hemisphere (SH) middle and high latitudes, we infer that the SH westerlies occupied a more southerly position from circa 1050 to 1400 CE (Common Era), coinciding with the Medieval Climate Anomaly (MCA). Subsequently, at ca. 1430 CE, the wind belt shifted equatorward, where it remained until the mid-to-late twentieth century. We find covariability between reconstructions of El NiñoâSouthern Oscillation (ENSO) and the mid-latitude westerly winds in the eastern Pacific, suggesting that centennial-scale circulation changes in this region are strongly influenced by the tropical Pacific. Further, we observe increased coarse particle deposition over the past 50 years, consistent with observations that the SH westerlies have been shifting southward and intensifying in recent decades
Rapid Microwave Preparation of Thermoelectric TiNiSn and TiCoSb Half-Heusler Compounds
The 18-electron ternary intermetallic systems TiNiSn and TiCoSb are promising for applications as high-temperature thermoelectrics and comprise earth-abundant, and relatively nontoxic elements. Heusler and half-Heusler compounds are usually prepared by conventional solid state methods involving arc-melting and annealing at high temperatures for an extended period of time. Here, we report an energy-saving preparation route using a domestic microwave oven, reducing the reaction time significantly from more than a week to one minute. A microwave susceptor material rapidly heats the elemental starting materials inside an evacuated quartz tube resulting in near single phase compounds. The initial preparation is followed by a densification step involving hot-pressing, which reduces the amount of secondary phases, as verified by synchrotron X-ray diffraction, leading to the desired half-Heusler compounds, demonstrating that hot-pressing should be treated as part of the preparative process. For TiNiSn, high thermoelectric power factors of 2 mW/mK^2 at temperatures in the 700 to 800 K range, and zT values of around 0.4 are found, with the microwave-prepared sample displaying somewhat superior properties to conventionally prepared half-Heuslers due to lower thermal conductivity. The TiCoSb sample shows a lower thermoelectric figure of merit when prepared using microwave methods because of a metallic second phase
Superheavy dark matter and ultrahigh energy cosmic rays
The phase of inflationary expansion in the early universe produces superheavy
relics in a mass window between 10^{12} GeV and 10^{14} GeV. Decay or
annihilation of these superheavy relics can explain the observed ultrahigh
energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff. We emphasize that
the pattern of cosmic ray arrival directions with energies beyond 20 EeV will
decide between the different proposals for the origin of ultrahigh energy
cosmic rays.Comment: Based on an invited talk given by RD at Theory Canada 1, Vancouver,
June 2-5, 200
Observation of Electron Clouds in the ANKA Undulator by Means of the Microwave Transmission Method
A superconducting undulator is installed in the ANKA electron storage ring. Electron clouds could potentially contribute to the heat load of this device. A microwave transmission type electron cloud diagnostic has been installed for the undulator section of the ANKA machine. We present the system layout with particular emphasis on the electron machine aspects. Hardware transfer function results and e-cloud data for different machine settings are discussed. Special care has been taken for front end filter design both on the microwave injection and pick-up side
Twentieth century dust lows and the weakening of the westerly winds over the Tibetan Plateau
Understanding past atmospheric dust variability is necessary to put modern atmospheric dust into historical context and assess the impacts of dust on the climate. In Asia, meteorological data of atmospheric dust is temporally limited, beginning only in the 1950s. Highâresolution ice cores provide the ideal archive for reconstructing preinstrumental atmospheric dust concentrations. Using a ~500âyear (1477â1982âA.D.) annually resolved calcium (Ca) dust proxy from a Tibetan Plateau (TP) ice core, we demonstrate the lowest atmospheric dust concentrations in the past ~500âyears during the latter twentieth century. Declines in late nineteenth to twentieth century Ca concentrations significantly correspond with regional zonal wind trends from two reanalysis models, suggesting that the Ca record provides a proxy for the westerlies. Twentieth century warming and attendant atmospheric pressure reductions over northern Asia have potentially reduced temperature/pressure gradients resulting in lower zonal wind velocities and associated dust entrainment/transport in the past ~500âyears over the TP
High Energy Neutrinos From Superheavy Dark Matter Annihilation
Superheavy ( GeV) particles produced during inflation may be the
dark matter, independent of their interaction strength. Strongly interacting
superheavy particles will be captured by the sun, and their annihilation in the
center of the sun will produce a flux of energetic neutrinos that should be
detectable by neutrino telescopes. Depending on the particle mass, event rates
in a cubic-kilometer detector range from several per hour to several per year.
The signature of the process is a predominance of tau neutrinos, with a
relatively flat energy spectrum of events ranging from 50 GeV to many TeV, and
with the mean energy of detected tau neutrinos about 3 TeV.Comment: 24 pages, 7 figure
Massless Metric Preheating
Can super-Hubble metric perturbations be amplified exponentially during
preheating ? Yes. An analytical existence proof is provided by exploiting the
conformal properties of massless inflationary models. The traditional conserved
quantity \zeta is non-conserved in many regions of parameter space. We include
backreaction through the homogeneous parts of the inflaton and preheating
fields and discuss the role of initial conditions on the post-preheating
power-spectrum. Maximum field variances are strongly underestimated if metric
perturbations are ignored. We illustrate this in the case of strong
self-interaction of the decay products. Without metric perturbations,
preheating in this case is very inefficient. However, metric perturbations
increase the maximum field variances and give alternative channels for the
resonance to proceed. This implies that metric perturbations can have a large
impact on calculations of relic abundances of particles produced during
preheating.Comment: 8 pages, 4 colour figures. Version to appear in Phys. Rev. D.
Contains substantial new analysis of the ranges of parameter space for which
large changes to the inflation-produced power spectrum are expecte
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
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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