We consider decaying dark matter with masses 107≲M≲1016 GeV as a source of ultrahigh energy (UHE) gamma rays. Using recent limits on UHE gamma-ray flux for energies Eγ>2×1014 eV, provided by ...extensive air shower observatories, we put limits on masses and lifetimes of the dark matter. We also discuss possible dark matter decay origin of tentative 100 PeV photon flux detected with the EAS-MSU experiment.
We examine the hypothesis of decaying heavy dark matter (HDM) in the context of the IceCube highest energy neutrino events and recent limits on the diffuse flux of high-energy photons. We consider ...dark matter (DM) particles X of mass 106?MX?1016 GeV decaying on tree level into X???¯, X?e+e?, and X?qq¯. The full simulation of hadronic and electroweak decay cascades and the subsequent propagation of the decay products through the interstellar medium allows us to determine the permitted values of MX. We show that for leptonic decay channels it is possible to explain the IceCube highest energy neutrino signal without overproducing high-energy photons for MX?5.5×107 GeV and 1.5×108?MX?1.5×109 GeV, while hadronic decays contradict the gamma-ray limits for almost the whole range of MX values considered. The leptonic hypothesis can be probed by operating and planned gamma-ray observatories: For instance, the currently upgrading Carpet experiment will be capable to test a significant part of the remaining parameter window within one year of observation.
The effects of generating pulsed radiation by a long spark discharge are important for the development of lightning models and applications related to lightning protection. In experiments with a Marx ...generator simulating a lightning discharge, we detected the radiation in the form of a single ultrawideband electromagnetic pulse (UWB EMP) about 200 ps in duration, and rising time about 100 ps. UWB EMP generation occurs during the breakdown of a “rod–rod” 4 m long gap. Pulses of almost unipolar shape are observed in more than half of all positive discharges. EMP emission occurs before the main stage, and corresponds to the start of the upward leader from a grounded electrode. In negative discharges, pulses are also observed, but less frequently and with a smaller amplitude. The UWB EMPs, given their large amplitude (more than 100 V/m at a distance of 90 m from the discharge), can be considered as possible new lightning damage factors.
Plain Language Summary
A Marx generator with an operating voltage of several megavolts is used to simulate a lightning stroke of a grounded object. The generation of a short electromagnetic pulse (several hundreds of picoseconds long) has been detected during a breakdown of a long air gap using special electromagnetic probes. Such a pulse is observed in the majority of positive discharges in the “rod–rod” gap. Subnanosecond pulse can be regarded as possible new damage factor of lightning strokes.
Key Points
Marx generator allows studying generation of ultra‐wideband electromagnetic pulses (EMPs) by long spark discharges that model lightning
Single EMP about 200 ps duration is generated by 4‐m long spark discharge in a rod‐rod geometry
Pulses of almost unipolar shape with up to 200 V/m amplitude are observed at 90 m distance from more than half of all positive discharges
We consider dark matter consisting of long-living particles with masses 10
7
GeV ≲
M
≲10
16
GeV decaying through hadronic channel as a source of high-energy neutrino. Using recent data on high-energy ...neutrino from IceCube and Pierre Auger experiments, we derive the upper-limits on neutrino flux from dark matter decay and constraints on dark matter parameter space. For the dark matter masses of order 10
8
GeV the constraints derived are slightly stronger than those obtained for the same dark matter model using the highenergy gamma-ray limits.
The single-walled carbon nanotube-based thin films with a thickness from 11 ± 3 to 157 ± 18 nm have been formed using vacuum filtration. The thermal conductivity of the thin films as a function of ...thickness and temperature up to 450 K has been studied by the 3ω technique. It has been found that, in the region of 49 nm, the supplied heat from a gold strip started propagating with the high efficiency to the thin film plane. The thermal conductivity of the thin films with a thickness of 49 ± 8 nm was measured using the 3ω technique for bulk samples. It has been found that the thermal conductivity of the single-walled carbon nanotube-based thin films strongly depends on their thickness and temperature. The thermal conductivity sharply (by a factor of ~60) increases with an increase in thickness from 11 ± 3 to 65 ± 4 nm. In addition, it has been observed that the thermal conductivity of the thin film with a thickness of 157 ± 18 nm rapidly decreases from 211 ± 11 to 27.5 ± 1.4 W m
–1
K
–1
at 300 and 450 K, respectively.
The layers of a high-temperature novel GaAs:Fe diluted magnetic semiconductor (DMS) with an average Fe content up to 20 at. % were grown on (001) i-GaAs substrates using a pulsed laser deposition in ...a vacuum. The transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy investigations revealed that the conductive layers obtained at 180 and 200 ºC are epitaxial, do not contain any second-phase inclusions, but contain the Fe-enriched columnar regions of overlapped microtwins. The TEM investigations of the non-conductive layer obtained at 250 ºC revealed the embedded coherent Fe-rich clusters of GaAs:Fe DMS. The x-ray photoelectron spectroscopy investigations showed that Fe atoms form chemical bonds with Ga and As atoms with almost equal probability and thus the comparable number of Fe atoms substitute on Ga and As sites. The n-type conductivity of the obtained conductive GaAs:Fe layers is apparently associated with electron transport in a Fe acceptor impurity band within the GaAs band gap. A hysteretic negative magnetoresistance (MR) was observed in the conductive layers up to room temperature (RT). MR measurements point to the out-of-plane magnetic anisotropy of the conductive GaAs:Fe layers related to the presence of the columnar regions. The studies of the magnetic circular dichroism confirm that the layers obtained at 180, 200 and 250 ºC are intrinsic ferromagnetic semiconductors and the Curie point can reach up to at least RT in case of the conductive layer obtained at 200 ºC. It was suggested that in heavily Fe-doped GaAs layers the ferromagnetism is related to the Zener double exchange between Fe atoms with different valence states via an intermediate As and Ga atom.
Results of the search for ∼(1016–1017.5) eV primary cosmic-ray photons with the data of the Moscow State University (MSU) Extensive Air Shower (EAS) array are reported. The full-scale reanalysis of ...the data with modern simulations of the installation does not confirm previous indications of the excess of gamma-ray candidate events. Upper limits on the corresponding gamma-ray flux are presented. The limits are among the most stringent published ones at energies ∼1017 eV.
•Pb50Se50-xTex have been fabricated by Plasma-enhanced chemical vapor deposition technique.•The morphology of PbSeTe thin films strongly depends as on the composition.•High thermoelectric ...characteristics of PbSeTe thin films are demonstrated.•Strong influence of the composition of the PbSeTe films on the thermoelectric properties.
Ternary chalcogenide PbSeTe thin films with sufficient excess of tellurium over the stoichiometry Pb50Se50-xTex have been fabricated by Plasma-enhanced chemical vapor deposition technique. High-pure elemental lead, tellurium, and selenium were the starting materials, that were supplied by the flow of high-pure argon into the 40 MHz inductively-coupled non-equilibrium plasma discharge at low pressure 13 Pa. Argon also served as the plasma-feed gas. The structural properties and chemical composition of the films were characterized by scanning electron microscope and X-ray diffraction analysis, respectively. The dependence of the Seebeck coefficient, resistivity and thermoelectric power factor on structural properties and composition were investigated. The influence of film composition on thermoelectric characteristics was studied.
The possibility of modifying the properties of a (Ga,Mn)As layer on the surface of a quantum-size InGaAs/GaAs-structure by laser annealing with the conservation of its emitting properties is studied. ...To perform these studies by a combination of the methods of MOC-hydride epitaxy and pulsed laser deposition, the structures have been prepared with four quantum wells InGaAs/GaAs (indium contents from 0.08 to 0.25) located at various distances from the (Ga,Mn)As layer. The radiation energy density of an LPX-200 pulsed excimer laser was varied during the experiments from 200 to 360 mJ/cm
2
, and the depth of the laser action was determined from the changes in the photoluminescence spectra of the quantum wells. The results are described using the laser annealing model based on the solution of the problem of heat propagation in a one-dimensional GaAs system, taking into account a (Ga,Mn)As layer on the surface. The changes in the structural and galvanomagnetic properties of the samples under action of laser irradiation are analyzed. It is shown that the pulsed laser irradiation with the laser radiation energy density 250–300 mJ/cm
2
enable one to conserve the emitting properties of the active region (quantum wells InGaAs/GaAs) disposed at the distances 10–12 nm from the (Ga,Mn)As layer and to modify the ferromagnetic properties of the (Ga,Mn)As semiconductor, namely, to increase the ferromagnet–paramagnet phase transition temperature to values no lower than 120 K. The results are promising for the development of the technology of devices of spin optoelectronics.
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