1,624 research outputs found
Fermionic vacuum currents in topologically nontrivial braneworlds: Two-brane geometry
The vacuum expectation value (VEV) of the fermionic current density is
investigated in the geometry of two parallel branes in locally AdS spacetime
with a part of spatial dimensions compactified to a torus. Along the toral
dimensions quasiperiodicity conditions are imposed with general phases and the
presence of a constant gauge field is assumed. Different types of boundary
conditions are discussed on the branes, including the bag boundary condition
and the conditions arising in -symmetric braneworld models. Nonzero
vacuum currents appear along the compact dimensions only. In the region between
the branes they are decomposed into the brane-free and brane-induced
contributions. Both these contributions are periodic functions of the magnetic
flux enclosed by compact dimensions with the period equal to the flux quantum.
Depending on the boundary conditions, the presence of the branes can either
increase or decrease the vacuum current density. For a part of boundary
conditions, a memory effect is present in the limit when one of the branes
tends to the AdS boundary. Unlike to the fermion condensate and the VEV of the
energy-momentum tensor, the VEV of the current density is finite on the branes.
Applications are given to higher-dimensional generalizations of the
Randall-Sundrum models with two branes and with toroidally compact subspace.
The features of the fermionic current are discussed in odd-dimensional parity
and time-reversal symmetric models. The corresponding results for
three-dimensional spacetime are applied to finite length curved graphene tubes
threaded by a magnetic flux. It is shown that a nonzero current density can
also appear in the absence of the magnetic flux if the fields corresponding to
two different points of the Brillouin zone obey different boundary conditions
on the tube edges.Comment: 33 pages, 7 figures, PACS numbers: 04.62.+v, 03.70.+k, 98.80.-k,
61.46.F
Fermionic currents in topologically nontrivial braneworlds
We investigate the influence of a brane on the vacuum expectation value (VEV)
of the current density for a charged fermionic field in background of locally
AdS spacetime with an arbitrary number of toroidally compact dimensions and in
the presence of a constant gauge field. Along compact dimensions the field
operator obeys quasiperiodicity conditions with arbitrary phases and on the
brane it is constrained by the bag boundary condition. The VEVs for the charge
density and the components of the current density along uncompact dimensions
vanish. The components along compact dimensions are decomposed into the
brane-free and brane-induced contributions. The behavior of the latter in
various asymptotic regions of the parameters is investigated. It particular, it
is shown that the brane-induced contribution is mainly located near the brane
and vanishes on the AdS boundary and on the horizon. An important feature is
the finiteness of the current density on the brane. Applications are given to
-symmetric braneworlds of the Randall-Sundrum type with compact dimensions
for two classes of boundary conditions on the fermionic field. In the special
case of three-dimensional spacetime, the corresponding results are applied for
the investigation of the edge effects on the ground state current density
induced in curved graphene tubes by an enclosed magnetic flux.Comment: 32 pages, 9 figures, PACS numbers: 04.62.+v, 03.70.+k, 98.80.-k,
61.46.F
Observation of Resonant Diffusive Radiation in Random Multilayered Systems
Diffusive Radiation is a new type of radiation predicted to occur in randomly
inhomogeneous media due to the multiple scattering of pseudophotons. This
theoretical effect is now observed experimentally. The radiation is generated
by the passage of electrons of energy 200KeV-2.2MeV through a random stack of
films in the visible light region. The radiation intensity increases resonantly
provided the Cherenkov condition is satisfied for the average dielectric
constant of the medium. The observed angular dependence and electron resonance
energy are in agreement with the theoretical predictions. These observations
open a road to application of diffusive radiation in particle detection,
astrophysics, soft X-ray generation and etc.. `Comment: 4pages, 4figure
Dependence of some physiological indicators of generative and vegetative organs of Sambucus nigra on habitat conditions
We carried out studies on water deficiency of Sambucus nigra L. (black elderberry) plant leaves extract and fruit juice, relative turgidity, sucrose content in fruits, chlorophyll content in the leaves, as well as gas exchange rates in the leaves, in particular, the changes in the concentrations of oxygen released during photosynthesis by leaves and carbon dioxide released during leaf respiration depending on habitat conditions (altitude of the habitat, position of slopes, average annual precipitation). Taking into account the difference in climatic conditions and the generality of some indicators, Vanadzor (1326–1600 m above sea level) and Stepanavan (1400–1830 m above sea level) regions of Lori Province, "Dilijan" National Park, Lake Parz, and the forest areas adjacent to the city of Dilijan (1240–1612 m above sea level) in Tavush Province in Armenia were chosen as the research sites. The test samples were taken from eight different test sites in Lori and Tavush provinces. Tavush Province is more humid than Lori Province. The highest rate of water deficiency in the leaves of the studied plants was recorded in S. nigra f. laciniata (L.) Zabel., and the lowest in S. nigra in Vanadzor forest. It can be assumed that the index of water deficiency in the leaves may depend on the position of the slopes of the test sites and the value of the filtration coefficient of the soil types. On the north-facing slope at the height of 1341 m above sea level and on the south-facing slope at the height of 1830 m above sea level, the values of water deficiency of the leaves are almost the same. Such data allows us to assume that the degree of water deficiency in the leaves is also affected by the filtration coefficient of habitat soil types. In two points that have north-facing positions, the water deficiency of leaves at the height of 1341 m above sea level is 2.14 times more than at the height of 1328 m above sea level. At 1328 m above sea level the filtration coefficient is small and the plant absorbs a lot of water. Based on the data we received we obtained rankings of plants according to the different criteria of the experimental plants: water deficiency in leaves tissues, relative turgidity, the sum of chlorophylls in the leaves, the sucrose content in the fruits, percentage of oxygen released as a result of photosynthesis, percentage of carbon dioxide released as a result of respiration. We found a certain dependence between the indicators of the water regime and the height above sea level of the researched habitats. We also found out that the water regime is influenced by certain microclimatic conditions such as the position of the slopes, whether they are north or south facing, as well as the average annual precipitation. Knowing the physiological processes taking place in the vegetative and generative organs of the S. nigra plant is important from the point of view of evaluating the intensity of growth and development of the generative organs. We consider that the results of the study will be applicable in the process of plant selection based on the indicators of the physiological processes taking place in the plant organism
Experimental Investigation of the Nature of the Knee in the Primary Cosmic Ray Energy Spectrum with the GAMMA experiment
We present preliminary results obtained by a novel difference method for the
study of the nature of the knee in the energy spectrum of the primary cosmic
radiation. We have applied this method to data from the GAMMA experiment in
Armenia. The analysis provides evidence for the possible existence of a nearby
source of primary cosmic rays in the Southern hemisphere.Comment: 17 pages, 5 figure
Thermoelectric Cooling at Cryogenic Temperatures
Experimental results demonstrating Peltier cooling below 10 K are reported, using crystals of the thermoelectric cerium hexaboride (CeB6). Direct measurements of the Peltier cooling showed δT up to ∼0.2 K in magnitude at T∼4–5 K. All three kinetic parameters: resistivity (ρ), heat conductivity (k), and Seebeck coefficient (S), characterizing the thermoelectric figure of merit ZT=S2T/ρk, were measured, giving high-confidence results
Laser-Powered Thermoelectric Generators Operating at Cryogenic Temperatures
A thermoelectric generator, operating in a cryostat at liquid helium temperatures, is described. Energy to the generator is supplied via an external laser beam. For this prototype device the associated heat load at permanent operation is comparable with the heat load associated with power delivery via metallic wires. Estimates indicate that still better performance can be enabled with existing thermoelectric materials, thereby far exceeding efficiency of traditional cryostat wiring. We used a prototype generator to produce electric power for measuring critical currents in Nb3Sn-films at 4K
Photon Physics in Heavy Ion Collisions at the LHC
Various pion and photon production mechanisms in high-energy nuclear
collisions at RHIC and LHC are discussed. Comparison with RHIC data is done
whenever possible. The prospect of using electromagnetic probes to characterize
quark-gluon plasma formation is assessed.Comment: Writeup of the working group "Photon Physics" for the CERN Yellow
Report on "Hard Probes in Heavy Ion Collisions at the LHC", 134 pages. One
figure added in chapter 5 (comparison with PHENIX data). Some figures and
correponding text corrected in chapter 6 (off-chemical equilibrium thermal
photon rates). Some figures modified in chapter 7 (off-chemical equilibrium
photon rates) and comparison with PHENIX data adde
Large scale cosmic-ray anisotropy with KASCADE
The results of an analysis of the large scale anisotropy of cosmic rays in
the PeV range are presented. The Rayleigh formalism is applied to the right
ascension distribution of extensive air showers measured by the KASCADE
experiment.The data set contains about 10^8 extensive air showers in the energy
range from 0.7 to 6 PeV. No hints for anisotropy are visible in the right
ascension distributions in this energy range. This accounts for all showers as
well as for subsets containing showers induced by predominantly light
respectively heavy primary particles. Upper flux limits for Rayleigh amplitudes
are determined to be between 10^-3 at 0.7 PeV and 10^-2 at 6 PeV primary
energy.Comment: accepted by The Astrophysical Journa
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