703 research outputs found
Electrostatic topology of ferroelectric domains in YMnO
Trimerization-polarization domains in ferroelectric hexagonal YMnO were
resolved in all three spatial dimensions by piezoresponse force microscopy.
Their topology is dominated by electrostatic effects with a range of 100 unit
cells and reflects the unusual electrostatic origin of the spontaneous
polarization. The response of the domains to locally applied electric fields
explains difficulties in transferring YMnO into a single-domain state. Our
results demonstrate that the wealth of non-displacive mechanisms driving
ferroelectricity that emerged from the research on multiferroics are a rich
source of alternative types of domains and domain-switching phenomena
Breit-Wheeler Process in Intense Short Laser Pulses
Energy-angular distributions of electron-positron pair creation in collisions
of a laser beam and a nonlaser photon are calculated using the -matrix
formalism. The laser field is modeled as a finite pulse, similar to the
formulation introduced in our recent paper in the context of Compton scattering
[Phys. Rev. A {\bf 85}, 062102 (2012)]. The nonperturbative regime of pair
creation is considered here. The energy spectra of created particles are
compared with the corresponding spectra obtained using the modulated plane wave
approximation for the driving laser field. A very good agreement in these two
cases is observed, provided that the laser pulse is sufficiently long. For
short pulse durations, this agreement breaks down. The sensitivity of pair
production to the polarization of a driving pulse is also investigated. We show
that in the nonperturbative regime, the pair creation yields depend on the
polarization of the pulse, reaching their maximal values for the linear
polarization. Therefore, we focus on this case. Specifically, we analyze the
dependence of pair creation on the relative configuration of linear
polarizations of the laser pulse and the nonlaser photon. Lastly, we
investigate the carrier-envelope phase effect on angular distributions of
created particles, suggesting the possibility of phase control in relation to
the pair creation processes.Comment: 13 pages, 8 figure
DarkSide status and prospects
DarkSide uses a dual-phase Liquid Argon Time Projection Chamber to search for WIMP dark matter. The current detector, DarkSide-50, is running since mid 2015 with a target of 50 kg of Argon from an underground sourc
Particle dynamics inside shocks in Hamilton-Jacobi equations
Characteristics of a Hamilton-Jacobi equation can be seen as action
minimizing trajectories of fluid particles. For nonsmooth "viscosity"
solutions, which give rise to discontinuous velocity fields, this description
is usually pursued only up to the moment when trajectories hit a shock and
cease to minimize the Lagrangian action. In this paper we show that for any
convex Hamiltonian there exists a uniquely defined canonical global nonsmooth
coalescing flow that extends particle trajectories and determines dynamics
inside the shocks. We also provide a variational description of the
corresponding effective velocity field inside shocks, and discuss relation to
the "dissipative anomaly" in the limit of vanishing viscosity.Comment: 15 pages, no figures; to appear in Philos. Trans. R. Soc. series
Properties of a ceramic pyroelectric X-ray generator as dependent on residual-gas pressure
The properties of X-ray emission from a pyroelectric accelerator based on ferroelectric ceramics have been experimentally studied, including the maximum energy of X-ray photons and their maximum yield generated by accelerated electrons at various pressures of residual gas in the generator chambe
Strain Hardening of Polymer Glasses: Entanglements, Energetics, and Plasticity
Simulations are used to examine the microscopic origins of strain hardening
in polymer glasses. While stress-strain curves for a wide range of temperature
can be fit to the functional form predicted by entropic network models, many
other results are fundamentally inconsistent with the physical picture
underlying these models. Stresses are too large to be entropic and have the
wrong trend with temperature. The most dramatic hardening at large strains
reflects increases in energy as chains are pulled taut between entanglements
rather than a change in entropy. A weak entropic stress is only observed in
shape recovery of deformed samples when heated above the glass transition.
While short chains do not form an entangled network, they exhibit partial shape
recovery, orientation, and strain hardening. Stresses for all chain lengths
collapse when plotted against a microscopic measure of chain stretching rather
than the macroscopic stretch. The thermal contribution to the stress is
directly proportional to the rate of plasticity as measured by breaking and
reforming of interchain bonds. These observations suggest that the correct
microscopic theory of strain hardening should be based on glassy state physics
rather than rubber elasticity.Comment: 15 pages, 12 figures: significant revision
Muon pair creation from positronium in a circularly polarized laser field
We study elementary particle reactions that result from the interaction of an
atomic system with a very intense laser wave of circular polarization. As a
specific example, we calculate the rate for the laser-driven reaction , where the electron and positron originate from a positronium
atom or, alternatively, from a nonrelativistic plasma. We distinguish
accordingly between the coherent and incoherent channels of the process. Apart
from numerical calculations, we derive by analytical means compact formulas for
the corresponding reaction rates. The rate for the coherent channel in a laser
field of circular polarization is shown to be damped because of the destructive
interference of the partial waves that constitute the positronium ground-state
wave packet. Conditions for the observation of the process via the dominant
incoherent channel in a circularly polarized field are pointed out
To the problems of modeling the brain ischemia in small animals
In the review article the problems of modeling cerebral ischemia in small mammals are consecrated. The advantages of experimental studies that are based on the similarity of the blood circulation of the brain in humans and animals are indicated. Classification of experimental models for the study of acute and chronic disorders of cerebral circulation, mechanisms of their development and preclinical approbation of new drugs is given. The authors indicate that all experimental models of brain ischemia can be divided into two groups: to study risk factors and pathophysiological studies of brain ischemia. And in the second case, the models of focal and global ischemia are described. In conclusion, the authors point out the difficulties and shortcomings of certain methods of ischemia reproduction, which await researchers to solve the above problems
Essential self-adjointness of magnetic Schr\"odinger operators on locally finite graphs
We give sufficient conditions for essential self-adjointness of magnetic
Schr\"odinger operators on locally finite graphs. Two of the main theorems of
the present paper generalize recent results of Torki-Hamza.Comment: 14 pages; The present version differs from the original version as
follows: the ordering of presentation has been modified in several places,
more details have been provided in several places, some notations have been
changed, two examples have been added, and several new references have been
inserted. The final version of this preprint will appear in Integral
Equations and Operator Theor
Tumor microenvironment: the formation of the immune profile
Tumor microenvironment (TME) is formed as a result of interaction and cross-linking between the tumor cell and different types of surrounding cells. Recent studies have shown that the tumor reprograms the microenvironment so that TME promotes the development of primary tumors, their metastasis and becomes an important regulator of oncogenesis. Under the influence of the tumor, the immune profile in the TME undergoes significant changes, “editing". An immunosuppressive network is formed, which suppresses the activity of the main effector of cellular immunity — T lymphocytes. T cells in TMA are in a state of anergy and exhaustion. T cells in TME are characterized by increased expression of inhibitory receptors, decreased secretion of cytokines and cytolytic activity. Blocking inhibitory receptors with specific antibodies can lead to the restoration of the functions of exausted T cells. Therefore, the restoration of the functional activity of T lymphocytes is one of the important strategies in cancer immunotherapy. The formation of the immune profile is influenced by genetic aberrations accumulating in the tumor. They play an important role in creating a specific, characteristic only for this tumor immune environment in the TME. Genetic changes in tumor cells lead to phenotypic and functional rearrangements of lymphocytes, which allows the tumor to escape the reaction of immune cells. Since many tumors occur after prolonged inflammation or exhibit characteristics of chronic inflammation as they progress, inflammation is considered an important factor in the formation of immune profile in TME. Immune infiltrates from different human tumors associated with inflammation may contain valuable prognostic and pathophysiological information. Macrophages in the TME now began to be regarded as descriptive marker and as a therapeutic target. One of the main mechanisms by which tumor cells reprogram surrounding cells is the release of exosomes — small vesicles that carry and deliver proteins and nucleic acids to other cells. When exosomal cargo is absorbed, molecular, transcriptional and translational changes occur in the recipient non-tumor cells in the TME. Therefore, tumor exosomes are an effective means by which the functions of immune cells in TME are purposefully changed. Thus, along with individual molecular and genomic testing of the tumor, attention should be paid to a deeper analysis of the immune profile of TME. It is a large resource of biomarkers and targets for immunotherapy
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