537 research outputs found
Experimental Study of the Effect of External Signal on Microwave Oscillations in a Nonrelativistic Electron Beam with Virtual Cathode
The effect of an external harmonic signal on the characteristics of microwave
generation in a nonrelativistic electron beam with virtual cathode (VC) formed
in a static retarding electric field (low-voltage vircator system) has been
experimentally studied. A significant increase in the vircator generation power
is observed when the frequency of the external signal is close to the frequency
of VC oscillations. At large detunings, a broadband chaotic generation is
observed.Comment: 3 pages, 2 figure
Self-similar solution of a nonsteady problem of nonisothermal vapour condensation on a droplet growing in diffusion regime
This paper presents a mathematically exact self-similar solution to the joint
nonsteady problems of vapour diffusion towards a droplet growing in a
vapour-gas medium and of removal of heat released by a droplet into a
vapour-gas medium during vapour condensation. An equation for the temperature
of the droplet is obtained; and it is only at that temperature that the
self-similar solution exists. This equation requires the constancy of the
droplet temperature and even defines it unambiguously throughout the whole
period of the droplet growth. In the case of strong display of heat effects,
when the droplet growth rate decreases significantly, the equation for the
temperature of the droplet is solved analytically. It is shown that the
obtained temperature fully coincides with the one that settles in the droplet
simultaneously with the settlement of its diffusion regime of growth. At the
obtained temperature of the droplet the interrelated nonsteady vapour
concentration and temperature profiles of the vapour-gas medium around the
droplet are expressed in terms of initial (prior to the nucleation of the
droplet) parameters of the vapour-gas medium. The same parameters are used to
formulate the law in accordance with which the droplet is growing in diffusion
regime, and also to define the time that passes after the nucleation of the
droplet till the settlement of diffusion regime of droplet growth, when the
squared radius of the droplet becomes proportionate to time. For the sake of
completeness the case of weak display of heat effects is been studied.Comment: 12 pages, 4 figure
NMR Study of Disordered Inclusions in the Quenched Solid Helium
Phase structure of rapidly quenched solid helium samples is studied by the
NMR technique. The pulse NMR method is used for measurements of spin-lattice
and spin-spin relaxation times and spin diffusion coefficient
for all coexisting phases. It was found that quenched samples are two-phase
systems consisting of the hcp matrix and some inclusions which are
characterized by and values close to those in liquid phase. Such
liquid-like inclusions undergo a spontaneous transition to a new state with
anomalously short times. It is found that inclusions observed in both the
states disappear on careful annealing near the melting curve. It is assumed
that the liquid-like inclusions transform into a new state - a glass or a
crystal with a large number of dislocations. These disordered inclusions may be
responsible for the anomalous phenomena observed in supersolid region.Comment: 10 pages, 3 figure
Investigation of the Chaotic Dynamics of an Electron Beam with a Virtual Cathode in an External Magnetic Field
The effect of the strength of the focusing magnetic field on chaotic dynamic
processes occurring inan electron beam with a virtual cathode, as well as on
the processes whereby the structures form in the beamand interact with each
other, is studied by means of two-dimensional numerical simulations based on
solving a self-consistent set of Vlasov-Maxwell equations. It is shown that, as
the focusing magnetic field is decreased,the dynamics of an electron beam with
a virtual cathode becomes more complicated due to the formation andinteraction
of spatio-temporal longitudinal and transverse structures in the interaction
region of a vircator. The optimum efficiency of the interaction of an electron
beam with the electromagnetic field of the vircator isachieved at a
comparatively weak external magnetic field and is determined by the
fundamentally two-dimensional nature of the motion of the beam electrons near
the virtual cathode.Comment: 12 pages, 8 figure
Local modes, phonons, and mass transport in solid He
We propose a model to treat the local motion of atoms in solid He as a
local mode. In this model, the solid is assumed to be described by the Self
Consistent Harmonic approximation, combined with an array of local modes. We
show that in the bcc phase the atomic local motion is highly directional and
correlated, while in the hcp phase there is no such correlation. The correlated
motion in the bcc phase leads to a strong hybridization of the local modes with
the T phonon branch, which becomes much softer than that obtained
through a Self Consistent Harmonic calculation, in agreement with experiment.
In addition we predict a high energy excitation branch which is important for
self-diffusion. Both the hybridization and the presence of a high energy branch
are a consequence of the correlation, and appear only in the bcc phase. We
suggest that the local modes can play the role in mass transport usually
attributed to point defects (vacancies). Our approach offers a more overall
consistent picture than obtained using vacancies as the predominant point
defect. In particular, we show that our approach resolves the long standing
controversy regarding the contribution of point defects to the specific heat of
solid He.Comment: 10 pages, 10 figure
Measurement of the Pion Form Factor in the Energy Range 1.04-1.38 GeV with the CMD-2 Detector
The cross section for the process is measured in the
c.m. energy range 1.04-1.38 GeV from 995 000 selected collinear events
including 860000 events, 82000 events, and 33000
events. The systematic and statistical errors of measuring the
pion form factor are equal to 1.2-4.2 and 5-13%, respectively.Comment: 5 pages, 2 figure
A Survey of Satisfiability Modulo Theory
Satisfiability modulo theory (SMT) consists in testing the satisfiability of
first-order formulas over linear integer or real arithmetic, or other theories.
In this survey, we explain the combination of propositional satisfiability and
decision procedures for conjunctions known as DPLL(T), and the alternative
"natural domain" approaches. We also cover quantifiers, Craig interpolants,
polynomial arithmetic, and how SMT solvers are used in automated software
analysis.Comment: Computer Algebra in Scientific Computing, Sep 2016, Bucharest,
Romania. 201
Test of exotic scalar and tensor interactions in K_e3 decay using stopped positive kaons
The form factors of the decay K+ --> pi0 e+ nu (K_e3) have been determined
from the comparison of the experimental and Monte Carlo Dalitz distributions
containing about 10^5 K_e3 events. The following values of the parameters were
obtained: lambda_+ = 0.0278 +- 0.0017(stat) +- 0.0015(syst), f_S/f_+(0) =
0.0040 +- 0.0160(stat) +- 0.0067(syst) and f_T/f_+(0) = 0.019 +- 0.080(stat) +-
0.038(syst). Both scalar f_S and tensor f_T form factors are consistent with
the Standard Model predictions of zero values.Comment: 10 pages, 5 figures, contributed to the proceedings of NANP
Conference, Dubna, June 19-23, 200
Synthesis, antibacterial and antitumor activity of methylpyridinium salts of pyridoxine functionalized 2-amino-6-sulfanylpyridine-3,5-dicarbonitriles
© 2018, © 2018 Taylor & Francis. A library of 29 2-amino-6-sulfanylpyridine-3,5-dicarbonitriles functionalized with a pyridoxine moiety was synthesized using a three-component one-pot reaction of aldehyde derivative of pyridoxine, malononitrile, and thiophenol. The obtained bipyridine structures were converted into methylpyridinium salts. Several compounds demonstrated expressed antibacterial activity with MICs (minimum inhibitory concentrations) in the range of 0.5–4 µg/mL against the three studied Gram-positive strains and 8–64 µg/mL against the Gram-negative E. coli strain, which was comparable or better than the activity of the reference antimicrobial agents. At the same time, all the synthesized compounds were inactive against the Gram-negative P. aeruginosa. Several compounds also demonstrated high cytotoxic activity against the studied tumor cells, but without selectivity for the normal HSF (human foreskin fibroblast) cells. Despite the preliminary character of the performed biological studies, the obtained results make the obtained structural chemotype a promising starting point for the design of physiologically active compounds
Defects and glassy dynamics in solid He-4: Perspectives and current status
We review the anomalous behavior of solid He-4 at low temperatures with
particular attention to the role of structural defects present in solid. The
discussion centers around the possible role of two level systems and structural
glassy components for inducing the observed anomalies. We propose that the
origin of glassy behavior is due to the dynamics of defects like dislocations
formed in He-4. Within the developed framework of glassy components in a solid,
we give a summary of the results and predictions for the effects that cover the
mechanical, thermodynamic, viscoelastic, and electro-elastic contributions of
the glassy response of solid He-4. Our proposed glass model for solid He-4 has
several implications: (1) The anomalous properties of He-4 can be accounted for
by allowing defects to freeze out at lowest temperatures. The dynamics of solid
He-4 is governed by glasslike (glassy) relaxation processes and the
distribution of relaxation times varies significantly between different
torsional oscillator, shear modulus, and dielectric function experiments. (2)
Any defect freeze-out will be accompanied by thermodynamic signatures
consistent with entropy contributions from defects. It follows that such
entropy contribution is much smaller than the required superfluid fraction, yet
it is sufficient to account for excess entropy at lowest temperatures. (3) We
predict a Cole-Cole type relation between the real and imaginary part of the
response functions for rotational and planar shear that is occurring due to the
dynamics of defects. Similar results apply for other response functions. (4)
Using the framework of glassy dynamics, we predict low-frequency yet to be
measured electro-elastic features in defect rich He-4 crystals. These
predictions allow one to directly test the ideas and very presence of glassy
contributions in He-4.Comment: 33 pages, 13 figure
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