579 research outputs found
Field-Dependent Critical Current in Type-II Superconducting Strips: Combined Effect of Bulk Pinning and Geometrical Edge Barrier
Recent theoretical and experimental research on low-bulk-pinning
superconducting strips has revealed striking dome-like magnetic-field
distributions due to geometrical edge barriers. The observed magnetic-flux
profiles differ strongly from those in strips in which bulk pinning is
dominant. In this paper we theoretically describe the current and field
distributions of a superconducting strip under the combined influence of both a
geometrical edge barrier and bulk pinning at the strip's critical current Ic,
where a longitudinal voltage first appears. We calculate Ic and find its
dependence upon a perpendicular applied magnetic field Ha. The behavior is
governed by a parameter p, defined as the ratio of the bulk-pinning critical
current Ip to the geometrical-barrier critical current Is0. We find that when p
> 2/pi and Ip is field-independent, Ic vs Ha exhibits a plateau for small Ha,
followed by the dependence Ic-Ip ~ 1/Ha in higher magnetic fields.Comment: 4 pages, 2 figures, Fig. 1 revised, submitted to Phys. Rev.
Critical temperature and giant isotope effect in presence of paramagnons
We reconsider the long-standing problem of the effect of spin fluctuations on
the critical temperature and isotope effect in a phonon-mediated
superconductor. Although the general physics of the interplay between phonons
and paramagnons had been rather well understood, the existing approximate
formulas fail to describe the correct behavior of for general phonon
and paramagnon spectra. Using a controllable approximation, we derive an
analytical formula for which agrees well with exact numerical solutions
of the Eliashberg equations for a broad range of parameters. Based on both
numerical and analytical results, we predict a strong enhancement of the
isotope effect when the frequencies of spin fluctuation and phonons are of the
same order. This effect may have important consequences for near-magnetic
superconductors such as MgCNiComment: 5 pages, 2 figure
Ab initio calculations of the physical properties of transition metal carbides and nitrides and possible routes to high-Tc
Ab initio linear-response calculations are reported of the phonon spectra and
the electron-phonon interaction for several transition metal carbides and
nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and
superconducting properties are calculated in detail at various pressures and
the normal-pressure results are found to well agree with the experiment.
Factors accounting for the relatively low critical temperatures Tc in
transition metal compounds with light elements are considered and the possible
ways of increasing Tc are discussed.Comment: 19 pages, 7 figure
ETEKOS experimental ecological system
The problem of changes in the ecology resulting, for example, in increases in water temperature because of discharges from large thermal power plants is considered. An experiment creating a model of such an ecological system is described
Manifestation of superfluidity in an evolving Bose-condensed gas
We study the generation of excitations due to an ''impurity''(static
perturbation) placed into an oscillating Bose-condensed gas in the
time-dependent trapping field. It is shown that there are two regions for the
position of the local perturbation. In the first region the condensate flows
around the ''impurity'' without generation of excitations demonstrating
superfluid properties. In the second region the creation of excitations occurs,
at least within a limited time interval, revealing destruction of
superfluidity. The phenomenon can be studied by measuring the damping of
condensate oscillations at different positions of the ''impurity''
Stationary striations in plasma, created by a short microwave pulse in a waveguide filled with a neutral gas
It was observed experimentally that after crossing a waveguide filled with a
neutral gas, a short powerful microwave pulse leaves a periodic glow of plasma
along the waveguide, persisting several tens of nanoseconds. A theoretical
model is presented which in combination with numerical simulations proposes a
possible explanation of this phenomenon.Comment: 15 pages, 9 figure
Geometrical edge barriers and magnetization in superconducting strips with slits
We theoretically investigate the magnetic-field and current distributions for
coplanar superconducting strips with slits in an applied magnetic field H_a. We
consider ideal strips with no bulk pinning and calculate the hysteretic
behavior of the magnetic moment m_y as a function of H_a due solely to
geometrical edge barriers. We find that the m_y-H_a curves are strongly
affected by the slits. In an ascending field, the m_y-H_a curves exhibit kink
or peak structures, because the slits prevent penetration of magnetic flux. In
a descending field, m_y becomes positive, because magnetic flux is trapped in
the slits, in contrast to the behavior of a single strip without slits, for
which m_y =0.Comment: 11 pages, 5 figures, revtex
Use of Nanocomposite Material Based on Graphene Oxide and Silver Nanoparticles in Research of Blood Erythrocytes in Various Diseases
The electrical and structural properties of a nanocomposite material based on silver nanoparticles and graphene oxide were investigated. Using electron and atomic force microscopy it was shown that the nanoparticles formed predominantly have sizes from 60 to 100 nm. Measurements of the current-voltage characteristics showed electrical conductivity values significantly exceeding the parameters of the initial graphene oxide. The prepared nanocomposites were used for scanning by electron microscopy (SEM) of blood samples of sick children with hematuria syndrome and patients of the radiological department of an oncologic dispensary diagnosed with cervical cancer. The formation of nanosize objects on the surface of erythrocytes is revealed. The size of these volumes is comparable to the size of the viruses. Results of these studies can indirectly confirm an assumption of authors about possible transportation of viruses by erythrocytes to various organs and viral etiology of renal diseases with the hematuria syndrome and cervical cancer.
Keywords: graphene, graphene oxide, silver nanoparticles, structural and electrical properties, nephropathy, diagnostics
Magnetic-field and current-density distributions in thin-film superconducting rings and disks
We show how to calculate the magnetic-field and sheet-current distributions
for a thin-film superconducting annular ring (inner radius a, outer radius b,
and thickness d<<a) when either the penetration depth obeys lambda < d/2 or, if
lambda > d/2, the two-dimensional screening length obeys Lambda = 2 lambda^2/d
<< a for the following cases: (a) magnetic flux trapped in the hole in the
absence of an applied magnetic field, (b) zero magnetic flux in the hole when
the ring is subjected to an applied magnetic field, and (c) focusing of
magnetic flux into the hole when a magnetic field is applied but no net current
flows around the ring. We use a similar method to calculate the magnetic-field
and sheet-current distributions and magnetization loops for a thin,
bulk-pinning-free superconducting disk (radius b) containing a dome of magnetic
flux of radius a when flux entry is impeded by a geometrical barrier.Comment: 10 pages, 13 figure
Some Algorithms for the Dynamic Reconstruction of Inputs
For some classes of systems described by ordinary differential equations, a survey of algorithms for the dynamic reconstruction of inputs is presented. The algorithms described in the paper are stable with respect to information noises and computation errors; they are based on methods from the theory of ill-posed problems as well as on appropriate modifications of N. N. Krasovskii's principle of extremal aiming, which is known in the theory of guaranteed control. © 2011 Pleiades Publishing, Ltd.This work was supported by the Russian Foundation for Basic Research (project no. 09-01-00378), by the Program for Fundamental Research of the Presidium of the Russian Academy of Sciences “Mathematical Theory of Control” (project no. 09-P-1-1014), by the Program for State Support of Leading Scientific Schools of the Russian Federation (project no. NSh-65590.2010.1), and by the Ural–Siberian Integration Project no. 09-S-1-1010
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