5,431 research outputs found
Multiple Andreev Reflections in Weak Links of Superfluid 3He-B
We calculate the current-pressure characteristics of a ballistic pinhole
aperture between two volumes of B-phase superfluid 3He. The most important
mechanism contributing to dissipative currents in weak links of this type is
the process of multiple Andreev reflections. At low biases this process is
significantly affected by relaxation due to inelastic
quasiparticle-quasiparticle collisions. In the numerical calculations,
suppression of the superfluid order parameter at surfaces is taken into account
self-consistently. When this effect is neglected, the theory may be developed
analytically like in the case of s-wave superconductors. A comparison with
experimental results is presented.Comment: 12 pages, 9 figures, RevTeX
Computational analysis method of the electromagnetic field propagation and deformation of conductive bodies
Introduction. The electromagnetic field is an integral attribute of the operation of many technical and technological systems. The action of an electromagnetic field leads to deformation, a change in temperature, a change in the physical properties of the materials. Problem. High-intensity electromagnetic fields can cause such a strong deformation of conductive bodies that it can lead to irreversible shape change or destruction. This fact is used in a class of technological operations: electromagnetic forming. Here, both the workpiece and the equipment are subjected to intense force action. As a result, equipment elements may become inoperable. Goal. Creation of a computational analysis method of the electromagnetic field propagation in systems of conductive bodies and subsequent analysis of deformation. Application of this method to the study of processes in electromagnetic forming systems in order to determine rational operational parameters that provide the result of a technological operation. Methodology. A variational formulation of the problems of an electromagnetic field propagation and deformation of conductive bodies systems is used. Numerical modeling and analysis are performed using the finite element method. Results. In a general form, a system of resolving equations for the values of the vector magnetic potential and displacements is obtained. The influence of the electromagnetic field is taken into account by introducing electromagnetic forces. The results of calculations for a technological system designed for electromagnetic forming of curved thin-walled workpieces are presented. Originality. For the first time, a method of computational analysis is presented, which involves modeling within the framework of one design scheme both the process of electromagnetic field propagation and the process of deformation. Practical significance. The proposed method of computational analysis can be used for various technological systems of electromagnetic forming in order to determine the rational parameters that ensure both the operability of the equipment and the purpose of the technological operation - the necessary shaping of the workpiece.Вступ. Електромагнітне поле є невід'ємним атрибутом роботи багатьох технічних і технологічних систем. Дія електромагнітного поля призводить до деформації, зміни температури, зміни фізичних властивостей матеріалів. Проблема. Електромагнітні поля високої інтенсивності можуть викликати настільки сильну деформацію провідних тіл, що це може призвести до незворотної зміни форми або руйнування. Цей факт використовується в класі технологічних операцій: електромагнітна обробка. Тут як заготовка, так і обладнання піддаються інтенсивній силовій дії. В результаті елементи обладнання можуть виходити з ладу. Мета. Створення методу розрахункового аналізу поширення електромагнітного поля в системах провідних тіл і подальшого аналізу деформування. Застосування цього методу для дослідження процесів в системах електромагнітної обробки з метою визначення раціональних робочих параметрів, що забезпечують результат технологічної операції. Методологія. Використано варіаційну постановку задач про поширення електромагнітного поля та деформування систем провідних тіл. Чисельне моделювання та аналіз виконано методом скінченних елементів. Результати. У загальному вигляді отримано систему визначальних рівнянь для значень векторного магнітного потенціалу та переміщень. Вплив електромагнітного поля враховується введенням електромагнітних сил. Наведено результати розрахунків для технологічної системи електромагнітної обробки вигнутих тонкостінних заготовок. Оригінальність. Вперше представлено метод розрахункового аналізу, який передбачає моделювання в рамках єдиної розрахункової схеми як процесу поширення електромагнітного поля, так і процесу деформування. Практичне значення. Запропонований метод розрахункового аналізу може бути використаний для різних технологічних систем електромагнітного формування з метою визначення раціональних параметрів, що забезпечують як працездатність обладнання, так і мету технологічної операції – необхідне формоутворення заготовки
Theory of AC Josepson Effect in Superconducting Constrictions
We have developed a microscopic theory of ac Josephson effect in short
ballistic superconducting constrictions with arbitrary electron transparency
and in constrictions with diffusive electron transport. The theory is valid for
arbitrary miscroscopic structure of the superconducting electrodes of the
constriction. As applications of the theory we study smearing of the subgap
current singularities by pair-breaking effects and also the structure of these
singularities in the constrictions between the composite S/N electrodes with
the proximity-induced gap in the normal layer.Comment: 11 pages, RevTex, 3 figures available on reques
Sausage oscillations of coronal loops
Aims. Analytical theory predicts the existence of trapped global (or fundamental) sausage fast magnetoacoustic modes in thick and dense coronal loops only, with the periods estimated as the ratio of double the loop length and the Alfvén speed outside the loop. We extend this study to the leaking regime, considering global sausage modes of long loops with small density contrasts.
Methods. Anti-symmetric fast magnetoacoustic perturbations (sausage, or m = 0 modes) of a low β plasma slab with the symmetric Epstein profile of plasma density are modelled numerically.
Results. It was found that long loops with sufficiently small density contrast can support global sausage leaky modes of detectable quality. The periods of the leaky modes are found to be approximately determined by the loop length and the external Alfvén speed. If the loop length can be estimated from imaging observations, the observed period of this mode provides us with the information about the Alfvén speed outside the loop. For typical flaring coronal loops, the estimated periods of the global sausage modes are
about 5−60 s
Contributions of spontaneous phase slippage to linear and non-linear conduction near the Peierls transition in thin samples of o-TaS_3
In the Peierls state very thin samples of TaS_3 (cross-section area \sim
10^{-3} mkm^2) are found to demonstrate smearing of the I-V curves near the
threshold field. With approaching the Peierls transition temperature, T_P, the
smearing evolves into smooth growth of conductance from zero voltage
interpreted by us as the contribution of fluctuations to the non--linear
conductance. We identify independently the fluctuation contribution to the
linear conductance near T_P. Both linear and non-linear contributions depend on
temperature with close activation energies \sim (2 - 4) x 10^3 K and apparently
reveal the same process. We reject creep of the {\it continuous} charge-density
waves (CDWs) as the origin of this effect and show that it is spontaneous phase
slippage that results in creep of the CDW. A model is proposed accounting for
both the linear and non-linear parts of the fluctuation conduction up to T_P.Comment: 6 pages, 5 Postscript figure, RevTeX, accepted for publication in PR
Point Contact Spectroscopy of Superconducting Gap Anisotropy in Nickel Borocarbide Compound LuNi2B2C
Point contacts are used to investigate the anisotropy of the superconducting
energy gap in LuNi2B2C in the ab plane and along the c axis. It is shown that
the experimental curves should be described assuming that the superconducting
gap is non-uniformly distributed over the Fermi surface. The largest and the
smallest gaps have been estimated by two-gap fitting models. It is found that
the largest contribution to the point-contact conductivity in the c direction
is made by a smaller gap and, in the ab plane by a larger gap. The deviation
from the one-gap BCS model is pronounced in the temperature dependence of the
gap in both directions. The temperature range, where the deviation occurs, is
for the c direction approximately 1.5 times more than in the ab plane. The
\Gamma parameter, allowing quantitatively estimate the gap anisotropy by
one-gap fitting, in c direction is also about 1.5 times greater than in the ab
plane. Since it is impossible to describe satisfactorily such gap distribution
either by the one- or two-gap models, a continuous, dual-maxima model of gap
distribution over the Fermi surface should be used to describe
superconductivity in this material.Comment: 10 pages, 14 Figs, accepted in PR
Proximity Effect in Normal Metal - High Tc Superconductor Contacts
We study the proximity effect in good contacts between normal metals and high
Tc (d-wave) superconductors. We present theoretical results for the spatially
dependent order parameter and local density of states, including effects of
impurity scattering in the two sides, s-wave pairing interaction in the normal
metal side (attractive or repulsive), as well as subdominant s-wave paring in
the superconductor side. For the [100] orientation, a real combination d+s of
the order parameters is always found. The spectral signatures of the proximity
effect in the normal metal includes a suppression of the low-energy density of
states and a finite energy peak structure. These features are mainly due to the
impurity self-energies, which dominate over the effects of induced pair
potentials. For the [110] orientation, for moderate transparencies, induction
of a d+is order parameter on the superconductor side, leads to a proximity
induced is order parameter also in the normal metal. The spectral signatures of
this type of proximity effect are potentially useful for probing time-reversal
symmetry breaking at a [110] interface.Comment: 10 pages, 10 figure
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