526 research outputs found
Anomalous Hall effect in granular ferromagnetic metals and effects of weak localization
We theoretically investigate the anomalous Hall effect in a system of
dense-packed ferromagnetic grains in the metallic regime. Using the formalism
recently developed for the conventional Hall effect in granular metals, we
calculate the residual anomalous Hall conductivity and
resistivity and weak localization corrections to them for both
skew-scattering and side-jump mechanisms. We find that, unlike for
homogeneously disordered metals, the scaling relation between and
the longitudinal resistivity does not hold. The weak localization
corrections, however, are found to be in agreement with those for homogeneous
metals. We discuss recent experimental data on the anomalous Hall effect in
polycrystalline iron films in view of the obtained results.Comment: published version, 10 pages, 6 figure
Surface impedance of superconductors with magnetic impurities
Motivated by the problem of the residual surface resistance of the
superconducting radio-frequency (SRF) cavities, we develop a microscopic theory
of the surface impedance of s-wave superconductors with magnetic impurities. We
analytically calculate the current response function and surface impedance for
a sample with spatially uniform distribution of impurities, treating magnetic
impurities in the framework of the Shiba theory. The obtained general
expressions hold in a wide range of parameter values, such as temperature,
frequency, mean free path, and exchange coupling strength. This generality, on
the one hand, allows for direct numerical implementation of our results to
describe experimental systems (SRF cavities, superconducting qubits) under
various practically relevant conditions. On the other hand, explicit analytical
expressions can be obtained in a number of limiting cases, which makes possible
further theoretical investigation of certain regimes. As a feature of key
relevance to SRF cavities, we show that in the regime of "gapless
superconductivity" the surface resistance exhibits saturation at zero
temperature. Our theory thus explicitly demonstrates that magnetic impurities,
presumably contained in the oxide surface layer of the SRF cavities, provide a
microscopic mechanism for the residual resistance.Comment: 9 pages, 3 figs; v2: published versio
Hall Transport in Granular Metals and Effects of Coulomb Interactions
We present a theory of Hall effect in granular systems at large tunneling
conductance . Hall transport is essentially determined by the
intragrain electron dynamics, which, as we find using the Kubo formula and
diagrammatic technique, can be described by nonzero diffusion modes inside the
grains. We show that in the absence of Coulomb interaction the Hall resistivity
depends neither on the tunneling conductance nor on the intragrain
disorder and is given by the classical formula , where
differs from the carrier density inside the grains by a numerical
coefficient determined by the shape of the grains and type of granular lattice.
Further, we study the effects of Coulomb interactions by calculating
first-order in corrections and find that (i) in a wide range of
temperatures T \gtrsim \Ga exceeding the tunneling escape rate \Ga, the
Hall resistivity and conductivity \sig_{xy} acquire logarithmic
in corrections, which are of local origin and absent in homogeneously
disordered metals; (ii) large-scale ``Altshuler-Aronov'' correction to
\sig_{xy}, relevant at T\ll\Ga, vanishes in agreement with the theory of
homogeneously disordered metals.Comment: 29 pages, 16 figure
Oscillations of Induced Magnetization in Superconductor-Ferromagnet Heterostructures
We study a change in the spin magnetization of a superconductor-ferromagnet
(SF) heterostructure, when temperature is lowered below the superconducting
transition temperature. It is assumed that the SF interface is smooth on the
atomic scale and the mean free path is not too short. Solving the Eilenberger
equation we show that the spin magnetic moment induced in the superconductor is
an oscillating sign-changing function of the product of the exchange field
and the thickness of the ferromagnet. Therefore the total spin magnetic
moment of the system in the superconducting state can be not only smaller
(screening) but also greater (anti-screening) than that in the normal state, in
contrast with the case of highly disordered (diffusive) systems, where only
screening is possible. This surprising effect is due to peculiar periodic
properties of localized Andreev states in the system. It is most pronounced in
systems with ideal ballistic transport (no bulk disorder in the samples, smooth
ideally transparent interface), however these ideal conditions are not crucial
for the very existence of the effect. We show that oscillations exist (although
suppressed) even for arbitrary low interface transparency and in the presence
of bulk disorder, provided that ( -- mean free path). At
low interface transparency we solve the problem for arbitrary strength of
disorder and obtain oscillating magnetization in ballistic regime () and nonoscillating magnetization in diffusive one () as
limiting cases of one formula.Comment: 10 pages, 2 figures, accepted for publication in Phys. Rev.
Проект ЭСПЦ по производству нержавеющих марок стали в условиях Урала производительностью 650 тыс. тонн стали в год
Реферат
Выпускная квалификационная работа 131 листов, 13 рисунков, 36 источников, 2 приложения.
Ключевые слова: электросталеплавильный цех, электродуговая печь, АКР, электрофильтр, МНЛЗ, блюм.
Объектом исследования является ЭСПЦ производительностью 650 тыс. тонн в год нержавеющих марок стали в условия Урала.
Цели исследования: выбор основного оборудования, расчет цеха и оборудования, баланса металла и шихты в цехе, социальная ответственность и экономическое обоснование строительства цеха.
Работа представлена введением, 5 разделами и заключением, список использованных источников.
В разделе 슫Объект исследования슻 описан проектируемый ЭСПЦ, рассмотрены основные пролеты и располагающееся в них оборудование.
Во разделе 슫Расчеты и аналитика슻 представлены расчет баланса металла в цехе и шAbstract
Abstract
The graduation work consists of 131 pages, 13 figures, 36 sources, 2 appendices.
Keywords: arc furnace plant, electric arc furnace, AOD, electrostatic precipitator, continuous casting machines, bloom.
The object of study is the EAF shop with the capacity of 650 thousand tons a year of stainless steels under the conditions of the Urals.
The aim of the study: choice of the basic equipment, calculation of plant and equipment, the balance of metal and charge in the plant, social responsibility and economic feasibility of the plant construction.
The work includes the introduction, 5 sections and conclusion, list of references.
In the section «Object of study» describe the designed EAF shop, considere the main spans and equipment located in them.
In the section «Calculation a
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
Order-Revealing Encryption and the Hardness of Private Learning
An order-revealing encryption scheme gives a public procedure by which two
ciphertexts can be compared to reveal the ordering of their underlying
plaintexts. We show how to use order-revealing encryption to separate
computationally efficient PAC learning from efficient -differentially private PAC learning. That is, we construct a concept
class that is efficiently PAC learnable, but for which every efficient learner
fails to be differentially private. This answers a question of Kasiviswanathan
et al. (FOCS '08, SIAM J. Comput. '11).
To prove our result, we give a generic transformation from an order-revealing
encryption scheme into one with strongly correct comparison, which enables the
consistent comparison of ciphertexts that are not obtained as the valid
encryption of any message. We believe this construction may be of independent
interest.Comment: 28 page
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