24 research outputs found
Hall resistivity of granular metals
We calculate the Hall conductivity \sig_{xy} and resistivity of
a granular system at large tunneling conductance . 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. The Coulomb interaction gives rise to logarithmic in
temperature correction to in the range \Ga \lesssim T \lesssim
\min(g_T E_c,\ETh), where \Ga is the tunneling escape rate, is the
charging energy and \ETh is the Thouless energy of the grain.Comment: 4 pages, 1 figur
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
Electron screening and excitonic condensation in double-layer graphene systems
We theoretically investigate the possibility of excitonic condensation in a
system of two graphene monolayers separated by an insulator, in which electrons
and holes in the layers are induced by external gates. In contrast to the
recent studies of this system, we take into account the screening of the
interlayer Coulomb interaction by the carriers in the layers, and this
drastically changes the result. Due to a large number of electron species in
the system (two projections of spin, two valleys, and two layers) and to the
suppression of backscattering in graphene, the maximum possible strength of the
screened Coulomb interaction appears to be quite small making the weak-coupling
treatment applicable. We calculate the mean-field transition temperature for a
clean system and demonstrate that its highest possible value
is extremely small
( is the Fermi energy). In addition, any sufficiently short-range
disorder with the scattering time would
suppress the condensate completely. Our findings renders experimental
observation of excitonic condensation in the above setup improbable even at
very low temperatures.Comment: 4+ pages, 3 figure
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
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
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Нарушения функции внешнего дыхания при различных формах легочной патологии
The most common lung function abnormalities in patients with respiratory diseases have been reviewed in the article. Ventilation disorders with change in the lung volumes, bronchial obstruction and hypoxemia are typical for patients with pneumonia. Lung compliance reduction, inconsistency of ventilation, and ventilation perfusion mismatch could be found in chronic non-obstructive bronchitis. Chronic obstructive pulmonary disease is associated with bronchial obstruction which is defined as decreased forced expiratory volume for 1 sec (FEV1) and forced expiratory flows at different levels of forced vital capacity (FVC) and increased airway resistance. Asthma is associated with reversible change in expiratory flows, such as FEV1 and peak expiratory flow, due to bronchial hyperresponsiveness. Residual volume (RV) could increase in patients with acute asthma attack. Emphysema is characterized by changes in lung volumes, primarily due to increased RV; lung diffusing capacity (DLCO) could decrease and the lung perfusion could change. Mixed (obstructive and restrictive) lung ventilation disorders could be diagnosed in patients with chronic purulent lung diseases. Patients with disseminated lung lesions could demonstrate decreased lung compliance, reduced lung volumes, decreased DLCO and hypoxemia without hypercapnia.Представлены наиболее типичные нарушения функции внешнего дыхания (ФВД) у больных пульмонологического профиля. При пневмонии нарушается вентиляция, страдают легочные объемы, имеют место нарушения бронхиальной проходимости, развивается артериальная гипоксемия. При хроническом необструктивном бронхите выявляются некоторое снижение растяжимости легких, нарушения равномерности вентиляции, признаки нарушения вентиляционно-перфузионных отношений. У пациентов с хронической обструктивной болезнью легких (ХОБЛ) бронхиальная обструкция имеет генерализованный характер, что выражается в снижении объема форсированного выдоха за 1-ю секунду (ОФВ1), уменьшении объемной скорости потока на различных уровнях форсированной жизненной емкости легких и увеличении бронхиального сопротивления. Существенные изменения претерпевают легочные объемы, особенно остаточный объем (ООЛ) и общая емкость (ОЕЛ) легких. При бронхиальной астме (БА) регистрируется преходящее лабильное изменение скоростных параметров спирометрии (ОФВ1, пиковая скорость выдоха), обусловленное бронхиальной гиперреактивностью. В период обострения БА также регистрируется увеличение ООЛ. У лиц с эмфиземой легких нарушения ФВД проявляются изменением структуры ОЕЛ за счет увеличения ООЛ. Снижается диффузионная способность легких (DLCO), отмечаются нарушения перфузии. У пациентов с хроническими нагноительными заболеваниями легких отмечаются нарушения вентиляции по смешанному типу (рестриктивные и обструктивные). Для диссеминированных заболеваний легких характерно уменьшение растяжимости легких и легочных объемов, снижение DLCO, развитие артериальной гипоксемии (без гиперкапнии)