Hall resistivity of granular metals

We calculate the Hall conductivity \sig_{xy} and resistivity $\rho_{xy}$ of a granular system at large tunneling conductance $g_{T}\gg 1$. 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 $\rho_{xy}=H/(n^* e c)$, where $n^*$ differs from the carrier density $n$ inside the grains by a numerical coefficient determined by the shape of the grains. The Coulomb interaction gives rise to logarithmic in temperature $T$ correction to $\rho_{xy}$ in the range \Ga \lesssim T \lesssim \min(g_T E_c,\ETh), where \Ga is the tunneling escape rate, $E_c$ 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 $\sigma_{xy}$ and resistivity $\rho_{xy}$ 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 $\rho_{xy}$ and the longitudinal resistivity $\rho_{xx}$ 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 $T_c^\text{max}\sim 10^{-7}\epsilon_F\lesssim 1 \text{mK}$ is extremely small ($\epsilon_F$ is the Fermi energy). In addition, any sufficiently short-range disorder with the scattering time $\tau \lesssim \hbar /T_c^\text{max}$ 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 $g_{T}\gg 1$. 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 $\rho_{xy}$ depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula $\rho_{xy}=H/(n^* e c)$, where $n^*$ differs from the carrier density $n$ 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 $1/g_T$ corrections and find that (i) in a wide range of temperatures T \gtrsim \Ga exceeding the tunneling escape rate \Ga, the Hall resistivity $\rho_{xy}$ and conductivity \sig_{xy} acquire logarithmic in $T$ 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-$N$ 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 $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ 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-$N$ 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 $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ 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-$N$ 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 $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ 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

X-ray Ptychographic Imaging and Spectroscopic Studies of Plasma-Treated Plastic Films

Polyethylene terephthalate (PET) is a thermoplastic polyester with numerous applications in industry. However, it requires surface modification on an industrial scale for printing and coating processes and plasma treatment is one of the most commonly used techniques to increase the hydrophilicity of the PET films. Systematic improvement of the surface modification by adaption of the plasma process can be aided by a comprehensive understanding of the surface morphology and chemistry. However, imaging large surface areas (tens of microns) with a resolution that allows understanding the surface quality and modification is challenging. As a proof-of-principle, plasma-treated PET films were used to demonstrate the capabilities of X-ray ptychography, currently under development at the soft X-ray free-electron laser FLASH at DESY, for imaging macroscopic samples. In combination with scanning electron microscopy (SEM), this new technique was used to study the effects of different plasma treatment processes on PET plastic films. The studies on the surface morphology were complemented by investigations of the surface chemistry using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). While both imaging techniques consistently showed an increase in roughness and change in morphology of the PET films after plasma treatment, X-ray ptychography can provide additional information on the three-dimensional morphology of the surface. At the same time, the chemical analysis shows an increase in the oxygen content and polarity of the surface without significant damage to the polymer, which is important for printing and coating processes

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)