Nonmonotonic magnetoresistance of a two-dimensional viscous electron-hole fluid in a confined geometry

Ultra-pure conductors may exhibit hydrodynamic transport where the collective motion of charge carriers resembles the flow of a viscous fluid. In a confined geometry (e.g., in ultra-high quality nanostructures) the electronic fluid assumes a Poiseuille-like flow. Applying an external magnetic field tends to diminish viscous effects leading to large negative magnetoresistance. In two-component systems near charge neutrality the hydrodynamic flow of charge carriers is strongly affected by the mutual friction between the two constituents. At low fields, the magnetoresistance is negative, however at high fields the interplay between electron-hole scattering, recombination, and viscosity results in a dramatic change of the flow profile: the magnetoresistance changes its sign and eventually becomes linear in very high fields. This novel non-monotonic magnetoresistance can be used as a fingerprint to detect viscous flow in two-component conducting systems.Comment: 10 pages, 8 figure

Counterflows in viscous electron-hole fluid

In ultra-pure conductors, collective motion of charge carriers at relatively high temperatures may become hydrodynamic such that electronic transport may be described similarly to a viscous flow. In confined geometries (e.g., in ultra-high quality nanostructures), the resulting flow is Poiseuille-like. When subjected to a strong external magnetic field, the electric current in semimetals is pushed out of the bulk of the sample towards the edges. Moreover, we show that the interplay between viscosity and fast recombination leads to the appearance of counterflows. The edge currents possess a non-trivial spatial profile and consist of two stripe-like regions: the outer stripe carrying most of the current in the direction of the external electric field and the inner stripe with the counterflow.Comment: 10 pages, 5 figure

Magnetoresistance of compensated semimetals in confined geometries

Two-component conductors -- e.g., semi-metals and narrow band semiconductors -- often exhibit unusually strong magnetoresistance in a wide temperature range. Suppression of the Hall voltage near charge neutrality in such systems gives rise to a strong quasiparticle drift in the direction perpendicular to the electric current and magnetic field. This drift is responsible for a strong geometrical increase of resistance even in weak magnetic fields. Combining the Boltzmann kinetic equation with sample electrostatics, we develop a microscopic theory of magnetotransport in two and three spatial dimensions. The compensated Hall effect in confined geometry is always accompanied by electron-hole recombination near the sample edges and at large-scale inhomogeneities. As the result, classical edge currents may dominate the resistance in the vicinity of charge compensation. The effect leads to linear magnetoresistance in two dimensions in a broad range of parameters. In three dimensions, the magnetoresistance is normally quadratic in the field, with the linear regime restricted to rectangular samples with magnetic field directed perpendicular to the sample surface. Finally, we discuss the effects of heat flow and temperature inhomogeneities on the magnetoresistance.Comment: 22 pages, 7 figures, published versio

Magnetoresistance in two-component systems

Two-component systems with equal concentrations of electrons and holes exhibit non-saturating, linear magnetoresistance in classically strong magnetic fields. The effect is predicted to occur in finite-size samples at charge neutrality in both disorder- and interaction-dominated regimes. The phenomenon originates in the excess quasiparticle density developing near the edges of the sample due to the compensated Hall effect. The size of the boundary region is of the order of the electron-hole recombination length that is inversely proportional to the magnetic field. In narrow samples and at strong enough magnetic fields, the boundary region dominates over the bulk leading to linear magnetoresistance. Our results are relevant for semimetals and narrow-band semiconductors including most of the topological insulators.Comment: 11 pages, 3 figure

Linear magnetoresistance in compensated graphene bilayer

We report a nonsaturating linear magnetoresistance in charge-compensated bilayer graphene in a temperature range from 1.5 to 150 K. The observed linear magnetoresistance disappears away from charge neutrality ruling out the traditional explanation of the effect in terms of the classical random resistor network model. We show that experimental results qualitatively agree with a phenomenological two-fluid model taking into account electron-hole recombination and finite-size sample geometry

A nonperturbative model for the strong running coupling within potential approach

A nonperturbative model for the QCD invariant charge, which contains no low-energy unphysical singularities and possesses an elevated higher loop corrections stability, is developed in the framework of potential approach. The static quark-antiquark potential is constructed by making use of the proposed model for the strong running coupling. The obtained result coincides with the perturbative potential at small distances and agrees with relevant lattice simulation data in the nonperturbative physically-relevant region. The developed model yields a reasonable value of the QCD scale parameter, which is consistent with its previous estimations obtained within potential approach.Comment: 14 pages, 4 figure

Dynamical boundary conditions for integrable lattices

Some special solutions to the reflection equation are considered. These boundary matrices are defined on the common quantum space with the other operators in the chain. The relations with the Drinfeld twist are discussed.Comment: LaTeX, 12page

Photometric and spectral variability of active star VY Arietis

Представлены квазиодновременные UBVRI фотометрические и H спектральные наблюдения хромосферно-активной двойной VY Ari. Ее фотометрическая переменность описывается зональной моделью запятненности, где пятна занимают до 32 % полной поверхности звезды. Пятна локализованы в низких и средних широтах, они холоднее спокойной фотосферы на 1 300 К. Спектральные наблюдения показывают в некоторые эпохи наличие в хромосфере звезды активных областей повышенной электронной плотности (факе- лов), сконцентрированных вблизи наиболее запятненных (активных) долгот. Отмечается цикличность в долговременных изменениях площади пятен и основных параметров чистой эмиссии H с характерным временем 9 10 лет.Quasi-simultaneous UBVRI photometric, and H spectroscopic observations of the chromospherically active binary star VY Ari are presented. The photometric variability of the star can be described by a zonal spottedness model. Spotted regions occupy up to 32 % of the total stellar surface. The temperature difference between the unspotted photosphere and starspots is about 1 300 K. Starspots are localized at middle-low latitudes. The spectroscopic observations show in some epochs the presence of chromospherically active regions with a higher electron density (plages), which concentrated near the mostly spotted stellar longitudes. There are the cyclic variations of the spot area and the H pure emission parameters with a typical time is about 9—10 years.Работа выполнена при частричной поддержке гранта РФФИ 16-02-00689, а также при финансовой поддержке Правительства Российской Федерации (постановление № 211, контракт 02.A03.21.0006)