Two-component systems with equal concentrations of electrons and holes exhibit nonsaturating, linear magnetoresistance in classically strong magnetic fields. The effect is predicted to occur in finite-size samples at charge neutrality due to recombination. 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 two-and three-dimensional semimetals and narrow band semiconductors including most of the topological insulators.

We present theoretical and experimental studies of the direct current effect on the detection of subterahertz and terahertz radiation in gated two-dimensional structures. We developed a theory of the current-driven detection both for resonant case, when the fundamental frequency of plasma oscillation is large compared to inverse scattering time, omega(0)tau >> 1, and for the nonresonant case, omega(0)tau << 1, when the plasma oscillations are damped. We predict that, in the nonresonant case, even a very small dc current would increase the detection amplitude up to two orders of magnitude. Physically, this increase is related to an abrupt transition from the linear to saturation region near the knee of the current-voltage characteristic. When the current increases up to the saturation value, the electron concentration near the drain becomes very low and can be strongly affected by a small external field. As a consequence, the two-dimensional channel becomes extremely sensitive to external perturbations. In the resonant case, the detection amplitude has maxima when the radiation frequency is equal to fundamental plasma frequency and its harmonics. We predict that the effective linewidths of the respective resonances would decrease with the increasing current. Physically, this happens because dc current shifts the system towards the plasma wave instability. At some critical current value, the width corresponding to the fundamental frequency would turn to zero, indicating the onset of plasma waves generation. Our experimental measurements performed on GaAs HEMT confirm the theoretical predictions

We report on room-temperature, resonant detection of 0.6THz radiation by 250nm gate length GaAs∕AlGaAs heterostructure field-effect transistor. We show that the detection is strongly increased (and becomes resonant) when the drain current increases and the transistor is driven into the current saturation region. We interpret the results as due to resonant plasma wave detection that is enhanced by increasing the electron drift velocity.

The magnetoresistance of classical two-dimensional electrons scattered by randomly distributed impurities is investigated by numerical simulation. At low magnetic fields, we find for the first time a negative magnetoresistance proportional to |B|. This unexpected behavior is shown to be due to a memory effect specific for backscattering events, which was not considered previously.

We show that the enhancement of backscattering responsible for the weak localization is accompanied by reduction of the scattering in other directions.A simple quasiclassical interpretation of this phenomenon is presented in terms of a small change in the effective differential cross-section for a single impurity. The reduction of the scattering at the arbitrary angles leads to the decrease of the quantum correction to the conductivity. Within the diffusion approximation this decrease is small, but it should be taken into account in the case of a relatively strong magnetic field when the diffusion approximation is no more valid.

The classical two-dimensional problem of non-interacting electrons scattered by short-range impurity centers in the presence of magnetic field is investigated both analytically and numerically. A strong magnetoresistance exists in such a system, due to freely circling electrons, which are not taken into account by the Boltzmann-Drude approach. A parabolic magnetoresistance is found at low fields.Comment: 4 pages, 3 figure

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.Recently, signatures of the hydrodynamic behavior of charge carriers have been observed in graphene 1-3 , palladium cobaltate 4 , and the Weyl semimetal WP 2 5 . This phenomenon occurs in the intermediate temperature regime, where the typical length scale of electronelectron interaction, ee , is much shorter than any other relevant scale in the problem including those characterizing scattering off potential disorder and electron-phonon scattering, ee dis , ph . In this case, the independent particle approximation is violated, the motion of charge carriers becomes collective, and transport properties of the system are determined by interaction 6,7 .Viscous electronic fluids exhibit unusual transport properties 6,7 , such as superballistic transport 3,8,9 , nonlocal resistivity 1,10 , and negative magnetoresistance 5,11-14 . The latter effect may also occur in two-component systems (e.g., semimetals or narrow-band semiconductors) near the charge neutrality point 15 . In such systems, response of the charge carriers to the external magnetic field is non-universal depending on the interplay between inelastic scattering processes and sample geometry.In the hydrodynamic regime, electronic transport can be described with the help of the linearized hydrodynamic theory 12-15 generalizing the standard Navier-Stokes equation 16 . The parameters of the theory, including the shear viscosity coefficient, η xx , and quasiparticle recombination time, τ R , can be derived, at least in principle, from the kinetic equation approach (for a particular case of graphene, see Ref. 17). Due to the above two processes, the electric current density in a finite-sized sample is nonuniform. In long samples (where the length is much larger than the width, L W ), viscous effects tend to form a Poiseuille-like flow. The actual profile of the current density depends on the ratio of the typical length scale describing the viscous effects, the so-called Gurzhi length 15 , G (B), and the sample width, W . In the limit where the Gurzhi length exceeds the width, G W , the current density profile is parabolic, simi-FIG. 1: Schematic plot of the inhomogeneous electric current density in the regime of fast recombination and strong enough magnetic field. The color map e...

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 electronhole 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.The theory of magnetotransport in solids 1,2 is a mature branch of condensed matter physics. Measurements of magnetoresistance and classical Hall effect are long recognized as valuable experimental tools to characterize conducting samples. Interpreting the experiments within the standard Drude theory 1,3,4 , one may extract many useful sample characteristics such as the electron mobility and charge density at the Fermi level. However, in materials with more than one type of charge carriers -e.g., semimetals and narrow band semiconductors -the situation is more complex. Indeed, already in 1928 Kapitsa observed unconventional magnetoresistance in semi-metal bismuth films 5 . More recently, interest in magnetotransport has been revived with the discovery of novel twocomponent systems including graphene 6-11 , topological insulators 12-16 , and Weyl semimetals [17][18][19][20][21][22][23][24][25][26][27] . A common feature of all such systems is the existence of the charge neutrality (or, charge compensation) point, where the concentrations of the positively and negatively charged quasiparticles (electron-like and hole-like, respectively) are equal and the system is electrically neutral.A fast growing number of experiments on novel twocomponent materials exhibit unconventional transport properties in magnetic field: (i) linear magnetoresistance (LMR) was reported in graphene and topological insulators close to charge neutrality [85][86][87] . In weak fields, resistivity of two-dimensional (2D) electron systems acquires an interaction correction 88 that is linear in the field.The extreme quantum limit of Refs. 85-87 has been realized in graphene 28 , Bi 2 Te 3 nanosheets 56 , and possibly in the novel topological material LuPdBi 57 . However, this mechanism is applicable...