Faraday rotation and depolarization of synchrotron radio emission are considered in a consistent general approach, under conditions typical of spiral galaxies, i.e. when the magneto‐ionic medium and relativistic electrons are non‐uniformly distributed in a layer containing both regular and fluctuating components of magnetic field, thermal electron density and synchrotron emissivity. We demonstrate that non‐uniformity of the magneto‐ionic medium along the line of sight strongly affects the observable polarization patterns. The degree of polarization p and the observed Faraday rotation measure RM are very sensitive to whether or not the source is symmetric along the line of sight. The RM may change sign in a certain wavelength range in an asymmetric slab even when the line‐of‐sight magnetic field has no reversals. Faraday depolarization in a purely regular magnetic field can be much stronger than suggested by the low observed rotation measures. A twisted regular magnetic field may result in p increasing with λ— a behaviour detected in several galaxies.  We derive expressions for statistical fluctuations in complex polarization and show that random fluctuations in the degree of polarization caused by Faraday dispersion are expected to become significantly larger than the mean value of p at λ ≳ 20 − 30 cm. We also discuss depolarization arising from a gradient of Faraday rotation measure across the beam, both in the source and in an external Faraday screen. We briefly discuss applications of the above results to radio polarization observations.  We discuss how the degree of polarization is affected by the scaling of synchrotron emissivity ɛ with the total magnetic field strength B. We derive formulae for the complex polarization at λ → 0 under the assumption that ɛ ∝ B2B2⊥, which may arise under energy equipartition or pressure balance between cosmic rays and magnetic fields. The resulting degree of polarization is systematically larger than for the usually adopted scaling ɛ ∝ B2⊥; the difference may reach a factor of 1.5.

Zero differential resistance state is found in response to direct current applied to 2D electron systems at strong magnetic field and low temperatures. Transition to the state is accompanied by sharp dip of negative differential resistance, which occurs above threshold value I th of the direct current. The state depends significantly on the temperature and is not observable above several Kelvins. Additional analysis shows lack of the linear stability of the 2D electron systems at I > I th and inhomogeneous, non-stationary pattern of the electric current in the zero differential resistance state. We suggest that the dc bias induced redistribution of the 2D electrons in energy space is the dominant mechanism leading to the new electron state.

The effect of a DC electric field on the longitudinal resistance of highly mobile two dimensional electrons in heavily doped GaAs quantum wells is studied at different magnetic fields and temperatures. Strong suppression of the resistance by the electric field is observed in magnetic fields at which the Landau quantization of electron motion occurs. The phenomenon survives at high temperature where Shubnikov de Haas oscillations are absent. The scale of the electric fields essential for the effect is found to be proportional to temperature in the low temperature limit. We suggest that the strong reduction of the longitudinal resistance is the result of a nontrivial change in the distribution function of 2D electrons induced by the DC electric field. Comparison of the data with recent theory yields the inelastic electron-electon scattering time τ in and the quantum scattering time τ q of 2D electrons at high temperatures, a regime where previous methods were not successful.

The effect of a microwave field in the frequency range from 54 to 140 GHz on the magnetotransport in a GaAs quantum well with AlAs/GaAs superlattice barriers and with an electron mobility no higher than 10 6 cm 2 /Vs is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in magnetic field being determined by the ratio of the radiation frequency to the cyclotron frequency. Earlier, such oscillations had only been observed in GaAs/AlGaAs heterostructures with much higher mobilities. When the samples under study are irradiated with a 140-GHz microwave field, the resistance corresponding to the main oscillation minimum, which occurs near the cyclotron resonance, appears to be close to zero. The results of the study suggest that a mobility value lower than 10 6 cm 2 /Vs does not prevent the formation of zero-resistance states in magnetic field in a two-dimensional system under the effect of microwave radiation. Current interest in studying the transport in twodimensional (2D) electron systems is related to the recent observation of resistance oscillations in magnetic field that arise in high-mobility GaAs/AlGaAs heterostructures under the effect of microwave radiation [1]. It was found that these oscillations are periodic in the inverse magnetic field (1/B) with a period determined by the ratio of the microwave radiation frequency to the cyclotron frequency. The photoresponse oscillations in magnetic field in a high-mobility 2D system (such oscillations were predicted more than 30 years ago [2]) fundamentally differed from the behavior of photoresponse in GaAs/AlGaAs heterostructures with lower mobilities [3]. The effect of microwave radiation on the magnetotransport in GaAs/AlGaAs heterostructures of moderate quality was found to manifest itself as a photoresistance peak caused by the heating of the 2D electron gas under the magnetoplasma resonance conditions [4]. Soon after the first experimental observation of the microwave radiation-induced resistance oscillations in magnetic field in high-mobility GaAs/AlGaAs heterostructures, it was shown that the minima of these oscillations may correspond to resistance values close to zero [5,6,7]. This unexpected experimental result initiated intensive theoretical studies of the aforementioned phenomenon [7,8,9,10,11,12,13,14,15,16]. However, despite the multitude of theoretical publications, the mechanisms responsible for the resistance oscillations under the effect of a microwave field in 2D systems with large filling factors remain open to discussion. The role of the mobility of charge carriers in the manifestation of microwaveinduced zero-resistance states arising in magnetic field in 2D systems also remains unclear. It is commonly believed that the mobility should exceed 3 × 10 6 cm 2 /Vs[17]. As for the experimental studies of the photoresponse to microwave radiation in 2D systems in classically strong magnetic fields, such studies, excluding a few of them [17,18,19,2...

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The magnetotransport of highly mobile 2D electrons in wide GaAs single quantum wells with three populated subbands placed in titled magnetic fields is studied. The bottoms of the lower two subbands have nearly the same energy while the bottom of the third subband has a much higher energy (E1 ≈ E2 << E3). At zero in-plane magnetic fields magneto-intersubband oscillations (MISO) between the i th and j th subbands are observed and obey the relation ∆ij = Ej −Ei = k·hωc, where ωc is the cyclotron frequency and k is an integer. An application of in-plane magnetic field produces dramatic changes in MISO and the corresponding electron spectrum. Three regimes are identified. Athωc ≪ ∆12 the in-plane magnetic field increases considerably the gap ∆12, which is consistent with the semi-classical regime of electron propagation. In contrast at strong magnetic fieldshωc ≫ ∆12 relatively weak oscillating variations of the electron spectrum with the in-plane magnetic field are observed. Athωc ≈ ∆12 the electron spectrum undergoes a transition between these two regimes through magnetic breakdown. In this transition regime MISO with odd quantum number k terminate, while MISO corresponding to even k evolve continuously into the high field regime corresponding tohωc ≫ ∆12.

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