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 low-temperature(4.2 < T < 12.5 K) magnetotransport (B < 2 T) of two-dimensional electrons occupying two subbands (with energy E 1 and E 2 ) is investigated in GaAs single quantum well with AlAs/GaAs superlattice barriers. Two series of Shubnikov-de Haas oscillations are found to be accompanied by magnetointersubband (MIS) oscillations, periodic in the inverse magnetic field. The period of the MIS oscillations obeys condition ∆ 12 =(E 2 −E 1 )=k · hω c , where ∆ 12 is the subband energy separation, ω c is the cyclotron frequency, and k is the positive integer. At T=4.2 K the oscillations manifest themselves up to k=100. Strong temperature suppression of the magnetointersubband oscillations is observed. We show that the suppression is a result of electron-electron scattering. Our results are in good agreement with recent experiments, indicating that the sensitivity to electron-electron interaction is the fundamental property of magnetoresistance oscillations, originating from the second-order Dingle factor.The Landau quantization in quasi-two-dimensional (2D) electron systems (with two or more occupied subbands) manifests itself in two or more sets of Landau levels. Resonance transitions of electrons between Landau levels corresponding to different two-dimensional subbands [1,2] causes the so-called magnetointersubband (MIS) oscillations of the resistance ρ xx [3][4][5]. The interaction between two subbands can be also significant for other phenomena such as cyclotron resonance [6]. The position of the maxima of the MIS oscillations obeys the condition ∆ 12 =E 2 −E 1 =k · hω c , where ∆ 12 is the intersubband energy gap, E i is the energy of the bottom of ith subband, ω c is the cyclotron frequency, and index k is the positive integer. The oscillations, similar to well-known Shubnikov-de Haas (SdH) oscillations, are periodic in the inverse magnetic field and appear in classically strong magnetic fields. The amplitude of SdH oscillations is limited by the broadening of Landau levels due to scattering and by thermal broadening of the Fermi distribution. With increasing temperature the thermal broadening of the Fermi distribution becomes the dominant factor, limiting the amplitude of SdH oscillations. MIS oscillations are significantly less sensitive to the electron distribution and their amplitude is predominantly determined by a quantum relaxation time τ q [4,5].MIS oscillations were recently observed in GaAs double quantum wells with AlAs/GaAs superlattice barriers with roughly equal electron densities in subbands (n 1 ≈ n 2 ) [7][8][9][10]. The quantum lifetimes of the electrons in subbands was also approximately equal (τ q1 ≈ τ q2 ) [11]. In the general case of two populated subbands the amplitude of the MIS oscillations of the resistance ∆ρ MISO depends on the sum of the quantum scattering rates in each subband [4,5] ),where 1/τ qi and n i are the quantum scattering rate and electron density in ith subband, and m is electron band mass. Parameter ν 12 is an effective intersubband scattering rate [5].In this ...

Abstrad. Negarive linear magnetoresistance o f two-dimensional (20) electrons has been found in a disordered m y of antidots. We suggest ffiat uajectories thal mll along Le array of anlidots exist in a magnetic field. These trajectories have a mean free path larger than the average value for electrons with ordinary diffusion.

We have studied negative magnetoresistance of a nonplanar two-dimensional electron gas. Effectively due to the curved AlGaAs/GaAs interface, electrons see a uniform in-plane magnetic field B as a random magnetic field ͑RMF͒. Small additional perpendicular B leads to a negative magnetoresistance predicted by a semiclassical treatment of the RMF problem beyond the relaxation-time approximation, in accordance with our observations.

We study a Al x Ga x−1 As parabolic quantum well ͑PQW͒ with GaAs/ Al x Ga x−1 As square superlattice. The magnetotransport in PQW with intentionally disordered short-period superlattice reveals a surprising transition from electrons distribution over whole parabolic well to independent-layer states with unequal density. The transition occurs in the perpendicular magnetic field at Landau filling factor Ϸ 3 and is signaled by the appearance of the strong and developing fractional quantum Hall ͑FQH͒ states and by the enhanced slope of the Hall resistance. We attribute the transition to the possible electron localization in the x-y plane inside the lateral wells, and formation of the FQH states in the central well of the superlattice, driven by electron-electron interaction.

We study Shubnikov de Haas ͑SdH͒ oscillations in a nonplanar stripe-shaped two-dimensional electron gas ͑2DEG͒. The effective-field normal to the nonplanar 2DEG is spatially modulated, when uniform external magnetic field is applied. We find that the amplitude of the SdH oscillations dramatically drops in the tilted magnetic field. From the Dingle plot of SdH oscillations we extract single-particle relaxation time. Reduction of this time in the tilted field, which leads to the enhanced damping of SdH oscillations, is shown to be due to the scattering of the electron by magnetic-field fluctuations. We calculate quantum lifetime of the electron in a tilted magnetic field. The agreement between these calculations and experimental result is found. In order to explain the damping of the SdH oscillations for magnetic field BϾ1 T we also take into account the spatial variation of the Landau filling factor.

Shubnikov-de Haas oscillations are measured in wide parabolic quantum wells with five to eight subbands in a tilted magnetic field. We find two types of oscillations. The oscillations at low magnetic fields are shifted toward higher field with the tilt angle increasing and can be attributed to two-dimensional Landau states. The position of the oscillations of the second type does not change with increasing the tilt angle which points to a three-dimensional character of these Landau states. We calculate the level broadening due to the elastic scattering rate ⌫ϭប/2, where is the quantum time, and the energy separation between two-dimensional subbands, ⌬ i j ϭE j ϪE i , in a parabolic well. For all levels we obtain ⌫ j ϳ⌬ i j , which means that the levels overlap, supporting the observation of three-dimensional Landau states. Surprisingly, we find that the lowest subband, which has a smaller energy separation from the higher level, does not overlap with these subbands and forms a two-dimensional state.