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Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields B = (B ⊥ , B ∥) tilted from the normal to the 2D electron layer at different temperatures T. The in-plane component (B ∥) of the field B induces magnetic entanglement between subbands, leading to beating in oscillating density of states (DOS) and to MISO suppression. Model of the MISO suppression is proposed. Within the model, a comparison of MISO amplitude in the entangled and disentangled (B ∥ = 0) 2D systems yields both difference frequency of DOS oscillations, d f , and strength of the electron–electron interaction, described by parameter ε F ∗ , in the 2D system. These properties are analyzed using two methods, yielding consistent but not identical results for both d f and ε F ∗. The analysis reveals an additional angular dependent factor of MISO suppression. The factor is related to spin splitting of quantum levels in magnetic fields. [ABSTRACT FROM AUTHOR]

We report on the optical realization of the magneto-intersubband oscillations that have been measured in the sub-terahertz transmittance of a GaAs quantum well with two subbands occupied. Following their dc analogue, the oscillations are periodic in the inverse magnetic field with the period governed by the subband gap. Their magnitude and polarization dependence accurately follow the presented simplified version of the dynamic magneto-intersubband oscillations equation that naturally combines dc magneto-intersubband oscillations with microwave-induced resistance oscillations. Simultaneously measured photoresistance also reveals its strong sensitivity to the sign of the circular polarization, proving the used theoretical modeling. [ABSTRACT FROM AUTHOR]

We address the temperature influence on the precessional motion of electron spins under a transverse magnetic field, studied in GaAs/AlGaAs triple quantum wells, using pump-probe Kerr rotation. In the presence of an applied in-plane magnetic field, the TRKR measurements show the robustness of carrier's spin polarization against temperature, which can be easily traced in an extended range up to 250 K. By tuning the pump-probe wavelength to the exciton bound to a neutral donor transition, we observed a remarkably long-lasting spin coherence (with dephasing time T 2 * > 14 ns) limited by the spin hopping process and exchange interaction between the donor sites, as well as the ensemble spread of the g-factor. The temperature dependent spin dephasing time revealed a double linear dependence due to the different relaxation mechanisms active in respective temperature ranges. We observed that the increase in sample temperature from 5 K to 250 K leads to a strong T 2 * reduction by almost 98%/97% for the excitation wavelengths of 823/821 nm. Furthermore, we noticed that the temperature increase not only causes the reduction of spin lifetime, but can also lead to the variation of the electron g-factor. Additionally, the spin dynamics were studied through the dependencies on the applied magnetic field and optical pump power. [ABSTRACT FROM AUTHOR]

Magneto-intersubband oscillations in a highly mobile two-subband electron system with one-dimensional periodic modulation of the potential under the conditions of overlapping Landau bands have been experimentally investigated. A significant modification of magneto-intersubband oscillations relative to the amplitude and phase is found: the amplitude decreases and the reversal of magneto-intersubband oscillations occurs in some ranges of magnetic fields. The obtained experimental data can be explained by a two-humped structure of the electron energy spectrum in Landau bands. [ABSTRACT FROM AUTHOR]

Low-temperature dependences of the amplitude of magneto-intersubband resistance oscillations () on the magnetic field T are studied in single GaAs quantum wells with the width () from 22 to 46 nm and two occupied quantum confinement subbands and . It is established that additional damping appears in dependences of on 1/ in the studied quantum wells, which is explained by the effect of large-scale fluctuations of the intersubband splitting on the amplitude of oscillations . It is found that the suppression of oscillations with the increase in 1/ is more efficient in "narrow" quantum wells. This experimental fact makes it possible to suppose that the main origin of fluctuations of in the studied narrow quantum wells is large-scale fluctuations of the well width . An expression taking into account the role of large-scale fluctuations of in the damping of is obtained. The comparison of theory and experiment has made it possible to determine the average amplitude of fluctuations of the intersubband splitting in the studied GaAs quantum wells. [ABSTRACT FROM AUTHOR]

Electron transport in single GaAs quantum wells of widths from 22 to 46 nm with two populated quantum-confinement subbands ES and EAS is investigated at a temperature of T = 4.2 K in tilted magnetic fields B < 2 T. The angle α between the applied magnetic field and the normal to the plane of the structure under study is varied from 0° to 90°. In a perpendicular magnetic field (α = 0), magnetointersubband oscillations with a period determined by the relation ΔSAS = EAS - ES = jħωc, where ωc is the cyclotron frequency and j is a positive integer, are observed in all investigated quantum wells. In tilted fields, the peaks of magnetointersubband oscillations are shifted toward higher fields B cos α. This shift is explained by an increase in the energy splitting ΔSAS with increasing component B sin α. In 46- and 36-nm-wide quantum wells, beats of magnetointersubband oscillations are observed at angles α > 72° and α > 85°, respectively. The origin of this unexpected behavior of magnetointersubband oscillations in tilted magnetic fields is discussed. [ABSTRACT FROM AUTHOR]

The electron properties of artificially disordered superlattices embedded in a wide AlGaAs parabolic well were investigated in a strong magnetic field. We demonstrated that in the extreme quantum limit the interlayer disorder results in formation of a new correlated phase. A nearly uniform electron distribution over the superlattice wells was found in a weak magnetic field. However, a nonuniform phase with partially localized electrons, representing well-developed fractional quantum Hall effect features, was observed in high magnetic field (at the filling factor ν < 1). A distinct magnetic field-induced transition separates these two phases. [ABSTRACT FROM AUTHOR]

The electron temperature of a two-dimensional electron gas heated by an electric field in DA-pHEMT heterostructures was determined by the Shubnikov - de Haas (SdH) oscillation method separately for each size-quantization subband. An analysis of the Fourier spectra of SdH oscillations showed that throughout the entire range of electron temperatures obtained, harmonics associated with different size-quantization subbands and transitions between the Landau levels of these subbands dominate in the spectra. To separate the SdH oscillations into individual harmonics, we used the approximation of the magnetic-field dependences of the oscillations by theoretical expressions using the ratio between the peak heights in the Fourier spectra. It is shown that this method allows one to determine the values of the electron temperature in case when the electron concentrations in the first and second subbands are close in magnitude. The obtained values of the electron temperature correspond to the literature data. [ABSTRACT FROM AUTHOR]

We report in detail oscillatory magnetoresistance in double quantum wells under microwave irradiation. The experimental investigation contains measurements of frequency, power and temperature dependence. In theory, the observed interference oscillations are explained in terms of the influence of subband coupling on the frequency-dependent photoinduced part of the electron distribution function. Thus, the magnetoresistance shows the interference of magneto-intersubband and conventional microwave induced resistance oscillations. [ABSTRACT FROM AUTHOR]

We have studied the quantum Hall effect in AlxGa1-xAs-double well structure with vanishing g-factor. We determined the density-magnetic field ns - B diagrams for the longitudinal resistance Rxx. In spite of the fact that the ns - B diagram for conventional GaAs double wells shows a striking similarity with the theory, we observed the strong difference between these diagrams for double wells with vanishing g-factor. We argue that the electron-electron interaction is responsible for unusual behavior of the Landau levels in such a system. [ABSTRACT FROM AUTHOR]

Here we present the results of magneto resistance measurements in tilted magnetic field and compare them with calculations. The comparison between calculated and measured spectra for the case of perpendicular fields enable us to estimate the dependence of the valley splitting as a function of the magnetic field and the total Landé g-factor (which is assumed to be independent of the magnetic field). Since both the exchange contribution to the Zeeman splitting as well as the valley splitting are properties associated with the 2D quantum confinement, they depend only on the perpendicular component of the magnetic field, while the bare Zeeman splitting depends on the total magnetic field. This information aided by the comparison between experimental and calculated gray scale maps permits to obtain separately the values of the exchange and the bare contribution to the g-factor. [ABSTRACT FROM AUTHOR]

Effect of an in-plane magnetic field on the features of the magnetoresistance of a narrow conducting channel placed in the bath of a macroscopic two-dimensional electron gas has been studied. These features are manifested in the hysteretic behavior of the magnetoresistance in the quantum Hall effect regime. It has been found that the hysteresis loops observed in different ranges of the filling factor may be separated into two groups that differ in both the response to the in-plane magnetic field and the temperature dependence. The basic features observed near the integer filling factors ν = 1 and 2 are almost independent of the in-plane magnetic field. Therefore, their origin is not associated with spin effects. At the same time, additional features that appear at ν ≈ 1.8 and 2.2 are suppressed by the in-plane magnetic field B| ≈ 6 T and almost temperature-independent from 45 mK to 1 K. [ABSTRACT FROM AUTHOR]

We have compared the properties of the nonequilibrium state of the two-dimensional electron gas observed in the samples of different types by means of magnetotransport and magnetization measurements in the quantum Hall effect regime at integer filling factors n. It has been found that the range of filling factors corresponding to the nonequilibrium state is universal for the samples of different types and different measurement techniques and varies from 0.1 to 0.3 for n changing from 1 to 4. The comparison indicates that the observed nonequilibrium state is not directly caused by the appearance of eddy currents and the dielectric phase in the two-dimensional electron gas but is probably associated with the magnetic field-induced phase transition. [ABSTRACT FROM AUTHOR]

The effect of microwave radiation on magnetotransport in single GaAs quantum wells with anisotropic mobility, whose maximum corresponds to the $$[1\bar 10]$$ direction and minimum to the [110] direction, is investigated using the Van der Pauw method. In samples shaped as squares with sides oriented along the $$[1\bar 10]$$ and [110] directions, giant oscillations of magnetoresistance arise under the effect of a microwave field for the both $$[1\bar 10]$$ and [110] orientations of the measuring current I ac. In the anisotropic two-dimensional system under study, the relative amplitude of microwave photoresistance oscillations in a magnetic field weakly depends on the orientation of I ac. At a temperature of 4.2 K and a microwave frequency of 130 GHz, magnetic field intervals characterized by close-to-zero resistance manifest themselves only for the case of the [110] orientation of I ac. The aforementioned experimental results are qualitatively explained by a quasi-one-dimensional potential modulation of the two-dimensional electron gas in the [110] direction. [ABSTRACT FROM AUTHOR]

The effect of microwave electromagnetic radiation on the resistance of the 2D electron gas in a GaAs/AlAs heterostructure in a strong magnetic field is investigated. It is shown that, under the nonequilibrium conditions caused by microwave radiation, the aforementioned 2D system exhibits giant oscillations of its resistance with varying magnetic field. When the measuring current density is small, an increase in the microwave power leads to the appearance of an absolute negative resistance at the main minimum of these oscillations, which lies near the cyclotron resonance. The experimental data are found to be in qualitative agreement with the theory of multiphoton photoinduced impurity scattering [J. Inarrea and G. Platero, Appl. Phys. Lett. 89, 052109 (2006)]. [ABSTRACT FROM AUTHOR]

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 106 cm2/V s is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in the 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 106 cm2/V s does not prevent the formation of zero-resistance states in a magnetic field in a two-dimensional system under the effect of microwave radiation. [ABSTRACT FROM AUTHOR]

Commensurability oscillations of thermopower in a square antidot lattice are observed. The oscillations are attributed to the geometrical resonances of the classical electron motion in a magnetic field and are much more pronounced than the corresponding magnetoresistance oscillations. The off-diagonal component of the thermopower tensor (the Nernst–Ettingshausen effect) changes sign at resonances. Additional measurements of magnetoresistance verify the correctness of the method used for thermopower measurements and provide information on the temperature distribution in the sample. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The results of studying the special features of the dependence of lateral photoconductivity in AlGaAs/InGaAs structures with quantum dots and quantum wells on the intensity of interband light at low temperatures are reported. It is found that there is a threshold for the increase in photoconductivity. Oscillations of photoconductivity are observed at relatively high pulling fields. The effects of the pulling field and temperature on the photoconductivity are studied. The results are analyzed in terms of the theory of percolation of nonequilibrium charge carriers over localized states, taking into account the relaxation of stresses around quantum dots. [ABSTRACT FROM AUTHOR]

The Aharonov–Bohm effect in submicron rings with narrow electron channels was studied in the range of magnetic fields from 0 to 15 T and temperatures from 0.1 to 10 K. It is found that the temperature dependences of the h/e-oscillation amplitude at low magnetic fields and in the situation of tunnel-coupled edge current states are different. The obtained experimental data are explained by the influence of Fermi-system chirality on the coherent transport in a ring interferometer. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We study electron transport in suspended semiconductor microstructures fabricated from AlAs/GaAs membranes containing a high mobility two-dimensional electron gas. In quantizing magnetic fields, a reflection of edge current channels from the border of suspended area is observed resulting in the absence of vanishing magnetoresistance in the quantum Hall effect (QHE) regime. Relocation of this border out of the Hall bar revives the QHE. We have also found that the critical current of the breakdown of QHE in suspended samples is three times lower than in non-suspended samples due to the peculiarity of heat transport in the membranes. [ABSTRACT FROM AUTHOR]

A photoluminescence (PL) study of the individual electron states localized in a random potential is performed in artificially disordered superlattices embedded in a wide parabolic well. The valence band bowing of the parabolic potential provides a variation of the emission energies which splits the optical transitions corresponding to different wells within the random potential. The blueshift of the PL lines emitted by individual random wells, observed with increasing disorder strength, is demonstrated. The variation of temperature and magnetic field allowed for the behavior of the electrons localized in individual wells of the random potential to be distinguished. [ABSTRACT FROM AUTHOR]

The effect of the measuring current Idc on the magnetoresistance (MR) of a high-mobility two-dimensional electron gas (2DEG) in a GaAs quantum well with AlAs/GaAs superlattice barriers has been studied. It has been found that, as Idc increases, the MR of the 2DEG in the studied structures becomes negative in the range of classically strong magnetic fields. It has been shown that the observed negative MR is due to the transport of the 2DEG in the nonlinear regime. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

We show that the electron spin polarization generated by an electrical current may have its direction controlled and magnitude amplified by periodic optical excitation. The electrical and optical spin control methods were combined and implemented in a two-dimensional electron gas. By Kerr rotation in an external transverse magnetic field, we demonstrate unexpected long-lived coherent spin oscillations of the current-induced signal in a system with large spin-orbit interaction. Using a single linearly polarized pulse for spin manipulation and detection, we found a strong dependence on the pulse optical power and sample temperature indicating the relevance of the hole spin in the electron spin initialization. The signal was mapped in a Hall bar as function of the position relative to the injection contact. Finally, the presence of an in-plane spin polarization was directly verified by rotating the experimental geometry. [ABSTRACT FROM AUTHOR]

Polarized magnetophotoluminescence is employed to study the energies and occupancies of four lowest Landau levels in a couple quantum Hall GaAs/AlGaAs double quantum well. As a result, a magnetic field-induced redistribution of charge over the Landau levels manifesting to the continuous formation of the charge density wave and direct evidence for the symmetric-antisymmetric gap shrinkage at v = 3 are found. The observed interlayer charge exchange causes depolarization of the ferromagnetic ground state. [ABSTRACT FROM AUTHOR]

We study nonlinear transport phenomena in a high-mobility bilayer system with two closely spaced populated electronic subbands in a perpendicular magnetic field. For a moderate direct current excitation, we observe zero-differential-resistance states with a characteristic 1/B periodicity. We investigate, both experimentally and theoretically, the Hall-induced resistance oscillations which modulate the high-frequency magnetointersubband oscillations in our system if we increase the current. We also observe and describe the influence of direct current on the magnetoresistance in the presence of microwave irradiation. [ABSTRACT FROM AUTHOR]

The influence of microwave irradiation on dissipative and Hall resistance in high-quality bilayer electron systems is investigated experimentally. We observe a deviation from odd symmetry under magnetic-field reversal in the microwave-induced Hall resistance ΔRxy, whereas the dissipative resistance ΔRxx obeys even symmetry. Studies of ΔRxy as a function of the microwave electric field and polarization exhibit a strong and nontrivial power and polarization dependence. The obtained results are discussed in connection to existing theoretical models of microwave-induced photoconductivity. [ABSTRACT FROM AUTHOR]

We observe zero-differential resistance states at low temperatures and moderate direct currents in a bilayer electron system formed by a wide quantum well. Several regions of vanishing resistance evolve from the inverted peaks of magneto-intersubband oscillations as the current increases. The experiment, supported by a theoretical analysis, suggests that the origin of this phenomenon is based on instability of homogeneous current flow under conditions of negative differential resistivity, which leads to formation of current domains in our sample, similar to the case of single-layer systems. [ABSTRACT FROM AUTHOR]

By measuring the thermoelectric effect in high-mobility quantum wells with two occupied subbands in perpendicular magnetic field, we detect magnetophonon oscillations due to interaction of electrons with acoustic phonons. These oscillations contain specific features identified as combined resonances caused by intersubband phonon-assisted transitions of electrons in the presence of Landau quantization. The quantum theory of phonon-drag magnetothermoelectric effect, generalized to the case of multisubband occupation, describes our experimental findings. [ABSTRACT FROM AUTHOR]