Magneto-intersubband Oscillations Research Articles

Resonance microwave photoresistance of a two-subband electron system at large filling factors

The microwave photoresistance of a double GaAs quantum well with two occupied size-quantization sub-bands E 1 and E 2 has been studied at the temperatures T = 1.6–4.2 K in the magnetic fields B < 0.5 T. The microwave photoresistance of such a system has been found to have a maximum amplitude when the maximum of the magneto-intersubband oscillations with the number k = (E 2 − E 1)ℏωc coincides with the maximum or minimum of the ω/ωc oscillations, where ω is the microwave frequency and ωc is the cyclotron frequency. It has been shown that the resonance photoresistance that appears in the kth maximum of the magneto-intersubband oscillations is determined by the condition ℏω/(E 2 − E 1) = (j ± 0.2)/k, where k and j are positive integers.

Interference of magneto-intersubband and phonon-induced resistance oscillations in single GaAs quantum wells with two populated subbands

Low-temperature electron magnetotransport in single GaAs quantum wells with two populated subbands is studied at large filling factors. Magneto-intersubband (MIS) and acoustic-phonon-induced oscillations of dissipative resistance are found to be coexisting but interfering substantially with each other. The experiments show that amplitude of the MIS oscillations enhances significantly by phonons, indicating ``constructive interference'' between the electron-phonon scattering and the intersubband electron transitions. Temperature damping of the quantum oscillations is found to be related to broadening of Landau levels caused by considerable electron-electron scattering.

Magnetoresistance oscillations in multilayer systems: Triple quantum wells

Magnetoresistance of two-dimensional electron systems with several occupied subbands oscillates owing to periodic modulation of the probability of intersubband transitions by the quantizing magnetic field. In addition to previous investigations of these magnetointersubband (MIS) oscillations in two-subband systems, we report on both experimental and theoretical studies of such a phenomenon in three-subband systems realized in triple quantum wells. We show that the presence of more than two subbands leads to a qualitatively different MIS oscillation picture, described as a superposition of several oscillating contributions. Under a continuous microwave irradiation, the magnetoresistance of triple-well systems exhibits an interference of MIS oscillations and microwave-induced resistance oscillations. The theory explaining these phenomena is presented in the general form, valid for an arbitrary number of subbands. A comparison of theory and experiment allows us to extract temperature dependence of quantum lifetime of electrons and to confirm the applicability of the inelastic mechanism of microwave photoresistance for the description of magnetotransport in multilayer systems.

Microwave induced magnetoresistance oscillations and inelastic scattering time in double quantum wells

The interference of microwave-induced resistance oscillations and magneto-intersubband oscillations in double quantum wells exposed to a continuous microwave irradiation is under study. By comparing experimental and theoretical magnetoresistance traces at different temperatures, we confirm that the inelastic mechanism of photoresistance explains our observations up to T ≃ 4 K . For higher temperatures, our results suggest a deviation of the inelastic scattering time τ in from the predicted T − 2 dependence.

Magneto-intersubband oscillations in triple quantum wells

We present magnetotransport studies of high-density triple quantum well samples with different barrier widths. Because of electron transitions between three occupied 2D subbands, the magnetoresistance shows magneto-intersubband oscillations whose periodicity is determined by the subband separation energies. Temperature-dependent measurements allow us to extract quantum lifetime of electrons. A theoretical consideration of the observed phenomenon is also presented.

Effect of electron-electron scattering on magnetointersubband resistance oscillations of two-dimensional electrons in GaAs quantum wells

The low-temperature $(4.2lTl12.5\text{ }\text{K})$ magnetotransport $(Bl2\text{ }\text{T})$ 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 ${\ensuremath{\Delta}}_{12}=({E}_{2}\ensuremath{-}{E}_{1})=k\ensuremath{\cdot}\ensuremath{\hbar}{\ensuremath{\omega}}_{c}$, where ${\ensuremath{\Delta}}_{12}$ is the subband energy separation, ${\ensuremath{\omega}}_{c}$ is the cyclotron frequency, and $k$ is the positive integer. At $T=4.2\text{ }\text{K}$ 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.

Magnetoresistance oscillations in triple quantum wells under microwave irradiation

The interference of magneto-intersubband oscillations and microwave-induced resistance oscillations is studied in high-density triple quantum wells. We give an introduction into magnetotransport in trilayer systems and focus on photoresistance measurements. The power and frequency dependence of the observed magnetoresistance oscillations can be described by the inelastic mechanism of photoresistance, generalized to the three-subband case.

Nonlinear transport and oscillating magnetoresistance in double quantum wells

The nonlinear regime of low-temperature magnetoresistance of double quantum wells in the region of magnetic fields below 1 T is studied both experimentally and theoretically. The observed inversion of the magnetointersubband oscillation peaks with increasing electric current and splitting of these peaks are described by the theory based on the kinetic equation for the isotropic nonequilibrium part of electron distribution function. The inelastic-scattering time of electrons is determined from the current dependence of the inversion field.

Zener tunneling between Landau levels in a double quantum well at high filling factors

Differential resistance r xx in a double GaAs quantum well with two occupied size-quantization subbands has been studied at a temperature of 4.2 K in magnetic fields B < 2 T. The oscillations of r xx with a period in the inverse magnetic field determined by the value of a dc bias current I dc have been discovered in the electron system under investigation at high filling factors in the presence of I dc. The amplitude of magneto-intersubband oscillations has been shown to increase in the r xx oscillation maxima, while the oscillation reversal has been observed in the minima. The discovered oscillations have been shown to be due to Zener tunneling of electrons between Landau levels tilted by a Hall electric field. The experimental data are qualitatively explained by the effect of intersubband transitions on the I dc-dependent component of the electron distribution function.

Interference oscillations of microwave photoresistance in double quantum wells

We observe oscillatory magnetoresistance in double quantum wells under microwave irradiation. The results are explained in terms of the influence of subband coupling on the frequency dependent photoinduced part of the electron distribution function. As a consequence, the magnetoresistance demonstrates the interference of magnetointersubband oscillations and conventional microwave induced resistance oscillations.

Magnetic oscillations of resistivity and absorption of radiation in quantum wells with two populated subbands

The dynamic (ac) conductivity tensor of quantum wells with two populated subbands in the presence of a magnetic field perpendicular to the well layer is calculated theoretically. The microscopic theory is based on the Kubo formalism assuming a detailed consideration of elastic scattering of electrons by the random disorder potential with arbitrary correlation length. The results describe the influence of magnetic field on the linear absorption of low-frequency electromagnetic radiation, and demonstrate the existence of magnetic oscillations that survive at high temperatures and whose maxima correspond to absorption of electromagnetic radiation at combined frequencies, determined by both the magnetic field and the subband separation. Different polarizations of the radiation field with respect to the quantum-well layer are considered. Analytical expressions are derived for the case of sufficiently weak magnetic field when the Landau levels are overlapping. Application of the theory to the static (dc) limit provides a consistent description of the magneto-intersubband oscillations of the resistivity in the systems with two populated subbands.

Influence of Intersubband Scattering on the Magnetic Field Dependence of the Conductivity Tensor

Transport properties of semiconductor structures with several subbands occupied have been attracting scientific interest for a long time because of, among other things, the appearance of intersubband scattering. It is known that this extra scattering mechanism affects both classical as well as quantum transport. In this paper we show that intersubband scattering can lead to apparent inconsistency of the experimental results obtained by means of classical and quantum transport measurements and this discrepancy is entirely connected with the usage of the classical fomulae to describe the magnetic field dependence of the conductivity tensor. We have studied electron transport of two-dimensional electron gas (2DEG) in the modulation Si δ-doped, MOVPE grown pseudomorphic GaAs/InGaAs/AGaAs quantum well (QW) at temperature T = 4.2 K and in magnetic field up to 7 T. The persistent photoconductivity effect was used to control the free electron concentration. Shubnikov de Haas (SdH) oscillations seen in ρxx indicated the existence of 2DEG. When electron sheet density exceeded the value of 1.3·10 12 cm -2 , the so-called magneto-intersubband (MIS) oscillations appeared. These oscillations are related to scattering between Landau levels of two occupied subbands. This means that quantum lifetimes of both first and second subband should be sufficiently high. However, at the same time classical analysis of the conductivity tensor revealed very low mobility of electrons on the second subband which seemed to rule out the possibility of the appearance of MIS oscillations. We prove that there is no contradiction in our observations. First we show that even if intersubband scattering is present in the sample with multi-subband occupation, one still can use the classical multilayer approach to analyse the non-oscillatory part of the conductivity tensor. However, one has to be very cautious in using this approach, since all the parameters describing the carriers of the contributing conduction channels (subbands) must be redefined in that case. This redefinition applies both to density and mobility of carriers. That is why a difference between electron density values obtained from quantum oscillatory effects and classical analysis of magnetic field dependence is usually observed in such cases. The same applies to the values of mobilities extracted from the classical analysis of magnetic field dependences of conductivity tensor – these values may substantially differ from the real ones determined by intraand intersuband scattering processes. Hence we show that there is really no contradiction in our observations. Low electron mobility obtained in classical analysis for the second subband in our sample does not imply low electron quantum lifetime for this subband. Thus the observation of MIS oscillations is absolutely possible.

Magnetic-field-enhanced quantum-cascade emission

We have observed an enhancement of terahertz intersubband electroluminescence in a quantum cascade structure in the presence of a magnetic field applied normal to the epitaxial layers. At a field of B=7.2 T the emission efficiency doubles. This effect is attributed to the suppression of nonradiative Auger–intersubband transitions caused by Landau-quantization of the in-plane electron motion. The magnetic field dependence of the luminescence intensity shows strong oscillations. These magnetointersubband oscillations are caused by the modulation of the transition rate via resonant inter-Landau-level transfer.

Magnetointersubband oscillations of conductivity in a two-dimensional electronic system.

Magnetoconductivity of the two-dimensional electron gas occupying two size-quantization subbands is studied theoretically. When the bottoms of subbands are separated by an integer number of Landau levels, the staircases of Landau levels in both subbands are completely aligned. For such values of magnetic field the intersubband scattering is enhanced. As it was pointed out by Polyanovsky, this results in additional Shubnikov--de Haas oscillations of conductivity with magnetic field, with period depending on subband separation, and amplitude depending weakly on temperature, provided that a large number of Landau levels in each subband are occupied. In the calculation of conductivity we make use of the self-consistent Born approximation generalized to the case of two subbands. An analytical theory for the case of strong disorder and numerical results for the case of weak disorder are presented.