Effective mass anisotropy of gamma electrons in GaAs/AlGaAs quantum wells

Resonant magnetotunneling in GaAs/Al(0.28)Ga0.72As double barrier structures is used to demonstrate that the effective mass of confined Gamma conduction electrons becomes anisotropic when an electric field is applied perpendicular to the interfaces. Although several authors have previously reported Gamma-related optical anisotropy, this is the first example of a corresponding electrical anisotropy. The results are explained using a quantum mechanical model involving interface band mixing that contains additional features not found in the optical case.     

Propagation of electrons in coupled quantum wells

Summary We study propagation of an electron wave in a double-quantum-well structure formed by alternate layers of GaAlAs and GaAs. In such a structure, electron states parallel to the layers are described by 2D plane waves and in the perpendicular direction by the bound states of the confining potential. We show that an electron, initially introduced in one well, will execute oscillations between the two wells of the structure. Although the frequency of oscillations depends primarily on the distance separating the wells and the confining potential, it is shown in this paper that the frequency also depends on the effective mass of the electron, if it is different within and outside the well. Expressions are derived for the frequency of oscillations, taking into account the difference in the effective mass of the electron.     

Two-dimensional electrons in spirally rolled-up quantum wells

An effective Hamiltonian is obtained to describe the motion of a one-dimensional quantum particle along an arbitrary plane curve. Calculations are made of the energy levels and the polarization dependence of the electromagnetic wave absorption in a spirally rolled-up quantum well.     

Effect of Coulomb interaction of electrons on the intersubband emission lineshape in quantum wells

We have used perturbation theory for a many-particle system to analyze intersubband emission in quantum wells, taking into account Coulomb interaction of electrons. We obtained approximate analytical expressions for the shape factor for homogeneous broadening for intersubband transitions. We analyzed the effect of the excitation level and the temperature on the spectral lineshapes. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 642–647, September–October, 2007.     

Parallel transport of electrons in double quantum wells

Calculations have been performed on the mobility of electrons in a double quantum well structure in which the barrier separating the two parts of the channel is sufficiently thin to allow tunnelling. The subband structure is completely taken into account in a self-consistent calculation and all significant scattering mechanisms (scattering on remote impurities, on selectively introduced channel impurities, on acoustic and optical phonons) are included in order to obtain quantitatively realistic results from low to room temperature. We discuss how the thickness of the barrier influences the conductivity of the channel vs density and show that it is possible to design structures having negative transconductance with a peak-to-valley ratio of 6 at T = 77 K.     

Mobility of Dirac electrons in HgTe quantum wells

The mobility of Dirac electrons (DEs) in HgTe quantum wells with the thickness close to the critical value corresponding to the transition from the direct to inverted spectrum has been studied experimentally and theoretically. The nonmonotonic dependence of this mobility on the electron density is found experimentally. The theory of DE scattering on impurities and fluctuations of the thickness of a well caused by its roughnesses is elaborated. This theory is in good agreement with experiment and explains the observed nonmonotonicity by the decrease in the ratio of the de Broglie wavelength of DEs to the characteristic size of the roughness with the increase in their concentration.     

Exciton-exciton interaction in quantum wells

Il Nuovo Cimento D - Fast initial decays of both the luminescence intensity and the circular luminescence polarization at high exciton densities, under resonant excitation, are reported for the...     

Magnetotunneling relaxation of electrons in double quantum wells

Electron tunneling relaxation in double quantum wells subject to a transverse magnetic field is studied. The resonant peaks in the tunneling relaxation rate appear when the energy splitting Δ of the tunnel-coupled pair of the left- and right- well electron states is a multiple of the cyclotron energy ℏω_c and two series of the Landau levels coincide. The shape of such resonant oscillations of the relaxation rate is determined by the Landau levels' broadening (which is associated with the intrawell scattering in the case of small tunnel coupling), but it is not expressed through the electron density of states directly. The dependence of the tunneling relaxation rate on ℏω_c and Δ is calculated taking into account elastic scattering of the electrons by the inhomogeneities of the structure in the limit when the scattering potential is slowly changing on the magnetic length scale.     

Monte Carlo study of hot electrons in quantum wells

We have performed an ensemble Monte Carlo calculation of the high field response at 77 K of two different two-dimensional systems, one a modulation doped (MD) single square well of GaAs/AlGaAs and the other a δ-doping layer formed by a sheet of donor impurities embedded in bulk GaAs. Results of this calculation show a reduced low field mobility and a decrease in the transient overshoot velocity in the δ-doping versus MD system due to the effect of impurity scattering. At high fields, however, the steady state saturated drift velocities appear to be similar in the two systems. Because of the higher carrier density, the performance of short-channel devices based on the δ-layer will be advantageous where high output currents are necessary.     

Spin-orbit interaction in coupled quantum wells

We theoretically investigate the spin-orbit interaction in GaAs/AlxGa1 x As coupled quantum wells. We consider the contribution of the interface-related Rashba term as well as the linear and cubic Dresselhaus terms to the spin splitting. For the coupled quantum wells which bear an inherent structure inversion asymmetry, the same probability density distribution of electrons in the two step quantum wells results in a large spin splitting from the interface term. If the widths of the two step quantum wells are different, the electron probability density in the wider step quantum well is considerably higher than that in the narrower one, resulting in the decrease of the spin splitting from the interface term. The results also show that the spin splitting of the coupled quantum well is not significantly larger than that of a step quantum well.     

Exciton-exciton interaction in coupled quantum wells

The exciton-exciton interaction is investigated for spatially indirect excitons in coupled quantum wells. The Hartree-Fock and Heitler-London approaches are improved by a full two-exciton calculation including the van der Waals effect. Using these potentials for the singlet and triplet channel, the two-body scattering matrix is calculated and employed to derive a modified relation between exciton density and blue shift. Such a relation is of central importance for gauging exciton densities on the way toward Bose condensation.     

Spin susceptibility of two-dimensional electrons in narrow AlAs quantum wells

We report measurements of the spin susceptibility in dilute two-dimensional electrons confined to a 45 A wide AlAs quantum well. The electrons in this well occupy an out-of-plane conduction-band valley, rendering a system similar to two-dimensional electrons in Si-MOSFETs but with only one valley occupied. We observe an enhancement of the spin susceptibility over the band value that increases as the density is decreased, following closely the prediction of quantum Monte Carlo calculations and continuing at finite values through the metal-insulator transition.     

Effective Coulomb interaction among electrons in cuprates

Analytical expressions for the charge susceptibility and permittivity of cuprates are obtained using the singlet-correlated conduction band model. The screening parameter caused by interband transitions is refined using experimental plasmon frequencies. A new branch of acoustic plasmons is predicted. The range of values in which acoustic plasmons do not experience Landau damping is determined for wave vectors in the Brillouin zone. Fourier images of the effective Coulomb interaction among charge carriers is calculated for different wave vectors.     

Tunneling of electrons in quantum wells with indirect gap semiconductor barriers

One of the features peculiar to GaAs-Ga_1−xAl_x As quantum wells with x ⩾0.43 are barriers formed by an indirect gap semiconductor. We make use of a simple one-dimensional tight-binding model to study the tunneling properties of such a system. Wave-functions and probabilities associated with an electron in each spatial region as a function of time are computed and compared with the results of a simple square barrier model. It is shown that the states related to the indirect conduction band minima of the barrier act as a new channel and increase the tunneling current between the wells. We suggest that these states are the origin of an unexplained structure observed in photoemission from a double quantum well. The effect of an external electric field is analyzed as well.