Observation of intersubband excitations in a multilayer two dimensional electron gas

We report the observation, by resonant inelastic light scattering, of intersubband excitations of the multilayer two dimensional electron gas, in modulation doped GaAsAlGaAs heterojunction superlattices. These are the first measurements of these transitions by any technique, and furnish intersubband energies in good agreement with calculated values. The spectral bands are broad, and nearly Lorentzian in shape: the implied relaxation rates scale linearly with band energy and are significantly faster than transport relaxation rates. Finally, the polarized spectra reveal differences between spin-flip and non spin-flip excitations which are unique to multilayer two dimensional electron gases.     

Differential negative resistance caused by inter-subband scattering in a 2-dimensional electron gas

Differential negative resistance has been observed in the range $\text{0.5～1.2 kV}\text{cm}$ at 77 K in a two-dimensional electron gas formed at a modulation-doped AlGaAs/GaAs heterojuction interface. This differential negative resistance is attributed to the sudden onset of inter-subband scattering when electrons achieve sufficient energy from the electric field.     

Modification of the intersubband excitation spectrum in a two-dimensional electron system under a perpendicular magnetic field

Under an external magnetic field, new branches of spin- and charge-density waves have been studied in a quasi-two-dimensional electron system whose ground state has more than one Landau level occupied by electrons. These collective excitations can be treated as manifestations of the multicomponent nature of the electron system in magnetic field, whereas the occupied Landau levels should be associated with degrees of freedom.     

Occurrence of intersubband polaronic repellons in a two-dimensional electron gas

Counteracting electron-phonon and electron-electron interaction in the optical absorption spectra of a quantum well intersubband system can have a significant impact on the line shape and line width of optical transitions at low temperatures. In particular, polaronic repellons can be formed due to an attractive intersubband polaron-polaron interaction. At low temperatures and for properly chosen densities; this effect can lead to a line broadening of the intersubband transition, even in a weak coupling regime.     

Shake-up processes in intersubband magneto-photoabsorption of a two-dimensional electron gas

I theoretically study shake-up processes in photoabsorption of an interacting low-density two-dimensional electron gas (2DEG) in magnetic fields. Such processes, in which an incident photon creates an electron–hole pair and simultaneously excites one electron to one of the higher Landau levels, were observed experimentally [Phys. Rev. Lett. 79 (1997) 3974] and were called combined exciton-cyclotron resonance (ExCR). The recently developed theory of ExCR [Phys. Rev. B 64 (2001) 241101] allows for a consistent treatment of the Coulomb correlations, establishes the exact ExCR selection rules, and predicts the high-field features of ExCR. In this work, I generalize the existing theory of high-field ExCR in the 2DEG to the case when the hole is excited to higher hole Landau levels.     

Light scattering by a 2D electron system with spin-orbit interaction in a strong magnetic field

We theoretically investigate light scattering by electrons of a 2D system in a strong magnetic field perpendicular to the plane of the system (the filling factor is smaller than two, i.e., only the states of the zeroth Landau level are filled). Analysis is carried out in the resonance approximation, in which the frequencies of the incident and scattered light are close to the effective separation between the conduction band and one of the branches of the valence band of the semiconductor. Spin splitting of the Landau level in the conduction band associated with Zeeman and the spin-orbit interactions is taken into account. The dynamic screening effects are considered in the random phase approximation (RPA).     

High-resolution study of quasi-elastic electron scattering from a two-layer system

In large-angle elastic scattering events of keV electrons a significant amount of momentum is transferred from the electron to a nucleus in the target. As a consequence kinetic energy is transferred from the energetic electron to the nucleus, and hence these processes can be referred to as ‘quasi-elastic’. How much energy is transferred depends on the mass of the nucleus. In this paper, we present measurements from a two-layer system (a germanium layer and a carbon layer), and at high energies the quasi-elastic peaks of Ge and C are clearly resolved. It is demonstrated that the sample geometry has a huge effect on the observed relative intensities. It is shown that the intensities are influenced by the elastic scattering cross-section of the atoms in the film, film composition and selective attenuation, due to varying amount of inelastic scattering for layers of the film. However truly quantitative agreement is not obtained.     

Ultrafast induced electron–surface scattering in a confined metallic system

Received: 20 September 1998     

Intensity of intersubband electron scattering by polar optical phonons in semiconductor quantum wires with account of energy level broadening

An approach describing the effect of energy level broadening in semiconductor quantum wires on the intensity of intersubband electron scattering by polar optical phonons is suggested. As a broadening mechanism, scattering by atomic thermal vibrations and by the roughness of the confining surfaces of a quantum system is considered. It is shown that in this case the dependence of the intensity of intersubband scattering of electrons on their kinetic energy has no singularities.     

Magnetic scattering effects on magnetoresistance in a quasi-two-dimensional disordered electron system

Magnetic-impurity-scattering effects in a quasi-2D disordered electron system have been investigated theoretically with the diagrammatic techniques in perturbation theory. The analytical expressions for magnetoconductivities due to weak-localization effects have been obtained as functions of elastic, inelastic and magnetic scattering times. The relevant dimensional crossover behavior from 3D to 2D with decreasing the interlayer coupling has been discussed, and the condition for the crossover has been obtained. Received 20 March 2001 and Received in final form 28 June 2001     

Hot electron effects in optically-pumped mid-infrared intersubband semiconductor laser

A theoretical investigation of an optically-pumped mid-infrared intersubband semiconductor laser is presented. The influence of electrons and dopant ions on the conduction band structure is simulated with a self-consistent Poisson–Schrödinger solver. Electron-polar optical phonon interactions are calculated by using a macroscopic phonon model with electromagnetic boundary conditions. In order to assess the influence of the electronic temperature on the device optical performances, electron dynamics under optical pumping are investigated within a rate equation model where particle and energy flow equations are derived from Boltzmann's equation with Fermi statistics. Our calculations show that population inversion between the first and second excited states can occur at 77 K under intersubband optical excitation.     

New branch of intersubband plasmons in a nonequilibrium two-layer system

The plasma oscillation spectrum for 2D electrons in a double quantum well is calculated. It is shown that an additional branch of intersubband plasmons can exist without experiencing Landau damping in the case of nonequilibrium population of subbands. In an asymmetric structure, this branch is responsible for both the emergence of instabilities and the possibility of amplification of plasma waves.     

Oscillatory magnetoconductivity of a two-dimensional electron system due to phonon scattering

Summary A quantum-statistical theory of magnetophonon resonance oscillations in two-dimensional systems has been developed, starting from the resolvent representation of Kubo's formula and its proper connected diagram expansion. Non-polar and polar optical-phonon scattering has been considered and the results show, as anticipated based on the physical considerations and experimental observations, conductivity oscillations as a function of magnetic field with the magnetophonon resonances occurring at the phonon frequencies {ie1539-1} {ie1539-2}=cyclotron frequency). Divergences occurring in the magnetoconductivity near the magnetophonon resonances are removed by using the full resolvent operator in the tetradic self-energy operator of an electron. These additional terms provide necessary damping of the magnetophonon resonance oscillations. The present results are also shown to be qualitatively similar to those obtained by others using quantum Boltzmann's equation approach to quantum transport theory.