Tunnel-coupled Quantum Wells Research Articles

Nonlinear regime of oscillatory relaxation of photoexcited electrons in tunnel-coupled quantum wells

We investigate the nonlinear coherent dynamics of the electrons in an asymmetrical double-quantum-well system photoexcited by an ultrashort laser pulse, taking into account phase breaking due to elastic scattering. The nonlinearity takes place when the density of the electrons is high enough to modify the level-splitting energy of the tunnel-coupled states due to the space-charge effect. It is found that the combined effect of nonlinearity and dephasing leads to transitions (accompanied with jumps of the amplitude and phase of the oscillations) between different oscillatory regimes of the electron system during its temporal evolution. The relaxation of the photoexcited electrons in this case can proceed much faster than in the linear regime.

Binding energy of shallow donors in asymmetrical systems of quantum wells

The dependence of the binding energy of a shallow donor impurity on its position in an asymmetrical system of tunnel-coupled quantum wells is mainly determined by the structure of the one-electron envelope functions and the difference between the dielectric constants of the quantum-well and barrier materials. An effective technique is suggested for calculating the binding energies and envelope functions of the shallow donor states in type-I heterostructures with narrow wells and barriers. We present the results of calculations for AlxGa1−xAs-GaAs structures with two or more quantum wells without imposing any restrictions on the ratios of their sizes.

Photovoltaic effect in coupled quantum wells under intersubband excitation

The photovoltaic effect in tunnel-coupled double quantum wells (DQWs) due to resonant infrared (IR) excitation of the electrons from the lower ground state to the excited state is considered. Intra- and interwell relaxation of electrons are studied self-consistently, taking into account modification of the electron states in DQWs due to the upper ground state occupation under IR pumping. The photoinduced potential is calculated for different energies of IR excitation, and the correlation between this potential and modification of the ground states in DQWs is discussed.

Photon drag effect in tunnel-coupled quantum wells.

Resonant photon drag current caused by transfer of momentum from absorbed far-infrared photons to electrons is calculated for transitions between coupled states in double quantum wells with nonsymmetric scattering. This system is found to exhibit both the reversal of current and complicated dependencies of the current on the level splitting and nonsymmetry of scattering. Numerical estimate demonstrates a possibility of application of this effect both for studies of the momentum-transfer kinetics and for detection of ultrafast pulses in the submillimeter spectral region. \textcopyright{} 1996 The American Physical Society.

Submillimeter radiation effect in the tunnel-coupled quantum wells under infrared excitation: A theoretical study

Redistribution of the electrons between the tunnel-coupled ground levels in the double quantum wells (DQWs) due to the resonant infrared excitation on the next level is considered. Intra- and interwell relaxation of electrons is studied self-consistently, taking into account both modification of the electron states in DQWs due to the upper ground states occupation and heating of the electrons due to IR pumping. The electron populations in the tunnel-coupled states is presented as a function of the DQWs parameters, doping, and IR pumping characteristics. The condition for origin of the inversive electron population, and possible value of the negative absorption coefficient in the submillimeter spectral region is found.

Quantum regime for in-plane magnetoresistance of double quantum wells.

Electrical conductivity of the tunnel-coupled double quantum wells under an in-plane magnetic field is studied theoretically. Comparison of the magnetic-field-induced suppression of the resistance resonance peak with the experimental data indicates the quantum regime for electron transport, when the tunnel coupling is partly suppressed by the scattering.

Ultrafast edge photoexcitation and coherent oscillations in tunnel-coupled double quantum wells.

Photoexcitation of electrons in double quantum wells is studied as a function of level splitting, duration of the light pulse ${\mathrm{\ensuremath{\tau}}}_{\mathit{p}}$, and light frequency. Dynamics of transitions between the upper states of the valence band and the tunnel-coupled pair of conduction band states is considered in the collisionless approximation. The nonmonotonic temporal dependence of the photoexcited electron concentration (the saturation value of this concentration determines the time-integrated interband photoluminescence intensity) is obtained for the times t\ensuremath{\sim}${\mathrm{\ensuremath{\tau}}}_{\mathit{p}}$. Transition from the nonperiodic (at t\ensuremath{\sim}${\mathrm{\ensuremath{\tau}}}_{\mathit{p}}$) to the periodic (at t\ensuremath{\gg}${\mathrm{\ensuremath{\tau}}}_{\mathit{p}}$) behavior of the dipole moment oscillations is described.

Interband optical transitions in double quantum wells with nonsymmetrical scattering.

The photoluminescence and photoluminescence excitation spectra near the fundamental absorption edge in the tunnel-coupled asymmetric double quantum wells are studied theoretically. When the scattering intensities in the left and right quantum wells are different, the spectral broadening depends significantly on the tunneling. The spectra shapes are calculated in an approximation where broadening energies are small in comparison with the level splitting. Two kinds of the scattering potentials, the long-range-correlated and the short-range-correlated potentials, are considered.

Resonant transport in coupled quantum wells: A probe for scattering mechanisms.

We present a microscopical theory and experimental results concerning resistance resonance in two tunneling coupled quantum wells with different mobilities. The shape of the resonance appears to be sensitive to the small angle scattering rate on remote impurities and to the electron--electron scattering rate. This allows the extraction of scattering parameters directly from the transport measurements. The negative resonance in a Hall coefficient is predicted and observed for the first time.

Magneto-oscillatory phenomena in double quantum wells with asymmetric scattering. II. Longitudinal transport coefficients.

Shubnikov--de Haas oscillations and cyclotron resonance absorption are considered for quasiclassical magnetic fields and very low temperatures in tunnel-coupled quantum wells with asymmetric short-range scattering. The two sets of Landau levels, separated by the splitting ${\mathrm{\ensuremath{\Delta}}}_{\mathit{T}}$ between the left and right levels, lead to two series of peaks with different periods and widths. In addition, the conductivity shows oscillations as function of ${\mathrm{\ensuremath{\Delta}}}_{\mathit{T}}$. The cyclotron resonance peak shows peculiar characteristics when both sets of Landau levels cross the Fermi energy.

Magneto-oscillatory phenomena in double quantum wells with asymmetric scattering. I. Density of states and thermodynamic quantities.

The one-particle Green's function, the density of states, and the thermodynamic quantities (susceptibility and specific heat) are evaluated for tunnel-coupled quantum wells with asymmetric scattering in the presence of a transverse quasiclassical magnetic field. The oscillations of the susceptibility and of the specific heat have different periods and widths and depend sensitively on the level splitting between the left and right levels. Peculiar characteristics appear in the density of states when the splitting due to tunneling is comparable to the difference in the broadening between wells of equal width. When the tunneling splitting is stronger than this difference two sets of peaks arise with the same width. In the opposite case, the peaks have different widths but are superposed.

Conductivity of coupled quantum wells with nonsymmetric scattering.

The Green function of an electron in tunnel-coupled quantum wells with different momentum relaxation times cannot be diagonalized when \ensuremath{\Delta}\ensuremath{\simeq}T is comparable to the collisional broadening of states (\ensuremath{\Delta} is the level splitting and T is the tunnel matrix element). In this case, a quasiparticle description of the energy spectrum is invalid and the conductivity \ensuremath{\sigma} is determined by the formulas of quantum transport theory [the dependence \ensuremath{\sigma}(\ensuremath{\Delta}) is obtained for the short-range scattering case]. The frequency dispersion of conductivity that may be realized in the low-frequency region is also discussed.