We show that a unified picture, namely, thermalization within the inhomogeneously broadened exciton band, can be used to explain the excitonic photoluminescence (PL) spectra in quantum wells independently of the presence, or lack thereof, of the low-energy shift of the PL line with respect to the absorption peak (Stokes shift). The Stokes shift itself turns out to be a mere consequence of the thermal equilibrium with a quadratic dependence on the absorption linewidth and a linear dependence on the inverse of the excitonic temperature. The predictions are found to be in excellent agreement with careful measurements in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As single-quantum-well structures.

We demonstrate the influence of lateral exciton transfer on the resolution in cathodoluminescence images and thereby the apparent island size in monolayer-flat (ML-flat) areas in quantum wells (QW’s). The QW used in this study exhibits two peaks originating in regions of two different QW thicknesses, differing by 1 ML. A variation of the exciton transfer is achieved by altering the temperature. When the temperature is increased from 20 to 120 K, the ratio between the low-energy and the high-energy peak is increased by over a factor of 5, caused by an increased lateral transfer. At 20 K the images of the two peaks show a high degree of complementary behavior and features of about 0.5 μm can be resolved. The contrast and the complementary behavior is reduced with increased temperature, i.e., increased transfer, and at 110 K the smallest features observed are about 1 μm. We also suggest a model, where the ML-flat areas in reality are clusters of microislands. These clusters appear to be extended islands of a single thickness, due to a one-way transfer of excitons from thinner to thicker microislands. The sizes of these apparently extended islands are discussed in terms of a diffusion-length-dependent critical density of microislands.

We present a study of the optical spin orientation and optical alignment relaxation of (e1:hh1)1S excitons in GaAs/${\mathrm{Al}}_{0.33}$${\mathrm{Ga}}_{0.67}$As narrow quantum wells at low temperatures. The photoluminescence was excited by ps pulses within the (e1:hh1) exciton band and its decay, as well as that of the degrees of the circular and linear polarizations [${\mathit{P}}_{\mathrm{cir}}$(t) and ${\mathit{P}}_{\mathrm{lin}}$(t)] were measured. The observed decay curves are analyzed by a model based on the in-plane wave-vector (${\mathbf{K}}_{\mathrm{\parallel }}$) dependence of the energy (${\mathrm{\tau }}_{\mathit{a}}$), spin (${\mathrm{\tau }}_{\mathrm{cir}}$), and alignment (${\mathrm{\tau }}_{\mathrm{lin}}$) relaxation times of delocalized excitons. These were found to vary in the range of 120<${\mathrm{\tau }}_{\mathit{a}}$<230 ps, 50>${\mathrm{\tau }}_{\mathrm{cir}}$>30 ps, and 15>${\mathrm{\tau }}_{\mathrm{lin}}$>5 ps for 0.019 A${\mathrm{̊}}^{\mathrm{-}1}$<${\mathit{K}}_{\mathrm{\parallel }}$<0.032 A${\mathrm{̊}}^{\mathrm{-}1}$. The model fitting is not sensitive to the relaxation times of the ${\mathit{K}}_{\mathrm{\parallel }}$=0 state, which are very long: 1000>${\mathrm{\tau }}_{\mathrm{lin}}^0$,${\mathrm{\tau }}_{\mathrm{cir}}^0$>300 ps. We further assume that the hole spin-relaxation rate is much faster than that of the electron. This allows us to estimate the relative contribution of each of these particles’ spin-relaxation rate to ${\mathrm{\tau }}_{\mathrm{cir}}$ and ${\mathrm{\tau }}_{\mathrm{lin}}$.

Binding energies and oscillator strengths of excitons in staggered-lineup heterostructures such as type-II quantum wells are calculated by a variational method, using a single parameter. This method involves the effective attractive potential imposed by the confined carrier to its unconfined companion. Contrary to previous comparable works, a three-dimensional trial function of the variable r (r=‖${\mathbf{r}}_{\mathit{e}}$-${\mathbf{r}}_{\mathit{h}}$‖) is used, instead of a two-dimensional function of the in-plane projection ρ. Due to the large spatial extension along z of the wave function of the unconfined carrier, the latter approximation commonly used up to date, is too drastic, even though it works reasonably well for type-I systems. This is demonstrated by comparison of both hypotheses for GaAs-AlAs systems: when using the 3D function, binding energies are increased by up to 52%, while electron-hole overlap integrals can be enhanced by one order of magnitude.

We report on the calculation of low-frequency Raman scattering by longitudinal acoustic phonons in superlattices. The phonons are treated as elastic waves propagating in a continuum and their mode patterns are calculated by a transfer-matrix method. The electromagnetic propagation is calculated neglecting reflections and refractions at the interfaces, through the use of the same index of refraction for both constituent materials. The use of a complex refraction index allows for absorption effects to be included. The suitability of the numerical calculation for modeling superlattices is demonstrated.

Confinement in semiconductor heterostructures induces optical anisotropy for light propagating perpendicular to the growth axis. By analyzing the intensity transmitted through crossed polarizers below the fundamental absorption edge, we have measured the wavelength-dependent linear birefringence of light propagating along the planes of wave guided GaAs/AlAs multiple quantum wells. This birefringence is dispersionless up to photon energies close to the ${\mathit{E}}_0$ gap where a resonance is observed, as expected. However, the birefringence in the dispersionless region is much larger than foreseen. We show that no isotropic point exists in the transparency region, thus preventing the separation of the birefringence from the optical gyration expected from symmetry grounds for light propagating with q parallel to a [100] direction. We analyze our results by evaluating the dielectric tensor components using two approaches, a two-dimensional critical point model with parabolic band approximation and a microscopic calculation using a tight-binding model for the energy bands of the multiple quantum well.

We investigate the edge structure of a system of composite fermions confined in a wire. Using the composite fermion picture of the fractional quantum Hall effect, we have studied the electronic edge structure of different fractional quantum Hall liquids. For strong confinement and in the case of the 2/3 quantum Hall state we find that a droplet of charge with filling factor 1 appears at the edge of the system. In the case of the 1/3 quantum Hall state the droplet of charge corresponds to filling factor 2/5. For very smooth boundaries, we obtain the existence of plateaus and droplets of charge at fractional filling factors. We present an expression for the fractional Hall resistance as a function of the composite fermion edge states, which is a generalization of the Landauer-Buttiker formula for the integer quantum Hall effect.

A rigorous quantum-mechanical treatment of the dynamical evolution of electronic real-space transfer from a quantum well is presented, based on the density-matrix theory. A continuous electron dynamical evolution (not an abrupt collision) corresponding to a phonon absorption is analyzed here. From the evaluation of the lowest-order correction to the unperturbed density matrix, the spatial and momentum distributions can be obtained. Results will also be shown for the Wigner and Husimi functions, and it will be discussed how the present results can be used for a semiclassical modeling of real-space transfer in a Monte Carlo simulation of this effect.

The dynamic conductivity of one-dimensional isolated finite rings is calculated with emphasis on the low-frequency domain. A quantum-mechanical linear-response formula is used for both ordered and weakly or strongly disordered rings. The configurational average of the conductivity shows a macroscopic behavior of either localized or metallic type, depending on the size, degree of disorder, and inelastic scattering parameter γ. The scaling behavior in the limit ω→0, γ→0 (ω/γ=const) is shown to fit a kinetic description corresponding to a Drude law for ordered systems and to a hoppinglike picture for the strongly localized ones.

We study and compare the behavior of the left- and right-tunneling times of electrons quasiconfined in asymmetric double-barrier structures as a function of the bias for two different doping configurations in the electrodes: uniform doping and gradual doping. The potential experienced by the electrons is calculated self-consistently for each case by simultaneously solving the Schrödinger and Poisson equations. Afterwards, the stabilization method is employed to calculate the corresponding tunneling times. The case of gradual doping is especially interesting because an accumulation layer appears when the voltage is high enough. To analyze the stabilization graphs in the range of voltages where the two-dimensional quasiconfined level in the emitter interacts with the quantum-well level we have introduced a three-level model. We present a comparison of the tunneling times calculated for the two doping profiles with the ones obtained with a model potential. Finally, charge-accumulation effects are analyzed in the gradually-doped case and critically discussed within the framework of available experimental data.

Experimental and theoretical studies of the energy diagram of magneto-optical transitions observed in both luminescence and luminescence excitation spectra of an n-type modulation-doped quantum well are presented. The transitions related to the lowest populated electronic subband are well understood in terms of interband transitions with many-body corrections included within the density-functional approach. The excited subbands are differently renormalized and show a two-particle excitonic correlation.

Nonadiabatic effects on the particle transport (PT) induced by a sliding potential in a one-dimensional system are investigated. A formal treatment for calculating both average-energy and quasienergy (QE) subbands is given. A relation between the band structure and the transport property is established. By varying the speed of the sliding potential, the authors demonstrate how the quantization of PT breaks down due to the emergence of QE band gaps once beyond the adiabatic regime. In the nearly adiabatic limit, the nonadiabatic correction to the quantized PT is found to obey an exponential law. Farther from the adiabatic condition, another type of nonadiabatic breakdown, due purely to the onset of soft crossings in the average energy spectrum, is identified. It is also shown that the current subject is closely related to the Wannier-Stark ladder and the nonadiabatic quantum Hall effect.

We present fully quantum-mechanical calculations of the magnetotransport properties of antidot superlattices. Our results are in quantitative agreement with experimental longitudinal resistance and Hall resistance data, which so far have been discussed within the framework of classical chaotic dynamics. Quantum oscillations in the density of states, discussed previously within Gutzwiller’s periodic orbit theory, are also confirmed by our calculations. Moreover, we predict an enhanced energy period in the density of states associated with the deformation of periodic orbits.

Using a perturbation theory based on the density-matrix formulation, we study the nonlinear optical responses of a noninteracting three-level model system to consecutive coherent pulsed excitations, as realized in several recent experiments involving excitonic transitions in quantum wells. The terahertz emission, which is a second-order response, and the four-wave mixing signal, a third-order response, are calculated within the rotating-wave approximation, in the presence of detuning, dephasing, and inhomogeneous broadening. We study the quantum beats in the photon echo from transient four-wave mixing experiments of a three-level system. We find that the temporal profile of the photon echo signal in the four-wave mixing experiments depends very strongly on the amount of inhomogeneous broadening of the energy levels involved. Both the position and the intensity of the peaks exhibit a smooth evolution from strong quantum beats to a conventional photon echo as the inhomogeneous broadening increases. These features from our noninteracting model system are compared with a recent experiment, and found to account for a number of experimental observations.

The positions and energies of four candidate adsorption sites for Cl on GaAs(110) have been investigated using the ab initio pseudopotential plane-wave method. We obtain general agreement with recent experimental studies on the distribution and pattern formation of Cl adatoms. An analysis of adatom height based on valence charge density agrees quantitatively with reported scanning tunneling microscopy values. For the four adatom sites considered, the relaxation of the first GaAs surface layer was similar to or less than the relaxation of the bare surface. In the case of the lowest energy site, the relaxation is nearly healed due to a change in character of the surface bonding. This is in contrast to conclusions based on core photoemission results and simple charge-transfer arguments.

The effects of inversion asymmetry on the anisotropic optical transition in the presence of electric field (Pockels effect) in GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum wells and superlattices are investigated theoretically. Within an 8×8 k⋅p effective mass Hamiltonian formalism, in which the linear k, ${\mathit{k}}^3$, ε (strain due to the piezoelectric effect), εk, and Rashba terms are taken into account, we have derived the renormalized effective Hamiltonian for the electron and hole, respectively. The effects of the inversion asymmetry and the hole mixing on the spin-split subband structures and the dependence of optical transition matrix elements on the polarization of incident light are fully presented. The microscopic model clearly shows that the inversion asymmetry is responsible for the Pockels effect. The roles played by the hole mixing in the enhanced quantum well Pockels effect mediated by the electron-hole pairs are shown by our calculations. Thus, we develop a theory with exciton states with eight spin components (due to the broken twofold degeneracy of the electrons) in the quantum well and calculate the excitonic spectra. The symmetry analysis and numerical results indicate that the excitonic anisotropic behavior results from the interference between different spin components of holes, which, owing to the heavy- and light-hole mixing, is enhanced significantly compared with the bulk material, especially for those exciton states whose dominant component does not coincide with its optically active component. Comparison between theory and experiment is discussed.

We present a systematic study of the electronic and structural properties of strained (ZnTe${)}_{\mathit{n}}$/(CdTe${)}_{\mathit{n}}$ (001) superlattices using the full-potential linearized-augmented-plane-wave method. We simulate three different growth conditions: a free-standing mode (no substrate is assumed) and the growth on ZnTe and CdTe substrates. The heat of formation of ultrathin superlattices and the valence-band lineup are determined in the different strain conditions. The agreement between the most recent experiments and the theoretical results is very good. The effects of the strain on the valence-band discontinuity are discussed.

We discuss the relationship between the edge-state properties of a laterally confined, periodically modulated two-dimensional electron system in a magnetic field and the topological quantum numbers, which characterize the bulk eigenenergy spectra of the corresponding strictly periodic system without confinement. In addition to general thermodynamic arguments, illustrative numerical results on confined systems are presented for two complementary limits, weak periodic potentials with strong magnetic fields and strong periodic potentials with weak magnetic fields. In both limits the confined systems exhibit two types of edge states, magnetic and, in addition, nonmagnetic, which do not contribute to the magnetization in the thermodynamic limit. From stability considerations, we argue that the inclusion of nonmagnetic edge states into a naive application of the Landauer-Büttiker approach to the quantized Hall conductance is inappropriate.

Low-temperature magnetoresistance measurements have been used to determine various electron scattering times in pure Pd and ${\mathrm{PdH}}_{0.3}$ thin films with different thicknesses, which have been prepared by electron-gun evaporation. From low-field magnetoresistance measurements, using localization theory, the spin-flip, electron-electron, and electron-phonon relaxation times can be deduced and they are found to depend on the electronic structure of both systems. It is also found that the increase of the disorder of the systems enhances the spin-orbit coupling, which is larger in ${\mathrm{PdH}}_{0.3}$ than in Pd. On the other hand, the high-field magnetoresistance allows the determination of both the Coulomb screening factor F and the Landé factor g for both systems. It is found that the F factor depends on the band structure and takes the mean values 0.63 and 0.53 for pure Pd and ${\mathrm{PdH}}_{0.3}$, respectively, while the g factor depends on the degree of disorder.

When an electron is transferred to ${\mathrm{C}}_{60}$, the electron-lattice interaction results in a distortion of the bond structure and a polaronlike state is formed. By solving the dynamical equations of molecules in a ${\mathrm{C}}_{60}$ cage, the time-dependent changes of the bond lengths are obtained. We find that the relaxation time to form a polaron is about 0.3 ps, and is consistent with the experimental results. The implication of this ultrafast evolution, with comparison to the soliton-antisoliton mechanism, is discussed.