Magneto-optical experiments have been used to study a range of InGaAsP-based multiple-quantum-well (MQW) structures containing biaxial strains, ranging from 1.6% tensile to 1.0% compressive. The observed excitonic transitions, involving both heavy and light holes, are studied in fields up to 15 T. Estimates of the hole effective masses are made, providing details of the valence-band nonparabolicities, and electronlike behavior is demonstrated for both heavy and light holes with different amounts of tensile strain. This is related to band crossings within the valence band and enables an estimate of 0.68±0.10 to be made of the heterojunction band offset in a strained ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/InGaAsP MQW, with approximately 1.25% tensile strain in the well region. The experimental data are compared to the results of k⋅p Hamiltonian calculations of the in-plane valence-band dispersion.

The average persistent current in an ensemble of mesoscopic metal rings at fixed chemical potential threaded by a magnetic flux in the diffusive regime is related to the presence of a component of the one-electron effective self-energy, which is odd with respect to the electron velocity. In a noninteracting model, the ‘‘velocity-odd’’ component of the self-energy, calculated within the self-consistent Born approximation, is found to be exponentially small. Hence, the average persistent current is also exponentially small. When the Coulomb interaction between electrons is included, a finite velocity-odd component of the self-energy is obtained. The microscopic reason for this is the enhancement of the average classical electrostatic energy of interaction between two electrons with almost opposite velocities. We show that the recent theory of Ambegaokar and Eckern can be reformulated in terms of a velocity-odd self-energy, and their results are reproduced. A simple model is then presented, which exhibits the main qualitative features of their result in an elementary way. Finally, the assumption that all rings have the same chemical potential is examined. It is shown that the error involved in this approximation is parametrically small for three-dimensional rings, scaling as the inverse of the volume of the ring.

A method for the self-consistent calculation of rectangular quantum wires is presented and used for the analysis of the classical (two-dimensional well) quantum wire embedded in an infinitely extended bulk. The method relies on the Fourier expansion of the wave functions and on using the structural symmetry properties for computation time savings. Furthermore, Löwdin’s perturbation method is also employed to increase efficiency. Calculations are performed at room temperature and at a number of dopant densities, and the influence of structure parameters is analyzed.

We use an optical probe of magnetization to explore the evolution of carrier-spin scattering and magnetic dynamics in quantum-confined geometries. II-VI magnetic heterostructures are studied using femtosecond-resolved Faraday rotation and exhibit both phase (∼1 ps) and spin scattering (∼6 ps) in concert with a field-tunable terahertz quantum beating of the total carrier spin. Spin-flip processes experienced by photoexcited carriers as they tunnel through nanometer-thick magnetic barriers produce a magnetic perturbation strongly sensitive to the initial magnetic state and the spin orientation of the carriers. Once these carriers have recombined (∼70 ps), the magnetic ions relax through completely different channels of significantly slower decay rate (100 ps–10 ns). The relaxation characteristics are found to be substantially influenced by exchange coupling between adjacent magnetic ions at low temperatures (T<13 K). These low-dimensional magnetic systems yield a rich array of spin phenomena absent in traditional semiconductor heterostructures.

An effective approach for calculating the (phonon) scattering rate in valence-band quantum wells is presented. The valence-subband structure and the wave functions are obtained using the multiband effective-mass approximation. The subband structure is modeled by piecewise second-order polynomials while its anisotropy is neglected. The effect of the central-cell symmetry is incorporated in the calculation by effectively including the overlap integral in the transition probability given by the Fermi golden rule. For a valence-band quantum well, the relaxation rates due to the polar LO-phonon-, nonpolar LO-phonon-, and LA-phonon-scattering mechanisms at zero temperature are calculated. Due to the higher density of states in valence-band quantum wells, the calculated scattering rates are generally higher than the rates in similar structures based on conduction-band quantum wells.

We study theoretically the asymptotic phase times of phonon packets scattered off the single- and double-superlattice structures which act as opaque barriers for phonons analogous to potential barriers for electrons. The analytical expressions for the transmission and reflection times are derived. For the systems consisting of semiconductor superlattices of the size of 1000 Å we find that the time advance or delay of the transmitted and reflected packets range from 10 ps for a single-superlattice structure to 1 ns at resonances for a double-superlattice structure. These effects should be observable in the phonon generation and detection experiments utilizing a picosecond laser technique.

We study theoretically exciton properties of quantum wires (QWR’s) with an anisotropic two-dimensional parabolic potential in a magnetic field. First, the analytical solution for the single-particle states of the QWR’s in the magnetic field is obtained. Then, the energy states are analyzed numerically including the Coulomb interaction between an electron-hole pair. The results show that the experimental results on magnetophotoluminescence (magneto-PL) of GaAs QWR’s [Y. Nagamune, Y. Arakawa, S. Tsukamoto, M. Nishioka, S. Sasaki, and N. Miura, Phys. Rev. Lett. 69, 2963 (1992)] can be well explained by this model, demonstrating the importance of the exciton effect to understand the magneto-PL properties of QWR’s. On the other hand, the diamagnetic energy shift can be understood even without considering the Coulomb interaction, because the change of the exciton binding energy due to the magnetic field is relatively small compared to the total energy shift.

Exact wave functions for a particle in a quantum well with infinite conduction-band offset are calculated for a well material described by the Kronig-Penney model. The exact wave-function envelopes are extremely sensitive to the position of the interfaces within certain ‘‘boundary-sensitive domains.’’ At certain ‘‘critical positions’’ for the interface within these domains the envelopes fulfull zero-slope boundary conditions. In the nth band there are normally n well defined boundary-sensitive domains. However, for narrow-gap materials the boundary-sensitive domains become wider and may fill the enitre unit cell. In each band, effective-mass theory reproduces adequately the low-lying energy eigenvlaues but not the wave-function envelopes, and may give misleading results for oscillator strengths.

A quantum-mechanical theory of the influence of disorder on the motion of free particles is presented. Taking into account the dominant pair correlations of the potential fluctuations an analytical formula is derived for the asymmetrically broadened and shifted spectral function. The results are in agreement with a complete numerical treatment of disorder. The theory is applied to excitons in quantum wells with rough interfaces. We show that mainly the center-of-mass motion is influenced. Both the effect of the correlation and the strength of the accompanying potential fluctuations on the optical absorption are discussed. We clarify the relationship between the asymmetry, the shift, and the width of exciton lines. In the limit that these effects are only due to static interface disorder, an analyticl relation between potential strength and correlation length is suggested.

Localization in a pair of degenerate Landau levels, coupled by random interlevel matrix elements, is discussed in a semiclassical picture appropriate to the case of smooth disorder. It is shown that there are two distinct energies on either side of the band center where delocalization occurs. Each transition belongs to the same universality class as that of a nondegenerate Landau level in a random potential. Using a simple physical picture for the eigenstates, I will further argue that, unique to this model, the localization length also diverges at the band center, in accordance with recent results of Hikami, Shirai, and Wegner for white-noise disorder [Nucl. Phys. B 408, 415 (1993)].

We propose a general approach to the problem of intervalley mixing of electron states in heterostructures in the effective-mass method. The method has been used to calculate electron-transmission spectra across GaAs(AlAs${)}_{\mathit{M}}$GaAs single-barrier structures taking into account the Γ-X mixing at interfaces. The spectra exhibit sharp peaks and dips connected with the electron resonant transmission through quasibound X-like states in the AlAs layer. The peak and dip sequence depends on the parity of M. The developed approach allows us to present the energy dependence of transmission probability in an analytical form. The low-temperature dc current-voltage characteristics of the single-barrier structure has been derived making allowance for the camel-back X-band structure in bulk AlAs and GaAs.

We calculate the distribution of incompressible and compressible regions at an electrostatically defined edge of a two-dimensional electron system (2DES) in a strong perpendicular magnetic field B for two models, one with a strictly 2D arrangement of gate and charges, and one with a realistic 3D structure. We ensure electrochemical equilibrium by self-consistent calculation of the electrostatic potential from the electron density and the boundary conditions defined by the metallic gates, and of the electron density in this potential, using the Thomas-Fermi approximation. Both models yield qualitatively the same results. In the limit of zero temperature and strong magnetic field, and at distances from the edge which are much larger than the effective Bohr radius, our 2D model yields quantitatively the same result for the density profile in the 2DES as recent work by Chklovskii, Shklovskii, and Glazman, who considered a purely electrostatic model, without requiring electrochemical equilibrium for B=0. We demonstrate explicitly how the incompressible regions are destroyed with increasing temperature.

We present full-scale microscopic band structure calculations of the linear and nonlinear optical response of L-valley ${\mathrm{Ga}}_{0.7}$${\mathrm{Al}}_{0.3}$Sb-AlSb and ${\mathrm{Ga}}_{0.7}$${\mathrm{Al}}_{0.3}$Sb-${\mathrm{Ga}}_{0.4}$${\mathrm{Al}}_{0.6}$Sb superlattices. We report the magnitude and spectral form of the first- and second-order optical susceptibilities. A maximum normal-incidence linear absorption coefficient of 2180 ${\mathrm{cm}}^{\mathrm{-}1}$, with a linewidth of 15 meV, is obtained at 10 μm. In addition, we predict that a large second-harmonic response can be induced by normally incident radiation. An account is given of the dependence of the response on the carrier concentration, and on microscopic features in the superlattice band structure. The significant enhancement of the normal-incidence response compared to other miniband absorption spectra reported in the literature is due to a combined effect of the large density of states, dispersion, and the degree of confinement.

We have studied numerically several statistical properties of the spectra of disordered electronic systems under the influence of an Aharonov-Bohm flux cphi, which acts as a time-reversal symmetry breaking parameter. The distribution of curvatures of the single-electron energy levels has a modified Lorentz form with different exponents in the Gaussian orthogonal ensemble (GOE) and the Gaussian unitary ensemble (GUE) regime. It has Gaussian tails in the crossover regime. The typical curvature is found to vary as ${\mathit{E}}_{\mathit{c}}$/Δ${\ln }^{1/2}$[Δ/(${\mathit{E}}_{\mathit{c}}$${\mathrm{\phi }}^2$)] (${\mathit{E}}_{\mathit{c}}$ is the Thouless energy and Δ the mean level spacing) and to diverge at zero flux. We show that the harmonics of the variation with cphi of single-level quantities (current or curvature) are correlated, in contradiction with the perturbative result. The single-level current correlation function is found to have a logarithmic behavior at low flux. The distribution of single-level currents is non-Gaussian in the GOE-GUE transition regime. We find a universal relation between ${\mathit{g}}_{\mathit{d}}$, the typical slope of the levels, and ${\mathit{g}}_{\mathit{c}}$ the width of the curvature distribution, as was proposed by Akkermans and Montambaux. We conjecture the validity of our results for any chaotic quantum system.

The third-order nonlinear-susceptibility ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-2ω;ω,ω,0) spectrum has been investigated for thin films of linear-Si-chain compounds, poly(dihexylsilane), by measurements of electric-field-induced second-harmonic generation (EFISHG). In the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ spectrum for EFISHG, two-photon resonance structures to the first and second levels of one-dimensional excitons were observed. The observed ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ spectrum can be accounted for by considering multiple optical processes in the one-dimensional excitonic series characteristic of the Si-chain system.

Acquisition of Brillouin spectra for light scattered by the (111) free surface of GaAs provides the experimental determination of pseudo-surface-mode velocities and their attenuation for all nonequivalent azimuthal in-plane directions. Observations were performed with p-p and p-s polarization configurations. Good agreement between measured and calculated velocities of the pseudo surface modes is achieved. A reasonable correlation between the azimuthal behavior of the pseudo-surface-mode attenuation and that of the linewidth of the Brillouin satellites corresponding to these modes is obtained.

We present results of linear muffin-tin-orbital band-structure calculations of ErAs in the local-spin-density approximation. While the exchange splitting and cyclotron masses are in good agreement with experimental data, we find the Fermi surface dimensions of this semimetal to be significantly overestimated. A good fit to recently measured Shubnikov-de Haas oscillations in ${\mathrm{Er}}_{\mathrm{x}}$${\mathrm{Sc}}_{1\mathrm{-}\mathrm{x}}$As is obtained when the ${\mathrm{\Delta }}_2^{\prime }$ band is shifted upward by a quasiparticle self-energy correction. The effects due to alloying with Sc are taken into account in a virtual-crystal approximation.

We show that polariton effects lead to time-delayed components in the time-resolved four-wave-mixing (FWM) signal. Hence we find that recent observations in FWM experiments that have been ascribed to many-body Coulomb interactions can also be explained from the formation of exciton polaritons. In addition, we find that due to the formation of polaritons, the rate of decay of the time-integrated FWM signal strongly depends on the oscillator strength and the propagation length.

We give a detailed procedure for the calculation of the second-harmonic-generation susceptibility tensor in single and multiple asymmetric quantum wells (AQW’s). The effect of nonhomogeneity is explicitly included. Exciton states and continuum states are considered. The contributions due to the asymmetry of the confining potential and to the inversion asymmetry of the bulk material are separately assessed. Numerical calculations are carried out for two typical AQW’s of GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As. The results indicate larger values than in the bulk, in agreement with preliminary experimental data.

The dynamics of a tunneling electron through a small-capacitance tunnel junction are studied by developing a method for self-consistently determining escape rate and barrier traversal time for those electrons that transfer a given energy to the electromagnetic environment. The transfer-energy-dependent escape rate lets us examine dissipative and nondissipative tunneling rates separately and its integral over the transfer energy gives the total escape rate that is obtained by the instanton technique. It is found that the escape rate increases exponentially with increasing the ratio of the elementary charging energy to the quantized energy of the environment, but that the traversal time peaks at one particular ratio. As the frequency of the electromagnetic mode increases, an effective potential is shown to change from linear (dominated by the zero-point fluctuations of charge on the junction) to parabolic (dominated by the static polarization of the charge). Possible experimental situations for observing the predicted effects are discussed.