We have investigated the impact-parameter dependence of stopping powers for axially channeled and semichanneled MeV He ions in Er-sheet-doped GaAs epitaxial layers grown by molecular-beam epitaxy. Ion channeling analysis using 2.0 MeV ${\mathrm{He}}^{+}$ coupled with the observation by transmission-electron microscope has revealed the formation of fine ErAs clusters, whose lattice constant shrinks and coincides exactly with that of the GaAs host. Thus the Er atoms take the position corresponding to the tetrahedral interstitial site of the GaAs lattice. The Er peak energies in the backscattering spectra strongly depend on the impact-parameter dependent stopping powers, in particular for the incidence along [110] and the directions slightly tilted from [110] and [100]. We divide the stopping power into two parts—contributions from outer electrons and from inner electrons of GaAs. The former is calculated from the dielectric response theory. For the latter, we consider four types of stopping powers dependent upon impact parameter; (1) Dettmann-Robinson theory, (2) the binary-encounter model, (3) the binary encounter combined with the local-density approximation, and (4) the extended-local-electron-density model. The Er peak energies observed are well reproduced employing model (3).

An approach describing the atomic positions around misfit dislocations lying along a planar interface is presented using both discrete (the atomic structural unit repeating along the interface) and continuous (anisotropic elasticity of heterogeneous media) information. An appropriate computer program illustrates the usefulness of the method when applied to high-resolution electron-microscopy images of the (001) CdTe/(001)GaAs and (1 1¯ 1¯) Au/(001)Au interfaces. Potentially, any two crystal structures can be taken into account.

Using the method of series expansion, interface-phonon vibrational modes are calculated in the dielectric continuum model for the graded quantum well of ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As with a ${\mathrm{Ga}}_{0.6}$${\mathrm{Al}}_{0.4}$As barrier. The intrasubband and intersubband scattering rates are obtained as functions of quantum-well width. The results reveal that the behavior of interface phonon modes is very different from that in a square quantum-well structure. It is found that the electron–interface-phonon scattering rates can be changed remarkably in a graded quantum-well structure compared with those in a square quantum-well structure, which is useful for some device applications.

We calculate analytically the energies of a class of N-electron quantum dots employing a recently introduced symplectic-group chain. The corresponding algebraic Hamiltonians lie beyond the N-electron Fock-Darwin Hamiltonian which only describes noninteracting particles in a perfect parabolic confining potential. Nonparabolicity and particle-particle interactions are simulated using quadratic Casimir operators within the symplectic-group chain. We calculate infrared optical absorption energies as a function of magnetic field, and analyze the breakdown of the generalized Kohn theorem. We also calculate the addition spectra associated with transport through the N-electron quantum dot.

We investigate theoretically the structure and electronic properties of the interface between aluminum and polyaniline in order to obtain information on the most favored adsorption sites and the nature of the interactions occurring between the metal and the polymer surface. For this purpose, quantum chemical calculations at the semiempirical and ab initio Hartree-Fock levels are performed on model systems consisting of short-chain molecules interacting with two metal atoms. The interaction of the metal with the polymer strongly modifies the geometrical structure of the organic backbone; electron charge transfer takes place from the metal atom to the organic system and the π electronic levels are strongly perturbed. Our calculations are compared to experimental x-ray photoelectron spectra taken during the initial stages of aluminum deposition on polyaniline.

While GaAs(001) surface reconstructions have been studied extensively in the ultrahigh-vacuum environment associated with molecular-beam epitaxy (MBE), comparatively little is known of these structures in the chemically rich environment associated with organometallic vapor-phase epitaxy (OMVPE). This work presents a structural study of the c(4×4) surface reconstruction stabilized in an arsenic-rich OMVPE environment. Measurements of the in-plane structure were performed in situ using grazing-incidence x-ray scattering with synchrotron radiation. Structural refinement confirms the presence of arsenic-arsenic dimers arranged with the c(4×4) symmetry. In concurrence with similar studies performed in the MBE environment, it is found that the surface is a mixture of structural domains composed of two- and three-dimer variants of the c(4×4) reconstruction. Atomic positions associated with these structures are presented. The size, aspect ratio, and orientation of the reconstructed regions are shown to be closely related to the atomic step geometry on the crystal surface.

Femtosecond photon-echo techniques are used to probe the dynamics of quantum-confined excitons in nanocrystals of CdSe. Using three-pulse photon echoes, the modulation of the echo signal from the LO-phonon mode is effectively suppressed, and both the electronic dephasing and the couplings to lattice vibrations are probed directly. Detailed measurements are reported as a function of both nanocrystal size and temperature. The dephasing times vary from 85 fs in nanocrystals of 20-Å diameter to 270 fs in 40-Å crystals. These rates are determined by several dynamical processes, all of which depend sensitively on the size of the nanocrystal. The time scale of the trapping of the electronic excitation to surface states increases with increasing size. The coupling of the excitation to low-frequency vibrational modes is strongly size dependent as well, in accordance with a theoretical model. The photon echo also gives information on the polar coupling between the electronic state and the LO-phonon mode. This coupling is found to peak at an intermediate size. This phenomenon is interpreted as a manifestation of coupling between the interior confined excitons and localized surface states, which destroys the spherical symmetry of the excited state. Using these data, all of the important contributions to the size-dependent homogeneous linewidths can be enumerated.

The effect of a strong magnetic field on the persistent current and current-current correlations in the Laughlin geometry are discussed. Due to the violation of time-reversal symmetry a single disordered sample may carry a current even in the absence of a threading flux. Once one averages over many realizations this current disappears. A theory is presented for the current-current correlations in the presence of a strong magnetic field and compared to the results of a numerical calculation. The results fit for the bulk states while discrepancies appear for edge states.

We have studied the photoluminescence (PL) emission from a GaAs/AlAs asymmetric-double-well superlattice with indirect band gap as a function of temperature and applied electric field. The PL spectrum consists of two peaks, with a separation of only 21 meV, originating from the recombination of wide-well heavy holes with either X electrons of the barrier or Γ electrons of the wide well, respectively. The PL intensity ratio of the two peaks shows a strong nonthermal population between Γ and X states near flatband, because in this sample the transfer from Γ to X after photoexcitation can be induced only by absorption of phonons with a large momentum. The small Γ-X separation and the long recombination time of X electrons results in the filling of the X states that are at the same energy as the Γ minimum, blocking Γ-X transfer by phonon emission or interface (roughness) scattering.

We show how macroscopic manifestations of P (and T) symmetry breaking can arise in a simple system subject to Aharonov-Bohm interactions. Specifically, we study the conductivity of a gas of charged particles moving through a dilute array of flux tubes. The interaction of the electrons with the flux tubes is taken to be of a purely Aharonov-Bohm type. We find that the system exhibits a nonzero transverse conductivity, i.e., a spontaneous Hall effect. This is in contrast to the fact that the cross sections for both scattering and bremsstrahlung (soft-photon emission) of a single electron from a flux tube are invariant under reflections. We argue that the asymmetry in the conductivity coefficients arises from many-body effects. On the other hand, the transverse conductivity has the same dependence on universal constants that appears in the quantum Hall effect, a result that we relate to the validity of the mean-field approximation.

An investigation has been made of the far-infrared cyclotron emission from electrically heated, magnetically quantized two-dimensional electron gases using a technique employing a broad-band detector. Spectral resolution is obtained by using the substrate as a Fabry-Pérot étalon. The experiments indicate that the system is well adapted for measuring the electron temperature as a function of magnetic field. Further spectroscopic information is obtained by including a low-pass filter. The system has been used to investigate the emission spectrum and its intensity in fields up to 3 T and currents up to 1 mA (current density of 5 mA/mm). The measurements demonstrate that 20–30 % of the emission occurs at frequencies less than ${\mathrm{\omega }}_{\mathit{c}}$.

We have demonstrated that ${\mathrm{\Phi }}_0$-periodic Aharonov-Bohm oscillations measured in an ensemble of rings may survive after ensemble averaging procedure. The central point is the difference between the preparation stage of the ensemble and the subsequent measurement stage. The robustness of the effect under finite temperature and nonzero charging energy of rings is discussed.

We present nonequilibrium calculations of the Coulomb-mediated carrier-capture time in electrically pumped quantum-well (QW) lasers and demonstrate that carriers are efficiently trapped into the QW at realistic densities. Furthermore, contrary to the common belief that the capture time oscillates strongly with well width, we argue that such oscillations must be heavily damped, since the confined carriers ultimately form a quasi-three-dimensional gas as the number of bound states increases.

We have investigated the transport properties of insulating phases in Si MOSFET’s at extremely low temperatures. It has been found that insulating phases in the quantum Hall regime behave in a way that is similar to a low-density insulating phase: for all phases we find similar behavior of both the activation energy for the resistance in the linear part of the current-voltage characteristics and the critical electric field corresponding to the onset of nonlinearity. A characteristic length extracted from measurements of the activation energy and critical electric field has been found to diverge near each metal-insulator phase boundary. These results support the localization and reject the Wigner crystal as the origin for any insulating phase. Nonlinear current-voltage characteristics of the insulating phases can be explained by electric-field-induced electron delocalization. We have obtained the critical index for the localization length s≊1, which is close to the value s=1.3 for classical percolation. We have tested that the effect of temperature can also be treated in terms of delocalization so that the temperature dependence of the width of peaks in the conductivity is explained by the thermal shift of the effective mobility edge of a Landau level. The experimental results point out the existence of the mobility edge in two-dimensional systems in zero magnetic field, in contrast with the predictions of scaling theory.

A detailed microscopic investigation of the effect of Coulomb correlations on the inelastic contribution to tunneling via localized states through thin amorphous barriers is presented. The theoretical calculation together with experimental results show that Coulomb correlations play an important role and also imply that localized states whose single-particle energies lie well below the Fermi level participate in transport due to the Coulomb correlations.

A uniform quantitative description of the properties of the one-dimensional Anderson model is obtained by mapping that problem onto an infinitely quasidegenerate master equation. This quasidegeneracy is identified as the source of the small-denominator problem encountered before in investigations of this problem. An appropriate quasidegenerate perturbation theory is developed to obtain a uniform asymptotic expansion, in powers of the strength of the noise, for the probability distribution function of the ratio of the value of the wave function at neighboring sites. Well known results, such as those obtained by Thouless, Kappus and Wegner, and Derrida and co-workers are reproduced and systematic corrections to these results as well as some more results are found. In particular, we find internal layers in the above-mentioned distribution function for values of the energy given by E=2 cosπα with α rational. We also find crossovers in the behavior of the distribution function (and consequently in quantities derived from it) near the band-edge and band-center regions. The properties of the model in the band-edge region were studied by us in detail in a previous publication [Phys. Rev. B 47, 1918 (1992)].

We have performed lateral-diffusion measurements of photoexcited electron-hole pairs at various excitation densities for nondoped and p-type-modulation-doped GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well structures at 77 K, using an all-optical time-of-flight technique with a single-mode optical-fiber probe. Systematic measurements at various electron and hole densities revealed remarkable deviation from the conventional ambipolar diffusion, indicating the important contribution of carrier-carrier scattering and excitonic effects to the diffusion.

We study theoretically the collective electronic excitations of quasi-one-dimensional cylindrical quantum-well wires, the intrasubband and intersubband plasmons in the random-phase approximation. Using a two-subband model we calculate the dispersion curves for the case that two subbands are occupied. We study the influence of the image forces on the intrasubband and intersubband plasmons in detail. These modes are split in two branches. The intrasubband plasmon branches can be classified in an optical and an acoustic branch which are influenced quite differently by the image forces. The intersubband plasmon branches behave like acoustic plasmons.

A 12-mode heterostructure theory employing an eight-band zone-center k⋅p model is formulated on a level comparable to the envelope-function approximation. The theory implements probability-flux-conserving interface boundary conditions, but also accounts for the often neglected material dependence of the zone-center Bloch functions. Such material differences lead to a coupling of the light- and heavy-particle states which is absent in many other schemes. Further, we find previously unidentified contributions to the eight-band-model transfer matrix, one describing the effect of spin-orbit coupling on interfacial scattering, another providing corrections required by the use of the finite eight-band bases. An important consequence of the theory is a unique perspective that exposes the relationship among, and implicit assumptions involved in, various schemes appearing in the literature. In particular, the usual envelope-function approximation results from adding simplifying assumptions to the theory.

Transport through a tunnel junction connecting a superconductor to a spin-aligned quantum Hall fluid at filling ν is studied theoretically. The dominant transport channel at low temperatures is the tunneling of Cooper pairs into edge states of the quantum Hall fluid. This process, which is greatly suppressed at low energies due to both Coulomb and Pauli exclusion effects, leads to a tunneling conductance which vanishes with temperatures as ${\mathit{T}}^{4/\nu \mathrm{-}2}$, for ${\nu }^{\mathrm{-}1}$ an odd integer. For integer filings with ν>1 the ‘‘Pauli blockade’’ is circumvented and a nonvanishing conductance is predicted.