We consider the complete probability distribution P({${\mathit{T}}_{\mathit{n}}$}) of the transmission eigenvalues ${\mathit{T}}_1$,${\mathit{T}}_2$,...,${\mathit{T}}_{\mathit{N}}$ of a disordered quasi-one-dimensional conductor (length L much greater than width W and mean free path l). The Fokker-Planck equation which describes the evolution of P with increasing L is mapped onto a Schrödinger equation by a Sutherland-type transformation. In the absence of time-reversal symmetry (e.g., because of a magnetic field), the mapping is onto a free-fermion problem, which we solve exactly. The resulting distribution is compared with the predictions of random-matrix theory (RMT) in the metallic regime (L≪Nl) and in the insulating regime (L≫Nl). We find that the logarithmic eigenvalue repulsion of RMT is exact for ${\mathit{T}}_{\mathit{n}}$’s close to unity, but overestimates the repulsion for weakly transmitting channels. The nonlogarithmic repulsion resolves several long-standing discrepancies between RMT and microscopic theory, notably in the magnitude of the universal conductance fluctuations in the metallic regime, and in the width of the log-normal conductance distribution in the insulating regime.

We study the classical dynamics of N fully interacting charged particles, confined to a plane and subjected to a transverse magnetic field. We solve for the exact motion numerically, and interpret our results with the aid of approximate analytical models. The cluster is dynamically bound by the magnetic field, and we find that for large initial density both the minimum density attained at short times and the final equilibrium density are independent of particle number.

We investigate the spectrum of interacting electrons at arbitrary filling factors in the limit of vanishing Zeeman splitting. The composite fermion theory successfully explains the low-energy spectrum provided the composite fermions are treated as hard core.

Recent studies of the magnetothermopower of a two-dimensional electron gas have concentrated on measurements in either the low-field weak localization regime or the high-field Landau quantization regime. In this paper we emphasize that for magnetic fields between these two limits there is an interesting intermediate regime in which the total thermopower is dominated by semiclassical effects. Detailed expressions are derived for both the diffusion and phonon-drag contributions to the thermopower and a number of important observations are made that should be directly amenable to experimental verification.

As and Si Auger parameters for As terminated Si(100) and Si(111) surfaces have been measured and analyzed in terms of ground state charge transfer across the interfaces and core hole screening efficiency. Our results indicate a small electron transfer (∼0.05 electrons per atom) from Si to As at these interfaces. We find that for the As/Si(100) system core holes in atoms in the interface layers are much better screened than in bulk Si, due to high polarizability in the As-As bond. The screening is more bulklike at the As terminated Si(111) surface, consistent with the unreconstructed ‘‘bulk-terminated’’ surface structure of this system.

The initial stages of the ${\mathrm{CoSi}}_2$ formation on Si(100) surfaces after room-temperature Co deposition and subsequent annealing were studied using Auger electron spectroscopy, low-energy electron diffraction, and valence-band and core-level photoemission spectroscopy with synchrotron radiation. ${\mathrm{CoSi}}_2$ formation does not take place at room temperature. A coverage dependence of the reaction temperature on Si(100) is observed which is attributed to a change of the Co adsorption sites at room temperature above a Co coverage of ∼2.5 monolayer. This is related to the formation of ${\mathrm{CoSi}}_2$ grains and of pinholes. Co-rich and Si-rich ${\mathrm{CoSi}}_2$ surfaces were observed and characterized for different annealing temperatures. A Si termination of the Co-rich surface is inferred from our data. A c(4×4) reconstruction of the Si-rich surface is observed.

The capture time for an electron in a graded index separate confinement heterostructure is calculated as a function of quantum-well width. The electron’s interactions with the other electrons in the well along with the phonons are included in the random-phase approximation using full multiple-subband and frequency-dependent screening. It is shown that at sufficiently high electron densities, there is a significant increase of the capture rate with electron density except for quantum-well widths near a phonon resonance.

Starting from a simple model of the double-well semiconductor heterostructure capable of generating continuous-wave quantum ‘‘Block’’ oscillations, we have calculated analytically their major statistical properties, including spectral density and amplitude distribution. Other characteristics of the structure, pertaining to the Bloch oscillation, such as the dc I-V curve, rf impedance, and dc response to an external rf signal, were also calculated. The results are used to discuss similarities and differences between the Bloch oscillations and other types of oscillatory processes including the Josephson oscillations, laser radiation, spontaneous radiation, and narrow-band random noise.

We calculate the optical absorption of a quantum point contact. We find that the absorption in the very center of the microconstriction is clearly separated in frequency from the absorption in the wide contact regions. Therefore optical point contact spectroscopy is possible and can be very useful in characterizing the shape of the laterally confining potential. We show how two different types of potentials produce entirely different spectra. Further, optical point contact spectroscopy is found to be sample independent since the microconstriction geometry has practically no influence on the absorption, as long as we consider adiabatic geometries.

We interpret SiC polytypes as natural superlattices, consisting of mutually twisted cubic layers. A method is presented to calculate the electron band structure of any polytype, based on an empirical pseudopotential description of cubic SiC. Bloch and evanescent waves, belonging to cubic layers are matched at interfaces in order to make up the wave functions of the respective polytypes. Band gaps of hexagonal and rhombohedral modifications are in excellent agreement with experimental data such that the nearly linear relationship between the indirect gap and the hexagonal nature is reproduced. A simple explanation of this relationship is given in terms of a Kronig-Penney-like model.

We present theoretical calculations on the electronic structures of (ZnSe${)}_{\mathit{m}}$(${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se${)}_{\mathit{n}}$ superlattices. First-principle pseudopotential calculations are performed to obtain the valence-band offset for CdSe grown on ZnSe. We then use an empirical nonlocal pseudopotential method, including the spin-orbit interaction to calculate the band structures and effective masses of (ZnSe${)}_{\mathit{m}}$(${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se${)}_{\mathit{n}}$ superlattices grown on ZnSe. The empirical pseudopotentials are taken to be linear combinations of Gaussian functions with a few adjustable parameters. The parameters of ZnSe and CdSe are fitted to available experimental data. The effects of strain due to lattice mismatch have been properly taken into account. We have also studied the band gap as a function of composition and layer thicknesses of constituent materials in superlattices. The results are in agreement with recent experimental data.

Quasi-one-dimensional excitons in a GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum well are studied; they are produced by an applied twin-split-gate potential which confines the particles laterally and allows free motion in one dimension. A variational approach is used to calculate the binding energies ${\mathit{E}}_{\mathrm{ex}}$ and oscillator strength ${\mathit{f}}_{\mathrm{ex}}$ of these excitonic transitions as functions of the applied voltage and width of the induced potential wells. In the limit of high electrostatic confinement the excitons are strongly polarized and the system resembles a type II structure where electron and hole are spatially separated. The resulting ${\mathit{E}}_{\mathrm{ex}}$ and ${\mathit{f}}_{\mathrm{ex}}$ show a strong dependence on applied voltage and structure width. Strong oscillations are found, which should be observed experimentally, as a consequence of subtle competition between confinement and Coulomb attraction.

Raman scattering spectra of GaAs/AlAs superlattices grown along the [311] direction are compared with results of lattice-dynamical calculations. Confined optical as well as folded acoustic-phonon modes are clearly observed at frequencies in excellent agreement with those predicted. Several of the bulk branches hybridize along the low-symmetry [311] direction, producing confined optical modes in the superlattice of mixed longitudinal/transverse character. This hybridization complicates also the assignment of a mode order to the confined phonons, since they encompass components deriving from several bulk branches with different effective wave vectors. We demonstrate that a proper assignment can only be made by comparison to the calculated eigenvector envelope functions. We discuss how the crystal symmetry affects the angular dispersion of the optic modes.

We study the dependence on laser frequency (${\mathrm{\omega }}_{\mathit{L}}$) of the Raman spectra of GaAs/AlAs superlattices (SL’s) grown along the [311] direction. The observed phonon modes follow the polarization selection rules predicted by the SL crystal symmetry and, at the highest ${\mathrm{\omega }}_{\mathit{L}}$, the intensities predicted by the bulk Raman tensor. For lower ${\mathrm{\omega }}_{\mathit{L}}$ we observe a complex variation in the intensities of the confined LO and TO modes which deviates sharply from these predictions. We interpret these deviations as due to interference between a background associated with the ${\mathit{E}}_1$ and ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$ gaps and a resonant contribution arising from the hh2→e2 transition of the SL, both induced by the deformation potential interaction. For the latter, the electron-phonon coupling with the hole bands is diagonal, in contrast to the off-diagonal (hh→lh) coupling of conventional (100) SL’s. Evidence for a Fröhlich induced resonance at the ${\mathit{E}}_0$+${\mathrm{\Delta }}_0$ gap of the SL’s is also obtained.

A theory of current noise in photoconducting devices, developed on the basis of a large set of measurements concerning the behavior of photoconductivity, photoresponsivity, and noise as a function of light intensity and wavelength in CdS-based photoconductors, is presented. The theory is based on a barrier-type photoconduction model and embodies, in a single general expression of the noise power spectrum, the contributions due to the generation recombination and to the flicker noise (intrinsic noise) as well as to the noise generated by the fluctuation of the potential barrier that controls the electron injection from the metal electrode into the conduction band of the photoconducting material (photoinduced noise). According to the model, the height of this barrier depends on a balance effect between the light-induced positive static charge (ionized deep donor centers or trapped holes) and the negative space charge due to the injected electrons. The fluctuation of the number of ionized centers or trapped holes causes a fluctuation of the potential barrier and thus of the conductance of the device.

The transport process is described as a stream of conduction electrons crossing the fluctuating potential barrier and undergoing thermally activated trapping-detrapping processes within the photoconductor bulk. General expressions for the behavior of the photoconductance vs light intensity and for the photoconductance fluctuation power spectrum are worked out on the basis of this model and checked in the following paper by comparison with the experimental results. As shown there, the theory accounts for the shape and the relative changes of the noise power spectra, when the light intensity and the wavelength are varied, without any free parameter. The absolute value of the noise power density is also well reproduced without free parameters in the low-frequency range, where the photoinduced noise dominates. At higher frequencies, where the intrinsic noise becomes important, the noise power spectrum is also nicely fitted with very reasonable values of those quantities that have not been directly measured on the device. The same values of the quantities giving the correct photoconductance fluctuation spectra also reproduce the photoconductance vs light intensity curve calculated according to the proposed barrier photoconductance model.

Many measurements of photocurrent noise and of other quantities related to the photoconduction processes were performed on CdS based devices to check the theory of current noise, developed on the basis of a barrier-type model of the photoconduction mechanism in the preceding paper. Noise power spectrum measurements have been carried out in a wide range of photon flux densities (8×${10}^{10}$≤${\mathrm{\phi }}_{\mathit{n}\mathit{f}}$≤2×${10}^{13}$ photons ${\mathrm{s}}^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}2}$) as a function of light wavelength in the range between 400 and 750 nm. The most interesting feature of these results is the abrupt variation of the noise power spectrum occurring in correspondence with the critical wavelength ${\lambda }_{\mathrm{gap}}$, even if the device conductance is kept constant by varying the light intensity. Measurements concerning the behavior of the photoconductance, the optical transmittance, and the photoresponsivity versus light intensity and wavelength were also taken and used to determine most of the parameters appearing in the theoretical expression of the noise power spectrum. The relative variations of the shape and amplitude of the noise power spectra with light intensity and wavelength are reproduced by the theory without introduction of free parameters. Also the absolute value of the noise power spectrum at medium to high illumination value (${\mathrm{\phi }}_{\mathit{n}\mathit{f}}$>${10}^{11}$ photons ${\mathrm{s}}^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}2}$) and at frequencies below 5 kHz, where the photoinduced noise component dominates, is reproduced by the theory without free parameters. Finally, by assuming a suitable value of the parameter ${\mathrm{\tau }}_{\mathit{s}}$ appearing in the distribution function of the free-electron lifetimes and representing the inverse of the cutoff angular frequency of the g-r Lorentzian component, the whole set of experimental results is completely fitted by the theory for any value of the light intensity and wavelength, in the explored range.

The adsorption of submonolayers ${\mathrm{C}}_{60}$ molecules on Si(111)-(7×7) surfaces, deposited at room temperature and then annealed to various elevated temperatures, is studied using scanning tunneling microscopy. For room temperature adsorption, the ${\mathrm{C}}_{60}$ molecules were observed to favor bonding to adatom bridge sites within a triangular half cell. After annealing to 600 °C, however, they were found to bond dominantly to adatom or restatom sites. The adsorbate-substrate interaction at room temperature is characterized by charge transfer from the substrate to the molecules. After the annealing process, however, the adsorbates bond covalently to the substrate. It is found that the desorption of the submonolayer adsorbates due to the annealing process is minimal. The surface diffusion of the adsorbates begins when annealing the sample to 700 °C, at which point the initial clustering of the adsorbates takes place. When annealing the sample to 850 °C, the ${\mathrm{C}}_{60}$ molecules decomposed on the sample surface and reacted with the Si atoms to form SiC islands.

Synchrotron-radiation photoelectron spectroscopy has been used to investigate the phases that are formed upon doping of ${\mathrm{C}}_{60}$ with Ba and to examine the electronic properties of those phases. Core-level analysis reveals a ${\mathrm{Ba}}_1$${\mathrm{C}}_{60}$ phase and confirms the formation of ${\mathrm{Ba}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Ba}}_6$${\mathrm{C}}_{60}$ at 300 K. A continuous range of solubility is indicated for 3<x<6. ${\mathrm{Ba}}_1$${\mathrm{C}}_{60}$ has states at the Fermi level but transport measurements indicate that these states are localized. The width of the ${\mathit{t}}_{1\mathit{u}}$-derived band for ${\mathrm{Ba}}_1$${\mathrm{C}}_{60}$ points to the continued importance of electron correlation. ${\mathrm{Ba}}_3$${\mathrm{C}}_{60}$ is an insulator with a gap of at least ∼0.35 eV. The insulating character of ${\mathrm{Ba}}_3$${\mathrm{C}}_{60}$ reflects transfer of two electrons per Ba atom to the fullerene and complete filling of the ${\mathit{t}}_{1\mathit{u}}$-derived band. ${\mathrm{Ba}}_6$${\mathrm{C}}_{60}$ exhibits a clear Fermi cutoff, as expected from its superconducting character, and resonant photoemission shows that the states near ${\mathit{E}}_{\mathit{F}}$ are ∼25% Ba in character with hybridization in both ${\mathit{t}}_{1\mathit{u}}$- and ${\mathit{t}}_{1\mathit{g}}$-derived bands.

An anisotropic body-centered solid-on-solid (BCSOS) model Hamiltonian of the fcc(110) surface is reexamined and studied for the description of phase transitions on nonreconstructed noble- and near-noble-metal surfaces. The limit case corresponding to the isotropic BCSOS model is examined and some results are obtained for the staggered order-parameter susceptibility. With parameters roughly appropriate for a nonreconstructed surface such as Ag(110), a preroughening-type phase transition associated with the in-plane degrees of freedom is detected. It takes place somewhat below the roughening transition temperature and presents critical exponents which differ significantly from the two-dimensional Ising values. The analysis of the model is carried out through standard Monte Carlo simulation techniques. Scattering intensities are also evaluated, both to give a deeper insight into the disordered flat phase lying between the two transitions and to suggest a possible experimental verification of the proposed phase structure for the nonreconstructed two-sublattice surfaces.

A methodology is developed for the simulation of solids in the grand-canonical ensemble at nonzero temperatures. Unlike canonical-ensemble simulations, this allows vacancies, interstitials, and substitutional impurities to be treated in a systematic way. The utility of the method is demonstrated by considering the reconstruction of the (110) surface of gold.