The Γ-X transition rate for electrons in type-II superlattices is calculated for the case of optical-phonon emission. The tight-binding method for electronic band structure and the dielectric continuum model for phonons are used. The relative strength of scattering due to different phonon modes is examined for varying superlattice dimensions. The scattering rate is highest when the energy separation between the Γ and X levels is smallest, and decreases quickly as the separation increases. It is found that the strongest scattering rate is due to the emission of AlAs confined modes. Changing of parity with layer thickness and its effect on scattering are discussed.

We study the level statistics of the quantum spectrum of a harmonic oscillator with biquadratic corrections. The distribution of interlevel spacings along the lines of constant scaled energy, whose values determine the classical dynamics, is obtained numerically. A transition from the Poisson to the Wigner character of the distribution is observed with the increase of the scaled energy. Using a quasi- classical approximation and assuming ergodic chaos we derive the level correlation function along the lines of constant scaled energy by employing the trace formula and keeping only the diagonal term. Our calculation yields a smoothed out tail of a Gaussian ensemble and is based on the exact cancellation of two ratios: the ratio of the mean level density along the line of constant scaled energy to that at a fixed bi- quadratic coupling and the ratio of the action to the period of a long periodic orbit. We further argue that the Wigner distribution will be found along any smooth line crossing the spectrum in the region of classical chaos. Applicability of our findings to quantum-dot structures at semiconductor interfaces is discussed.

A random-matrix analysis is presented for a four-terminal device consisting of one-dimensional mesoscopic wires. Interest is focused on the potential difference v measured between the two sides of a weakly disordered conductor. Various interference effects are found in the statistical distribution w(v) of v. Both the centroid and width of w(v) oscillate as a function of the probe separation, the size of the oscillations depending on the coupling constant ε between the probes and the wire. For ε≪1, w(v) is wide enough to yield a nonzero probability to find a potential rise instead of a potential drop.

We study Raman scattering by spin flips of acceptor-bound holes in p-type GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells in normal and tilted magnetic fields. It is shown that different mechanisms are responsible for the scattering under excitation in resonance with the following complexes; exciton bound to neutral acceptor (${\mathit{A}}^0$X) and a localized exciton neighboring as a neutral acceptor. It is demonstrated that in the Faraday backscattering geometry the ${\mathit{A}}^0$X-mediated scattering process can be considered as a double spin flip because it includes an acoustic-photon-induced spin flip of an electron in the photocreated ${\mathit{A}}^0$X complex. In tilted magnetic fields an additional satellite line ${\mathit{A}}^0$X’ appears in the Raman spectrum due to the Zeeman interaction of the electron spin with the in-plane field component. The neutral-acceptor and electron g factors are directly measured as a function of the quantum-well width. Two other lines of the ${\mathit{A}}^0$X-related scattering are attributed to the bound-hole interlevel transitions ±3/2→∓1/2, which allow the determination of the interlevel splitting and an analysis of its inhomogeneous broadening induced by fluctuations in the well thickness.

The destabilization of a flat liquid-metal surface subjected to a normal dc electric field is investigated both theoretically and experimentally, taking into account the influence of the finite distance of an upper electrode and the confinement of the liquid to a vessel of circular boundaries. The theoretical approach consists of a hydrostatic linear perturbation analysis giving the critical values of the applied electric field for which the basic planar configuration coexists with other possible solutions. Experimental observations are in good agreement with the predicted modes of destabilization and critical fields.

Optical properties of the Ga(010), Ga(111), and Ga(112) surfaces have been studied. Single crystals have been grown at UHV conditions and in situ ellipsometric techniques have been applied. On the Ga(112) surface the formation of a quasiliquid layer has been detected at temperatures below the bulk melting point ${\mathit{T}}_{\mathit{m}}$. The optical properties of the layer are close to the ones of the crystal bulk. The thickness of the quasiliquid layer increases with increasing temperature T. The functional character of the growth law of the quasiliquid layer of the Ga(112) surface is in agreement with the prediction of mean-field theory. In the temperature range 0.5<${\mathit{T}}_{\mathit{m}}$-T<3.5 K the increase in layer thickness can be described by a logarithmic growth law with a correlation length ξ=0.8±0.2 nm. For temperatures 0<${\mathit{T}}_{\mathit{m}}$-T<0.2 K the increase in layer thickness can be described by a power-law growth with a Hamaker constant W=(4.8±2.0)×${10}^{\mathrm{-}18}$ mJ. There is a crossover from a logarithmic to a power-law growth of the layer in the temperature range 0.2<${\mathit{T}}_{\mathit{m}}$-T<0.5 K. The crossover thickness is comparable with the correlation length within the disordered or quasiliquid layer: ${\mathit{l}}_{\mathit{c}}^{\mathrm{*}}$=0.7±0.1 nm. Ga(010) and Ga(111) surfaces are stable against thermal disordering up to ${\mathit{T}}_{\mathit{m}}$. The highest stability has been observed at the Ga(010) surface. On the Ga(111) surface, changes in the refractive index have been detected while cycling the temperature up to the bulk melting temperature, whereas the extinction coefficient remains constant. Close to the melting point no drastical changes in n or k have been observed indicating that no enhanced disordering takes place close to ${\mathit{T}}_{\mathit{m}}$.

Conversion-electron Mössbauer spectroscopy in ultrahigh vacuum has been employed to study in situ the average Fe-spin orientation and hyperfine fields in zero external field in uncoated and Tb-coated ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ films deposited either onto a clean Tb(0001) single-crystal surface or onto (bulklike) polycrystalline Tb or Ag films. At 30 K, the spectra of 15-Å-thick uncoated Fe films on Tb(0001) indicate structurally disordered (amorphous) Fe layers, whereas spectra of 30- or 35-Å-thick uncoated and coated Fe films are typical for bcc Fe. We show that the orientation of Fe spins in 30–35-Å-thick uncoated Fe films is preferentially in the film plane for temperatures ranging from 30–300 K for all substrates used. In striking contrast to uncoated films Tb-coated 30–35-Å-thick Fe films exhibit a preferential Fe-spin orientation perpendicular to the film plane at ∼30 K irrespective of the substrates used. This demonstrates (i) that the perpendicular spin orientation in these films originates from an interface magnetic anisotropy and (ii) that the dominant effect in inducing perpendicular spin direction originates from the interface formed by the Tb-coating layer and the Fe film and to a smaller exent to the Fe/Tb-substrate interface. A temperature-dependent reversible Fe-spin reorientation was observed near 150–200 K which is qualitatively compatible with recent models including spin fluctuations.

We report an all-electron density-functional-theory study of the interaction of hydrogen with Pd(100) and Rh(100) surfaces. We use the local-density approximation for the exchange-correlation energy functional and the full-potential linear-muffin-tin-orbital method. Various coverages, between Θ=0.25 and Θ=2 are considered. In particular, we discuss the adsorption energies, stable adsorption sites, adsorption-induced surface relaxations, and the work-function changes. The results show that for coverages Θ≤1 at Pd(100) the fourfold hollow site is energetically favorable. For higher coverages it is predicted that the additional hydrogen goes subsurface. The work function is found to increase with coverage up to Θ=1 and for higher coverages (i.e., when the subsurface sites get occupied) it remains roughly at the Θ=1 level. Hydrogen adsorption at Rh(100) is found to be very similar to Pd(100) up to Θ=1. The results are compared to available experimental data as well as to other calculations.

Molecular-dynamics simulations using a Pauling type pairwise potential have been carried out in order to study the structure of the surface of $\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3$. Starting from an ideal (100) face, a reconstruction process occurs in which the unbalanced coordination of surface ions tends to fulfill their coordination capabilities. Oxygen ions come to the surface and pore formation with a concomitant increase of apparent volume takes place. The structure of the surface of the resulting material corresponds to a porous amorphouslike phase in which only short-range order is present. Analysis of the radial distribution function agrees with experimental x-ray-diffraction data.

It is shown theoretically that there is competition between core-hole decay and delocalization of a resonantly excited electron from the adsorbate to the substrate metal with simultaneous substrate-to-ligand charge-transfer core-hole screening [relaxation from the resonantly core excited neutral state to the x-ray-photoemission-spectroscopy (XPS) core ionized state] or between the core-hole decay and the charge-transfer core-hole screening from the substrate metal to the adsorbate (relaxation from the XPS core ionized state to the resonantly core excited neutral state). In the former case a spectral feature due to the Auger decay from the ionic core-hole state appears in the autoionization spectrum, while in the latter case a spectral feature arising from the spectator and participant decay from the neutral screened core-hole state appears in the Auger spectrum measured high above the core ionization limit. The relaxation time can be determined from the relative Auger and autoionization spectral intensity. Applications of the Auger-photoelectron coincidence spectroscopy and x-ray-emission spectroscopy to adsorbates are also discussed.

We report results from ab initio cluster-model calculations on the O(1s) binding energy (BE) in the alkaline-earth oxides, MgO, CaO, SrO, and BaO; all these oxides have a cubic lattice structure. We have obtained values for both the initial- and final-state BE’s. A simple point-charge model, where an ${\mathrm{O}}^{2\mathrm{-}}$ anion is surrounded by point charges, accounts for the observed shift in the O(1s) BE by about 2.5 eV to lower energy going from MgO (largest BE) to BaO (smallest BE). This point-charge model only describes the effect of the Madelung potential at the 1s ionized ${\mathrm{O}}^{2\mathrm{-}}$ anion; it does not allow any covalent bonding between the metal (M) cations and the oxygen anions. Once the effect of the ${\mathrm{O}}^2$→${\mathit{M}}^{2+}$ covalent bonding is included explicitly by using ${\mathrm{O}}_{\mathit{m}}$${\mathit{M}}_{\mathit{n}}$ cluster models of the MO crystal, the trend in the O(1s) BE to smaller values for heavier metals does not change significantly. This shows that the main contribution to the shift is not the different amount of covalent mixing in the alkaline-earth oxides but rather the change in the Madelung potential along the group.

The energetics and electronic structure of a series of neutral and charged oxygen and magnesium vacancy defects (F, ${\mathit{F}}^{+}$, ${\mathit{F}}^{2+}$, V, ${\mathit{V}}^{\mathrm{-}}$, ${\mathit{V}}^{2\mathrm{-}}$, and P centers) in MgO have been computed using the stationary total-energy functional. We find that contrary to the charge-compensation model for anion-cation defect pairs in ionic materials it is energetically unfavorable for an isolated neutral oxygen vacancy (F center) to transfer electrons to an isolated neutral magnesium vacancy (V center), and form isolated ${\mathit{F}}^{2+}$ and ${\mathit{V}}^{2\mathrm{-}}$ centers. Charge compensation is unfavorable because additional electrons at the V center induce new occupied states in the gap, which increase the energy of the defect. This result is consistent with the interpretation of spectroscopic experiments on MgO, in which the ground-state defects are either neutral or singly charged. The computed formation energies of both the F and V centers are larger than the cohesive energy of MgO per formula unit, but the binding energy of the defects in the P center configuration is 12.16 eV. This attraction between the F and V centers is enhanced when the defects carry a net charge. The position of the vacancy defect state in the fundamental energy gap of MgO is found to be in qualitative agreement with a model for optical absorption and emission, and is used as a simple model for the formation energies of the defects. The contribution of the band-structure energy to the stationary functional is found to account for more than 90% of the defect energies. This component of the defect formation energy is computed directly, using the recursion method, rather than as the difference between the total energies of our 8000-atom cluster with and without the defect.

Moleclar-dynamics studies of water aggregation on (111) surfaces of ${\mathrm{BaF}}_2$ and ${\mathrm{CaF}}_2$ show that at a critical coverage of about 0.5 monolayer, regular chain and ring patterns develop on ${\mathrm{BaF}}_2$ and ${\mathrm{CaF}}_2$, respectively, governed by the free energy of the system. At one monolayer, the structure on ${\mathrm{CaF}}_2$ randomizes, whereas on ${\mathrm{BaF}}_2$ it forms a two-dimensional pattern, commensurate with the substrate ${\mathrm{F}}^{\mathrm{-}}$ sublattice. Beyond one bilayer, regular ice-Ih grows on ${\mathrm{BaF}}_2$, stable even at room temperature. Vibrational frequencies at low coverages and low-energy electron diffraction patterns for high coverages are predicted.

The reconstruction of the C(100) surface is studied by a cluster model at several theoretical levels. It is found that the calculated surface-dimer bond length is very sensitive to the level of theoretical treatment, the spin state, and the degree of constraints in the geometry optimization process. A single-determinant self-consistent field (SCF) treatment gives a closed-shell singlet state, higher in energy than the triplet state, and with a dimer length of 1.401 Å, 0.279 Å shorter than the triplet. The dimer is found to be symmetric for both singlet and triplet states at the SCF and complete active space (2,2) levels of theory. The correct ground state is a singlet, but a multideterminant wave function is required for its description. At the configuration-interaction level, the surface-dimer bond length in the ground state is found to be 1.508 Å and the energy decrease on dimer formation with respect to the ideal C(100) 1×1 surface is 2.28 eV per dimer.

We have investigated the dynamics of electrons in a series of ultrathin copper films (sandwiched between protective layers of either germanium or silicon) in the weak-localization regime. Using a contactless technique, we have measured the changes in the ac conductivity at a fixed microwave frequency (ω∼${10}^{11}$ ${\mathrm{s}}^{\mathrm{-}1}$) as a function of applied static magnetic field, for different temperatures. By decreasing the temperature below 5 K we can tune the dephasing rate below our measurement frequency, which produces a difference between the real part of the ac magnetoconductivity and the dc magnetoconductivity for small static fields. Corresponding changes occur in the imaginary part of the ac magnetoconductivity as the temperature or magnetic field are varied. We obtain similar results on samples with different thicknesses, and we find that the magnitude of the ac effect remains constant even as we vary the sheet resistance by a factor of 3. The magentoconductivty data for all of our samples can be well described by frequency-dependent weak-localization theory provided that contributions from superconducting (Maki-Thompson) fluctuations are taken into account. The fits to the data provide three ways to estimate the phase and spin coherence times in our samples, and the estimates agree to within 20%. We have also directly observed superconducting transitions near 0.4 K for Ge-Cu-Ge samples, and we see strong evidence for superconductivity in the Si-Cu-Si samples, with a transition temperature below 0.3 K. The temperature variation of the dephasing rate ${\mathrm{\tau }}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}^{\mathrm{-}1}$, which we extract from the theoretical fits to the dc data, fully confirms the picture that the rates become smaller than ω for temperatures below about 5 K. We find an approximately linear temperature depedence for ${\mathrm{\tau }}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$(T) below 6 K, which is consistent with theories of enhanced electron-electron inelastic scattering, but the theories do not give the correct magnitude for ${\mathrm{\tau }}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$(T).

A symmetry analysis of the 2a phase recently observed in some samples of ${\mathrm{C}}_{60}$ is presented. This phase is described by a unit cell with eight molecules in inequivalent orientations. We first show that if this structure is assumed to be exactly cubic, there are only three allowed space groups, none of which corresponds to the PA3¯ arrangement of threefold axes previously established for ${\mathrm{C}}_{60}$ by several groups. Our calculated powder diffraction spectra for these space groups are not consistent with existing experimental data. Second, if the symmetry of the PA3¯ structure is lowered by a doubling of the unit cell, we show that the resulting structure is trigonal, space group R3¯. We calculate powder diffraction spectra for this scenario and thereby place upper limits on both the angular distortion and the trigonal lattice distortion. Third, since the microscopic origin of this distortion probably involves defects of some presently unknown type, we consider a phenomenological scenario for the origin of this trigonal distortion. Within this scenario, we study the symmetry of the interactions needed to explain this structure. We start by giving an analysis of the structural distortion within harmonic lattice dynamics. However, to obtain the correct (R3¯) symmetry structure we were forced to study the cubic coupling between zone-corner librons and macroscopic strains. In this way we relate the development of R3¯ symmetry from the PA3¯ structure in terms of a phenomenological model of lattice dynamics. Fourth, we extend the above arguments to construct a Landau theory for the hypothesized PA3¯→R3¯ phase transition, which occurs as a function of the concentration of the presumed defects. The resulting free energy has no cubic terms (so the transition can be continuous) but has five fourth-order invariants.

Cross sections for quasiatomic excitation by exchange scattering of electrons are calculated in the Born-Ochkur approximation for 3p-3d and 3d-3d transitions in transition-metal systems, and for 4d-4f and 4f-4f transitions in rare earths. The energy dependence of the spin polarization of 3p-3d and 4d-4f losses in reflection electron-energy-loss spectroscopy from ferromagnetic surfaces involves a balance of small-angle spin-dependent inelastic processes accompanied by a high-angle elastic scattering, and large-angle spin-flip exchange scattering without the need for elastic scattering. Both 3d-3d excitations, e.g., in transition-metal compounds, and 4f-4f excitations in rare earths involve spin-flip transitions whose scattering amplitudes g fall off with momentum transfer q such that the full width at half maximum ${\mathit{q}}_{1/2}$ (in a.u.) is given by ${\mathit{q}}_{1/2}$〈${\mathit{r}}_{\mathit{n}\mathit{l}}$〉∼2, where 〈${\mathit{r}}_{\mathit{n}\mathit{l}}$〉 is the expectation value of r for the 3d or 4f electron. The angular width of the spin-flip differential cross section is then much greater than for dipole transitions, a pattern that helps to account for how these intra-atomic transitions compete with dipole processes for primary energies in excess of 100 eV.

The Hubbard model with bond alternation is used to calculate the hyperpolarizability γ of polymers. Exact numerical solutions are found for short chains of N sites. The hyperpolarizability is represented as γ∝${\mathit{N}}^{\mathit{n}}$. The exponent n is over six for no bond alternation, and declines slowly with increasing bond alternation. This behavior is compared with the measurements in polythiopehene.

An ESR study performed on a thick ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As epilayer with removed GaAs substrate is presented. The measurements were performed on LPEE-grown ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As (x=0.41) heavily doped with Te. The detailed photo-ESR investigations of the light-induced conversion of the DX centers into the shallow Te donor states are presented. The mechanism of an enhanced photosensitivity of the Te-related electron-spin-resonance signal in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As is discussed. The observed anisotropy of the signal allows us to estimate the ${\mathit{A}}_{1\mathrm{-}}$E valley-orbit splitting for Te donor. The ODMR experiment indicates that the E-symmetry state of the Te donor participates in electron trapping to the ${\mathit{A}}_1$ ground state.

We report the experimental observation of nonparabolicity in the conduction band of a II-VI semiconductor. Optically detected cyclotron resonance in ZnTe shows a spin splitting which is proportional to the square of the applied magnetic field. The observed spin splittings at two different far-infrared energies are in excellent agreement with theoretical calculations.