The constitutive equation for Frenkel exciton polarization in a bounded medium is derived from both macroscopic Lagrangian theory and microscopic coherent-wave theory. The result corresponds to the Hopfield and Thomas model previously thought to apply to the Wannier exciton. We show that the Frenkel exciton must be regarded as a finite-volume object and so the change of the exciton binding energy near a surface must be included in the optics problem of transmission/reflection near its resonance (the so-called additional boundary condition or ABC problem). The combination of the exciton polarization equation and the wave equation is then solved by a new mathematical method in wave-vector space (k space). We show that the truly macroscopic nonlocal susceptibility must contain surface terms that previous macroscopic theories did not include. We obtain a different solution to the problem with this theory that requires no boundary conditions. By transforming to real space, we also derive the ABC needed for a macroscopic real-space approach and point out the important (but often confused) difference between macroscopic and microscopic boundary conditions.

By means of temperature-dependent surface-extended x-ray-absorption fine structure (SEXAFS) measurements we characterize two different chemisorption states of atomic O/Cu(100), one corresponding to a reconstructed surface and the other to an unreconstructed surface, respectively. The unreconstructed state is characterized by an O-Cu nearest-neighbor (NN) bond length of 1.88(3) Å, the O atoms being located 0.8(3) Å above the first Cu layer. Upon reconstruction, the O-Cu NN bond is shortened by 0.020(6) Å, the local geometry as determined by SEXAFS supporting a missing-row model for the reconstruction, with the adsorbate atoms being located 0.2(1) Å above the first Cu layer. Static and dynamical contributions to the local disorder (Debye-Waller factor) as well as anharmonic contributions could be separated. Evidence for the existence of surface strain upon occurrence of a surface reconstruction is found as the static part of the local disorder for the NN O-Cu bond increases by 7.8(4) ${10}^{\mathrm{-}3}$ Å for the reconstructed state. The static part of the anharmonic contributions to the disorder varies as well upon reconstruction. Through the systematic comparison of the effective pair potentials, obtained using the dynamic part of the disorder, of the two O/Cu chemisorption states with the ones of other reconstructed (N/Ni) and unreconstructed (O/Ni) surface systems, we find evidence that the bond is predominantly ionic for the unreconstructed case and more covalent for the reconstructed case. For the case of the ionic systems the charge transfer to the adatom is found to be 0.92(3) electrons based on a Madelung form of the pair potential, in agreement with theoretical calculations.

The structural and dynamical properties of molecular oxygen in submonolayer coverage adsorbed on Cu(100) were investigated by means of oxygen K-edge photoabsorption. The adsorption geometry of the ${\mathrm{O}}_2$ molecules, as well as the disorder and anharmonic contributions to the pair distribution function for the first-nearest-neighbor O-Cu shell were quantitatively determined for well-defined sequential molecular states of physisorption and chemisorption. The ${\mathrm{O}}_2$ molecules are found to adsorb in a tilted configuration. Furthermore, the O-Cu interatomic pair potentials are estimated. It is found that the nearest-neighbor O-Cu bond varies only slightly as the strength of chemisorption increases, in opposition to a strong O-O bond length and other O-Cu pair-potential parameter variations.

The capability of atomic-force microscopy (AFM) to localize both individual adsorbates and aggregates of adsorbed molecules was demonstrated a few years ago. More recently submonolayers of fullerene molecules deposited on a gold substrate have been imaged using such devices. In this paper, simulations of the atomic force between a thin probe tip and a set of adsorbed molecules is presented. The long-range part of the interaction is determined from a whole self-consistent procedure in which many-body effects are accounted for at all orders. In this description the probe tip interacts with the molecules and the surface through many-body dispersion forces. Short-range interactions are then included by using an atom-atom semiempirical pairwise potential. Simulations of AFM images of ${\mathrm{C}}_{60}$ adsorbed molecules are presented in two different modes of imaging: the constant-tip-height mode and the constant-force mode.

We have investigated the effect of photoinduced electron transfer on the photoconductivity (PC) of conducting polymer-${\mathrm{C}}_{60}$ films by comparing the photoconductivity (carrier generation and carrier transport) of the conducting polymer sensitized with ${\mathrm{C}}_{60}$ with that of the conducting polymer alone. We present time-resolved transient PC results, subnanosecond to 0.5 μs, obtained from poly[2-methoxy,5-(2’-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly(3-octylthiophene) (P3OT), and from conducting polymer films sensitized with several concentrations of ${\mathrm{C}}_{60}$. Both the magnitude and the lifetime of the transient PC increase substantially on increasing the concentration of ${\mathrm{C}}_{60}$. The results imply that quantum efficiency for photogeneration of charge carriers is increased by photoinduced electron transfer and that early time recombination is inhibited by the spatial separation of the electron and hole on the ${\mathrm{C}}_{60}$ and the conducting polymer, respectively. The greater enhancement of the transient PC in MEH-PPV (nearly two orders of magnitude with the addition of a few percent of ${\mathrm{C}}_{60}$) than in P3OT (nearly one order of magnitude with the addition of a comparable amount of ${\mathrm{C}}_{60}$) suggests a higher probability of early time recombination in pristine MEH-PPV. As a result of the enhancement in the photogeneration efficiency and the carrier lifetime, the spectral response of the steady-state PC in both MEH-PPV/${\mathrm{C}}_{60}$ and P3OT/${\mathrm{C}}_{60}$ films is significantly enhanced throughout the entire spectral range from the near infrared to the ultraviolet.

The Hubbard model with unit concentration of electrons per one site in the external magnetic field H is considered. In the H→∞ limit the ground state is ferromagnetic and dielectric. A decrease of H results in the phase transition to a new ground state. The electric properties of this state depend on (1) the lattice dimensionality and (2) the U/t ratio (U,t are the standard parameters of the Hubbard model). The linear [one-dimensional (1D)], square (2D), and simple cubic (3D) lattices are considered. In the 1D and 2D cases the new ground state is always dielectric. In the 3D case this state is dielectric only at U/(12t)>${\mathit{W}}^{\mathrm{-}1}$, where W=1.516. . . is the Watson integral. At U/(12t)<${\mathit{W}}^{\mathrm{-}1}$ it becomes a conducting one. The line of phase transition with respect to the field is constructed in the H-U coordinates. It is found that the transition to the dielectric state results in the transformation of the ferromagnetic order to the spin-flip phase. The transition to the conducting state results only in a decrease of the average local spin.

The ground and the first two excited states of an extrinsic surface optical polaron bound to a Coulomb impurity near the surface are studied by using a variational method for all ranges of the coupling parameters. It is shown that for large electron-impurity coupling the phonon-induced Lamb shift is quite significant and is opposite in sign as compared to the corresponding shift in the bulk material.

The electronic eigenenergies and the magnetization of multiple quantum-well structures under high magnetic fields have been calculated numerically. The magnetization shows a characteristic oscillating behavior as a function of the chemical potential with much enlarged magnitude compared to the single-well case, suggesting that the present system can be a suitable candidate for the experimental verification of current theories on the magnetization of quantum structures. The oscillating pattern consists of discontinuous steplike jumps under tilted magnetic fields while, under the magnetic field parallel to the layers, it reveals smooth envelope oscillations and, superimposed upon these, several distinct peaks. These arise as a consequence of hybrid magnetoelectric quantization of electron energies arising from the coupling of magnetic confining effects and band-structure effects.

A theoretical investigation has been made of the collective (bulk and surface) excitations in a semi-infinite n-i-p-i superlattice system in the presence of a magnetic field oriented perpendicular to the interfaces and to the direction of propagation. The thicknesses of the constituent layers is assumed to be sufficiently large so that the quantum-well effects can be ignored. The material layers are characterized by frequency- and magnetic-field-dependent dielectric functions. The magnetoplasma modes are defined by electromagnetic fields that are localized at and decay exponentially away from the interfaces and from the end of the superstructure. The general dispersion relations are derived using a fully retarded theory in the framework of a transfer-matrix method. Numerical examples are presented for several illustrative cases.

Experimental observation of the ground-state biexciton transition in ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{-}\mathit{x}}$ quantum dots in glass is presented in a three-beam experiment, involving a probe, a pump, and a saturating laser pulse. The observed quantum-dot biexciton ground state has a strongly enhanced binding energy compared to the bulk as theoretically predicted. The biexciton binding energy is measured as a function of quantum-dot size and the results are compared with calculations.

Step motion on surfaces can now be measured quantitatively. We present a formalism for analyzing equilibrium step fluctuations and apply it to real-time reflection electron microscope observations of step motion on Si(111). The time correlation functions of the step positions and of their Fourier components are compared with predictions from Langevin equations for two extreme mechanisms for step motion: edge diffusion and terrace exchange. At 900 °C, the dominant mechanism is terrace exchange with a time constant ${\mathrm{\tau }}_{\mathit{a}}$ of ∼1 μs. The significance of ${\mathrm{\tau }}_{\mathit{a}}$ for atomic mechanisms of surface mass transport is discussed.

Photoelectron and near-edge spectroscopies are used to characterize a ${\mathrm{C}}_{60}$ film on a Ta(110) surface. Like similar studies of ${\mathrm{C}}_{60}$ on a variety of metal surfaces, the valence band, C 1s core level, and photoelectron-derived work-function measurements show that ${\mathrm{C}}_{60}$ molecular states are aligned with the Fermi level and the charge is transferred from the metal to the fullerene. The photoelectron and near-edge spectra of the ${\mathrm{C}}_{60}$ peaks closest to the Fermi level show significant changes in intensity identifying those π-like states as interacting with the substrate. A careful examination of the C 1s and Ta 4f core levels suggests that some fraction of the fullerene overlayer decomposes on the clean metal surface leading to or resulting in, a thin layer of carbon between the assembled film and the metal substrate.

We investigate the equilibrium geometry of very large carbon clusters with special emphasis on nested multishell fullerene derivatives of spherical, cylindrical, and conic shape. For cluster sizes above 20 atoms, we find spherical shapes to be most stable. For very large sizes, a transition from single-shell fullerenes to multishell structures, locally similar to graphite, is energetically favored by the weakly attractive interaction between the shells.

In a recent paper [Phys. Rev. B 46, 7632 (1992)], Peschel and Lederer consider a nonlinear planar resonator filled by a negative Kerr medium which is local but not instantaneous. In this Comment, we show that the loops reported by Peschel and Lederer, for large oblique incidence, do not correspond to bistable cycles. Indeed the noninstantaneous response of the nonlinear material and the dynamics of the fields do not lead to the feedback mechanism required for optical bistability.

An analysis [Phys. Rev. B 44, 3277 (1991)] linking surface resistivity, surface reflectance, adsorbate vibrational damping, and infrared antiabsorption resonances to the scattering of electrons by adsorbates overlooks a crucial distinction between elastic and inelastic scattering. When the proper distinction is made, the relation between resistivity and reflectance is preserved, but the connection to damping and antiabsorption resonances must be significantly modified.

The criticism raised by Tobin is refuted.

We report on the observation and numerical calculations of femtosecond gain dynamics in an optically excited, CdSe epitaxial sample at low temperature. Spectral hole burning is clearly observed around the pump as it is tuned through the gain region. The pump-probe experiments agree well with our theory that involves numerical evaluation of the semiconductor Bloch equations in the presence of carrier-carrier and carrier–LO-phonon scattering.

Nonlinear magneto-optical properties of Wannier excitons in GaAs are studied at 4 K using four-photon nearly degenerate parametric coupling. The results show a large enhancement in the nonlinear optical response related to a field-induced decrease in exciton mobility due to magnetic freezeout. High-resolution cw nonlinear spectroscopy is used to measure the relative strength of inter-Landau-level coupling and shows that the nonlinearity is dominated by Coulomb effects.

Employing millimeter-wave spectroscopy, we have studied the magnetoplasma excitations of a two-dimensional electron gas confined in a ring geometry. The observed spectrum decomposes into several high- and two low-frequency branches. For large magnetic fields the latter are identified as edge magnetoplasmons localized at the inner and outer boundary of the rings. With decreasing magnetic field the inner edge mode exhibits a crossover from antidotlike to dotlike behavior. The resonances can also be analyzed in terms of plasmons in a strip with periodic boundary conditions.

High-resolution transmission electron microscopy coupled with a STEM nanoprobe reveals that the Au/Si (100) interface deposited at ∼80 °C is not abrupt and contains a disordered Si-rich Au-Si alloy decorated with pure Au nanocrystallites. The presence of oxygen at the Au surface enhances the outdiffusion of Si atoms through the Au overlayer, resulting in the phase transition from a disordered interface to an ordered metastable ${\mathrm{Au}}_4$Si phase. Instead, the Au/Si (100) capped with a Ge layer shows that the amorphous structure remains relatively stable. These results allow explanations of some unresolved experimental enigmas.