We report on a quantitative determination of the band-gap renormalization in type-II GaAs/AlAs superlattices under high excitation. The time-resolved luminescence after picosecond high excitation is analyzed using a detailed luminescence line-shape fit. We find the renormalization of the type-I transition to be comparable to that in type-I superlattices, whereas the type-II transition reveals a negative-differential renormalization. This is attributed to the space-charge effects caused by the carrier separation in agreement with quantitative calculations.

A simple method for obtaining a good approximation of the ground state of a quantum disk is presented in the case where the electron radial confinement is substantial. Developed in the envelope-function formalism, the method is derived from the effective-index technique well known to dielectric wave-guide theory. Its validity is discussed by comparing its results with those obtained using a numerical relaxation technique.

The structure of the low-coverage (√3 × √3 )R±30° Pd phase, produced upon mild annealing of a Pd layer deposited at room temperature, has been examined by scanning tunneling microscopy (STM). The STM images have revealed a rather inhomogeneous surface that contains large (√3 × √3 )R±30° domains, mixed with features assigned to a transition state from a (√3 × √3 )R±30° to a (3 √3 ×3 √3 )R±30° structure, and areas of disordered Si phase. The structural units of both the (√3 × √3 )R±30° and (3 √3 ×3 √3 )R±30° structures are protrusions of ≃5.5 Å average diameter described as Pd trimers chemisorbed on a (1×1) Si(111) surface. A structural model for the possible surface atomic arrangements in the (√3 × √3 )R±30° and (3 √3 ×3 √3 )R±30° phases is suggested and discussed.

Molecular-dynamics simulations with a corrected-effective-medium potential have been applied to study the kinetics of intermixing in Au/Ag(110) heteroepitaxy. Significant intermixing of Au and Ag at the interface has been observed at room temperature. A variety of interesting kinetic mechanisms have been revealed. Our study demonstrates that the kinetics of intermixing have a complex dependence on the local geometry of the Ag substrate, Au-Au interactions, and a long-range coupling of adatom motions via adsorbate-induced relaxation of the surface atoms.

Enhanced second-order Raman modes of ${\mathrm{C}}_{60}$ molecules adsorbed on Ag and In surfaces are reported. This unusual observation for the metal surfaces is believed to result from the unique surface-enhancement mechanism of the Raman spectrum of adsorbed ${\mathrm{C}}_{60}$ and from the high relative intensity of the second-order modes previously observed for bulk ${\mathrm{C}}_{60}$ samples. Differences in line shapes and a modified resonant behavior, relative to results for ${\mathrm{C}}_{60}$ single crystals, are attributed to interactions between the adsorbed ${\mathrm{C}}_{60}$ and the metal surfaces, which have been shown in previous studies to include charge transfer to ${\mathrm{C}}_{60}$.

The bonding geometry of nitrogen adsorbed on Rh(110) has been determined by means of low-energy-electron-diffraction I-V analysis. The nitrogen atoms are found to be adsorbed in the long bridge site with rows of Rh atoms missing in the [001] direction, and with a metal-nitrogen bond length similar to that found on other metal surfaces. This gives an overall structure in which the Rh and the adsorbate form metal-nitrogen-metal chains. Ultraviolet photoemission shows that the valence-band emission induced by nitrogen consists of a single peak which disperses strongly to higher energy in one azimuth. This is consistent with a band due to nitrogen p states which overlap most strongly along the chains.

A detailed structure determination of a complex surface alloy formed by alkali-metal deposition on a metal substrate yields a type of alloy consisting of an ordered assembly of substructures. The (3×3) structure formed on Cu(001) by Li deposition is found to consist of small pyramids of four Cu atoms capped by single Li atoms, the pyramids being joined by pairs of Li atoms.

Certain adsorbates A on (100) surfaces of bcc metals M induce a large expression of the near-surface interlayer spacing of up to 23%, if compounds of stoichiometry AM and with fcc-type NaCl structure exist. The adlayer of A and the expanded first two layers of M are an incipient surface compound of type AM, thus necessitating a substantial lattice expansion. We illustrate this behavior with new data for two surfaces phases. an incomplete CrN and a completed CrC on an Fe-15% Cr(100) crystal.

The molecules ${\mathrm{C}}_{59}$X (X=B,N) are investigated by the extended Su-Schrieffer-Heeger model. The obtained results for the energy levels and molecular structures are compared with those from a self-consistent-field molecular-orbital (SCF-MO) method. We have found that by choosing suitable empirical parameters, most of the features included in the results of the SCF-MO method can be well reproduced. Other structural and electronic properties such as the excess electron density and the amplitude of the midgap states have also been studied.

The unique property of the open 4f energy shell in the lanthanide metals is used to show that the initial-state energy shift gives an insufficient description of surface core-level shifts. Instead a treatment, which fully includes the final-state screening, account for the experimentally observed surface shifts of the occupied as well as the unoccupied 4f states. The surface energy shift of the initial state corresponds approximately to the average of the shifts for the occupied and unoccupied 4f levels.

Simulations of the Van der Drift model for polycrystalline growth are described. These simulations have been carried out for conic and cubic crystals and the exponent p describing the growth of the linear monocrystalline domain size has been determined. For both crystal types p=0.4 is found, in accordance with previously obtained mean-field results. Moreover, the angular distribution of surviving crystals as a function of time matches the mean-field result very well.

We present a realistic theoretical study of the surface-phonon spectra of diamond (100) surfaces. Our results reproduce the major features of the vibrational spectra of clean and hydrogenated diamond (100)-(2×1) surfaces obtained from experiments. The reconstruction of the diamond (100)-(2×1) surface yields a rich spectrum of surface modes and we identify surface features involving excitations of the surface dimer. We detail connections between atomistic surface modes and peaks observed in surface probes measuring vibrational excitations.

The exactly soluble 1/r Hubbard chain is studied at half filling. Two classes of variational wave functions are introduced, one representing the metallic phase, the other the insulating phase. An iterative scheme is found to converge rapidly towards the exact results both for the ground-state energy and the double occupancy. The insulating and metallic phases are connected by a duality relation, which allows one to locate exactly the critical value of the Hubbard parameter U.

We show that the zero-temperature susceptibility of the one-dimensional, dimerized Hubbard model at quarter-filling can be accurately determined on the basis of exact diagonalization of small clusters. The best procedure is to perform a finite-size scaling of the spin velocity ${\mathit{u}}_{\mathrm{\sigma }}$, and to calculate the susceptibility from the Luttinger-liquid relation χ=2/π${\mathit{u}}_{\mathrm{\sigma }}$. We show that these results are reliable by comparing them with the analytical results that can be obtained in the weak- and strong-coupling limits. We have also used quantum Monte Carlo simulations to calculate the temperature dependence of the susceptibility for parameters that should be relevant to the Bechgaard salts. This shows that, used together, these numerical techniques are able to give precise estimates of the low-temperature susceptibility of realistic one-dimensional models of correlated electrons.

We report on the observation of strong electroluminescence and photoluminescence from a Si/Ge quantum structure involving a two-monolayer-thick Ge layer in a bulk Si matrix. It has been proposed that when a conduction-band electron is scattered at the Si-Ge interface, the normally forbidden no-phonon (NP) interband transition is allowed due to mixing of different bulk momenta. The luminescence spectra of our samples consist of a NP line and a transverse-optical (TO) phonon replica originating from the Ge layers. The TO-phonon energy coincides with a Si-Ge mode, confirming the importance of the interface.

The Fermi-edge singularity (FES) in modulation-doped GaAs quantum wells is a unique many-body feature that arises from strong Coulomb correlations within a dense sea of electrons. Using intense ultrashort excitation, we show experimentally and theoretically that the nonresonant nonlinearity (ac Stark shift) of the FES is dramatically different than that of the excitonic ac Stark shift of undoped-quantum-well excitons.

We consider how x-ray-edge singularity modification occurs in a class of nonequilibrium systems with more than one Fermi sea at different chemical potentials coupled to one other. The problem is solved by generalizing the Noziéres–De Dominicis solution for equilibrium systems to the present nonequilibrium situation. We find a much stronger orthogonality catastrophe effect associated with potential differences between different Fermi seas. Some consequences of the present result are considered.

We present numerical results for ballistic-electron quantum transport through weakly open integrable circle and chaotic stadium billiards. The geometry of the pair of conducting leads is chosen in accordance with recent experiments for semiconductor microstructures [Marcus et al., Phys. Rev. Lett. 69, 506 (1992)]. The conductance as a function of the Fermi wave number displays characteristic noisy fluctuations for both the integrable and the chaotic systems. We show that structures in the conductance autocorrelation function as a function of the Fermi wave number are related to short-length classical orbits. This correspondence permits incorporation of effects of phase decoherence due to incoherent scattering into the quantum calculation.

Assuming that the phase transition between the Wigner solid and the Laughlin liquid is first order, we compare ground-state energies to find features of the phase diagram at fixed ν. Rather than use the Coulomb interaction, we calculate the effective interaction in a square quantum well, and fit the results to a model interaction with length parameter λ roughly proportional to the width of the well. We find a transition to the Wigner-solid phase at high density in very wide wells, driven by the softening of the interaction at short distances, as well as the more well-known transition to the Wigner solid at low density, driven by Landau-level mixing.

Scanning-tunneling-microscopy (STM) images were taken from the surface of a Si layer grown at a rate of 0.6 nm/min on a Si(111)-7×7 substrate held at 250°C in order to study the surface morphology change in terms of the thin-film-growth process. Each STM image was obtained from the Si layer whose growth was terminated at an arbitrary value of the specular reflected high-energy electron-beam intensity (I). At the initial growth stage on the native Si(111)-7×7 substrate, the amplitude of I does not oscillate regularly, and three-dimensional (3D) islands nucleate and grow layer by layer. When a second bilayer has a mean thickness between 13/4 and 21/4 bilayers, the layer growth starts with the nucleation and growth of 2D islands, and is followed by regular oscillation in I. The transition from a 3D to a 2D growth mode can be deduced from the difference in the nucleation and growth processes of islands on the native Si(111)-7×7 substrate and the growing Si layer; this is because the latter surface is composed of small domains with metastable 2×1, 2×2, 5×5, 7×7, and 9×9 superlattices.