We derive numerical tight-binding hopping parameters to describe conduction-band dispersion in arbitrary orientational phases of ${\mathit{A}}_3$${\mathrm{C}}_{60}$. The parameters are obtained by direct Fourier inversion of the spectra from self-consistent electronic-structure calculations for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ using the local-density approximation, including the effects of orientational dependence. Using these parameters, we reconsider several earlier investigations of the orientational ordering in ${\mathit{A}}_3$${\mathrm{C}}_{60}$; some of the earlier results are substantiated, while others are slightly modified.

The dielectric functions of ${\mathrm{CeSn}}_3$ and ${\mathrm{LaSn}}_3$ were remeasured to correct errors in previous measurements. The partial-sum rule for the optical conductivity shows that ${\mathrm{CeSn}}_3$ has about 0.54 electrons per formula unit more than ${\mathrm{LaSn}}_3$, contributing to the conductivity in the 1.5–5.4-eV region.

The electron and positron chemical potentials and their volume derivatives are calculated in Al using the first-principles pseudopotential technique and the frozen-core approximation method, respectively. The positron deformation potential of -8.03 eV obtained by this method agrees very well with that found in other first-principles calculations. The variation of the positronium work function with temperature is calculated by including the thermal lattice vibrations into the electron and positron potentials. It is observed that the temperature dependence of the positronium work function does not arise entirely from the volume expansion of the lattice as has been suggested in the past. Proper inclusion of the nonzero-temperature derivative of the positronium work function at constant volume leads to good agreement between the positron deformation potential as obtained from positronium work function data and present theoretical values.

Measurements of the two-photon absorption coefficient in semiconductor-doped glasses were performed. The imaginary part of the third-order optical susceptibility in the semiconductor particles is derived by combining nonlinear-optical-transmission data with small-angle neutron-scattering (SANS) results, which give the volume fraction of the nanocrystals. We find that the absorptive nonlinearity of the nanocrystals (with radii, also measured by SANS, in the range 2–11 nm) is close to that of the bulk semiconductor, in contrast to recent predictions and data.

We report the observation of second-harmonic generation (SHG) in reflection from a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As asymmetric quantum-well (AQW) structure and a GaAs(001) face. SHG intensities are measured as a function of sample azimuthal angle for several polarization configurations using a pulsed 775-nm fundamental beam. Isotropic contributions to the p-polarized SHG intensity are found for both samples, which provides clear evidence of SHG arising from both the AQW structure and the GaAs(001)/oxide buried interface. The experimental results are consistent with a strong SHG contribution from the AQW structure with magnitude close to that of the bulk contribution; this interpretation is supported by previous theoretical predictions. Given the magnitude of the second-harmonic susceptibility for an AQW, strong SHG enhancement for visible and/or near-infrared light by full implementation of quasi-phase-matching is feasible.

Amorphous-silicon double Schottky switching diodes, with one contact of vanadium, can show discrete steps in the room-temperature current-voltage characteristics in the ON state at quantized resistance values. Additional steps at half-integer values appear in a magnetic field. We have found that a threshold field is required and that the field must be at an angle to the conducting filament. The step separation does not appear to change with field. A small-scale statistical analysis shows that the occurrence of steps at half-integer values appears to be real. Mechanisms for this are suggested.

Quantized conductance measurements on InAs/AlSb split-gate ballistic constrictions with a varying perpendicular magnetic field are reported. The magnetic depopulation of the one-dimensional subbands is used to determine the energy spacing of the subbands in the constriction. Assuming a parabolic lateral-confining potential, a theoretical fit to our data reveals subband spacings as large as 10 meV. For fields greater than ∼2 T, conductance plateaus emerge at odd integer multiples of ${\mathit{e}}^2$/h, indicating the lifting of the spin degeneracy. The magnetic-field dependence of the G=3${\mathit{e}}^2$/h conductance step width is used to obtain a rough estimate of the effective g factor in the constriction.

Theoretical calculations of free-exciton magnetic-polaron energies in semimagnetic quantum-well structures have been performed. The dependence of the polaron energy on well width, magnetic field, and temperature has been calculated and compared with the experimentally determined values. In this manner conclusions have been drawn with regard to the structural characteristics of the quantum-well systems.

The set of phenomenological model equations which we proposed to determine the scaling behavior of the low-density Rabi oscillations [Phys. Rev. B 48, 17 811 (1993)] is justified from the coherent part of the semiconductor Bloch equations in lowest order in the light-matter coupling. In contrast to similar short-time studies, it is shown that for resonant excitation of an excitonic state the Rabi oscillations can be described by a Ginzburg-Landau type equation which includes only the exciton-exciton interaction of the resonantly excited state but no state-filling terms or contributions from detuned excitonic or continuum resonances. Explicit calculations are performed for the bulk two-band jellium model and for a one-dimensional two-band Hubbard model.

We calculate the I-V characteristics of a tunnel junction containing impurities in the barrier. We consider the indirect resonant tunneling involving the impurities. The Coulomb repulsion energy ${\mathit{E}}_{\mathit{c}}$ between two electrons with opposite spins simultaneously residing on the impurity is introduced by an Anderson Hamiltonian. At low temperatures T≪${\mathit{E}}_{\mathit{c}}$ the I-V characteristic is linear in V both for V<${\mathit{E}}_{\mathit{c}}$ and for V>${\mathit{E}}_{\mathit{c}}$ and changes slope at V=${\mathit{E}}_{\mathit{c}}$. This behavior reflects the energy spectrum of the impurity electrons—the finite value of the charging energy ${\mathit{E}}_{\mathit{c}}$. At T∼${\mathit{E}}_{\mathit{c}}$ the junction reveals an Ohmic-like behavior as a result of the smearing out of the charging effects by the thermal fluctuations.

The behavior of emitted LA phonons when an electric field is applield to δ-doped 〈001〉 GaAs is investigated by the time-of-flight method. The characteristics show peculiar features due to specific interaction processes involved, i.e., direct emission by heated carriers as well as resonance absorption of nonequilibrium LA phonons besides decay and conversion processes of high-energy phonons. The latter contributions are in accordance with Monte Carlo simulations.

Depressurization of Si from its metallic β-Sn modification results in a dense metastable form (Si-III), which is semimetallic and persists indefinitely at ambient pressure. We have investigated possible reconstructions of the Si-III (001) surface using ab initio pseudopotential calculations and find that a configuration comprised of adatoms and symmetric dimers has, to the best of our knowledge, a lower surface enrgy than any other previously reported surface of silicon.

The effects of the image potential due to dielectric mismatch on the impurity states in GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum wells are investigated using the variational method. The results show that the effects of the image potential on the impurity binding energies in GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum wells are important, especially when the width of quantum well becomes small. The comparison with the results in the GaAs quantum well with an infinitely high confining potential and the dependence of the effects of the image potential on the composition x are discussed.

We calculate the binding energies of shallow donors and acceptors in a spherical GaAs-${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum dot for both a finite barrier and an infinitely high barrier using the variational approach, including the spatial variation of dielectric screening. The results show that when the spatial variation of dielectric screening is considered, the impurity binding energies increase noticeably, especially when the radius of the quantum dot is small. The results also show that the effects of spatial variation of dielectric screening on acceptors are larger than those on donors. The dielectric mismatch in this structure is also discussed.

We have carried out numerical investigations of transmittance fluctuations in disordered chains in the presence of external electric fields. We have obtained an almost constant fluctuation in a length scale smaller than the localization length. However, the value of the fluctuation in the plateau region is dependent on the external electric field and the strength of the disorder. We have also studied the transmittance autocorrelation as a function of external electric field to probe the nonlinearity in the transmittance.

The ionization energy for a metallic cluster of radius R is I(R)=W+α${\mathit{e}}^2$/R+O(${\mathit{R}}^{\mathrm{-}2}$), where W is the work function. Two different classical values for α have been derived: 1/2 from the spherical-capacitor approach, and 3/8 from the image-potential approach. We present a ‘‘classical jellium’’ or Wigner-crystal model in which α=1/2 and W=9/10${\mathit{e}}^2$/${\mathit{r}}_{\mathit{s}}$. In the image-potential approach to the ‘‘perfect-conductor’’ model, we discover an additional work 1/8${\mathit{e}}^2$/R, resolving the contradiction in favor of α=1/2. The experimental deviation from α=1/2 is a quantum effect.

We report the results of a comprehensive study of the temperature dependences of the resistance of one-dimensional narrow gold films with respect to the effects of weak localization (WL) and electron-electron interaction (EEI) in a wide temperature range. The electron wave-function coherence for such samples is limited by electron-electron collisions with small energy transfer (the Nyquist phase-breaking mechanism). It is shown that the temperature dependence of the WL contribution to the resistance, obtained for such a case, is in excellent agreement with the theory by Altshuler, Aronov, and Khmelnitskii. The experimental dependences ΔR(T) for one-dimensional samples are described quantitatively by the sum of the WL contribution, the contribution of the singlet part of the diffusion channel of EEI, and the contribution of the quantum interference between electron-phonon and electron-impurity scattering.

The multiplet structure induced by the Coulomb interactions in ${\mathrm{C}}_{60}$${\mathrm{}}^{\mathit{n}\mathrm{-}}$ (n=1–12) is analyzed. The lowest partially occupied molecular orbital gives rise to a set of levels that fill a relatively wide band (≊2–3 eV). A large (∼1 eV) intramolecular effective repulsion is found. The anions are also shown to be highly polarizable. The optical-absorption and photoemission spectra are calculated. The probability of exciting the molecule upon the addition or removal of one electron is shown to be significant, and this effect may give rise to the insulating behavior of ${\mathrm{K}}_4$${\mathrm{C}}_{60}$. The role of intermolecular interactions in suppressing the net repulsion within the ${\mathrm{C}}_{60}$ molecules is discussed.

We present a method to compute high-order derivatives of the total energy of a periodic solid with respect to a uniform electric field. We apply the 2n+1 theorem to a recently introduced total energy functional which uses a Wannier representation for the electronic orbitals and we find an expression for the static nonlinear susceptibility which is much simpler than the one obtained by standard perturbative expansions. We show that the zero-field expression of the nonlinear susceptibility can be rewritten in a Bloch representation. We test numerically the validity of our approach with a 1D model Hamiltonian.

A theoretical study of single-electron capacitance spectroscopy in quantum dots is presented. Exact diagonalizations and the unrestricted Hartree-Fock approximation have been used to shed light over some of the unresolved aspects. The addition spectra of up to 15 electrons is obtained and compared with experiment. We show evidence for understanding the decrease of the single-electron tunneling rates in terms of the behavior of the ω→0 spectral weight function.