We report the results of neutron-diffraction and low-energy neutron-inelastic-scattering experiments on high-purity solid ${\mathrm{C}}_{70}$ between 10 and 640 K. Thermal hysteresis effects are found to accompany structural changes both on cooling and on heating. The observed diffuse scattering intensity does not change with temperature. At 10 K broad librational peaks are observed at 1.82(16) meV [full width at half maximum=1.8(5) meV]. The peaks soften and broaden further with increasing temperature. At and above room temperature, they collapse into a single quasielastic line. At 300 K, the diffusive reorientational motion appears to be somewhat anisotropic, becoming less so with increasing temperature. An isotropic rotational diffusion model, in which the motions of adjacent molecules are uncorrelated, describes well the results at 525 K. The temperature dependence of the rotational diffusion constants is consistent with a thermally activated process having an activation energy of 32(7) meV.

The open-system boundary conditions for the one-dimensional Schrödinger equation are derived by dividing the unbounded domain into a finite system and two semi-infinite reservoirs. The resulting boundary conditions on the system are non-Markovian, as they contain a convolution over the history of the system. Thus, time-irreversibility arises in a pure-state problem. The propagator which appears in the boundary condition is derived for a simple discrete model. The correctness of the boundary conditions is verified and the usefulness of the discrete model is demonstrated by a numerical calculation of the time-evolution of a wave packet.

The shapes of the luminescent lines corresponding to transitions in the localized iron impurities of the system InP:${\mathrm{Fe}}^{2+}$ are studied and adjusted. The starting point of the analysis is the set of wave functions obtained after considering the vibronic coupling. Six different experimental spectra available in the literature are considered. Gaussian shapes are found to explain better transitions to zero-phonon lines, while Lorentzian shapes are better for explaining lines that involve more vibronic mixing. The vibronic coupling by means of a linear Jahn-Teller Hamiltonian fully explains the general structure of the spectra.

We present a high-resolution electron-energy-loss-spectroscopy study of the electronic and dielectric properties of the (1×1) and (1×2) Bi/GaSb(110) interfaces, in the energy region of the semiconductor bulk gap. Among the semimetal/III-V(110) interfaces, we found this as the unique ordered system showing a semimetallic character when one monolayer of semimetal is deposited on the (110) surface, at room temperature. Electronic loss structures induced by transitions among localized states have been identified for the (1×1) interface, the most prominent lying at about 1 eV. The (1×2) superstructure, produced through an appropriate thermal treatment, is still semimetallic, while the electronic loss structures shift to lower energies and present a clear dependence on the symmetry direction of the surface Brillouin zone.

Room-temperature luminescence observed in porous silicon has been attributed either to direct radiative recombination in nanometer-size silicon in the porous silicon network, or to chemical complexes of silicon, hydrogen, and/or oxygen. In this paper, we attempt to correlate results of infrared and photoluminescence spectroscopy of freshly prepared and chemically treated porous silicon layers such that hydrogen or oxygen related species can be selectively enhanced on the surface of the porous silicon network. Our results indicate that it is difficult to establish a direct correlation between the visible luminescence in porous silicon and any particular chemical specie(s) such as silicon hydride(s), oxide, or siloxene.

At a heterojunction the constituents of which share neither common anion nor common cation, the interface can be considered as a very thin interlayer of a third material. Depending on the band alignments this built-in interface layer may be a thin quantum well for both electrons and holes. In this case it can trap carriers at the interface. We show why this effect is mainly significant at type-II heterojunctions. The effect is thus twofold: modification of the apparent band offset (related to the effective band gap), and enhancement of the overlap integrals between conduction and valence states, which leads to high transition probabilities. Here we first review a lot of heterojunctions likely to present this phenomenon. Then we show that this can be observed for ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{\mathit{y}\mathrm{-}\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As/InP systems, and we detail the case of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/InP. We provide a semiempirical tight-binding calculation which includes strain effects at the interface. The resulting local density of states clearly demonstrates whether electrons or holes are trapped at the heterojunction.

The influence of the conduction-band nonparabolicity on the collective spin and charge-density excitation spectra in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells is discussed theoretically in the framework of the local-density approximation. Emphasis is placed on estimating the importance of band population effects at finite temperature. A line-shape calculation shows that the nonparabolicity shifts resonance energies and leads to the additional broadening of the excitation spectra. Qualitatively different behavior of the linewidth (as a function of the electron concentration) is found for the charge- and spin-density excitations.

Using our recently developed ab initio schemes for generating optimally smooth pseudopotentials, we have carried out the first fully relaxed structural study of a silicon/transition-metal-silicide interface, ${\mathrm{NiSi}}_2$/Si(111), by allowing the atoms to be fully mobile in the direction perpendicular to the interface. We found that the contraction of the silicide triple layer immediately adjacent to the interface is responsible for most of the interface relaxation. This is in disagreement with the assumption that the relaxation occurs primarily at the interface Si-Si bond. Such an assumption has been used to interpret the experimental results as well as being used in most of the electronic-structure calculations and may be responsible for the disagreement between the calculated and measured results for the Schottky barrier heights. We also confirm that the type-A interface is more energetically favored than type B, while the type-B interface relaxes more than type A.

Two variational states, the displaced state and the displaced-squeezed state, are compared for a two-state system with super-Ohmic dissipation, and their dependence on the form of the coupling strength ${\mathit{g}}_{\mathit{l}}$∼${\mathrm{\omega }}_{\mathit{l}}^{\lambda }$ (${\mathrm{\omega }}_{\mathit{l}}$ is the phonon frequency) is analyzed. We find that, in the strong-coupling region, the displaced-squeezed state is more stable and the ground-state properties depend not only on the spectral density but also on the index λ. Moreover, we find that the stable area of the displaced-squeezed state increases as λ decreases.

We study the polarization of the long-lived component of the band-edge photoluminescence in undoped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells at low temperatures. A high degree of both linear and circular polarizations (${\mathit{P}}_{\mathrm{lin}}$ and ${\mathit{P}}_{\mathrm{cir}}$) persists over more than 100 nsec and is attributed to separately localized e-h pairs. We show that the long-lived ${\mathit{P}}_{\mathrm{lin}}$ is not due to optical alignment but to resonant excitation and subsequent emission of e-h pairs oriented parallel to the light polarization. The spectral dependence of ${\mathit{P}}_{\mathrm{lin}}$ and ${\mathit{P}}_{\mathrm{cir}}$ on excitation energy indicates that during energy relaxation the e-h pairs preserve their spin orientation but their linear polarization is destroyed.

The electron correlation effect in a double-quantum-well structure is investigated. The local-field correction and static structural factors are calculated for these structures. It is found that the static structural factors show an anomalous behavior at small q due to the coupled quantum-well-plasma excitation and the interlayer structural factor is negative at large q. We also found that the long-wavelength acoustic plasma mode is further softened due to short-range electron correlations. The minimum critical well separation for this mode to exist increases by an amount ∼1/${\mathit{k}}_{\mathit{f}1}$, where ${\mathit{k}}_{\mathit{f}1}$ is the Fermi wave vector of the well with higher electron concentration.

The dramatic enhancement of the silicon room-temperature oxidation rate by a Na overlayer has been attributed to the spectator catalytic action of the alkali metal. In contrast, we show evidence that Na participates in the oxidation process binding an oxygen atom in a substoichiometric intermediate oxidation state. This causes preferential saturation of the semiconductor dangling bonds on the surface and a subsequent promotion of the oxide growth. We have, in fact, detected the Na 2p photoemission chemical shifts when bonds with an oxygen atom are established in a hybrid configuration.

We have tested Haldane’s ‘‘fractional-Pauli-principle’’ description of excitations around the ν=1/3 state in the fractional quantum Hall effect, using exact results for small systems of electrons. We find that Haldane’s prediction β=±1/m for quasiholes and quasiparticles, respectively, describes our results well with the modification ${\mathrm{\beta }}_{\mathit{q}\mathit{p}}$=2-1/3 rather than -1/3. We also find that this approach enables us to better understand the energetics of the ‘‘daughter’’ states; in particular, we find good evidence, in terms of the effective interaction between quasiparticles, that the states ν=4/11 and 4/13 should not be stable.

We have shown that the one-dimensional interacting electron gas carrying dc current I should exhibit single-electron singularities at frequency ${\mathrm{\omega }}_{\mathit{s}}$=2πI/e for arbitrary strength of electron-electron interaction. These singularities arise due to Coulomb or Fermi correlations between electrons in the gas. Depending on the strength of the interaction the frequency ${\mathrm{\omega }}_{\mathit{s}}$ can be interpreted either as a frequency of the single-electron oscillations or the cutoff frequency of the shot noise associated with the current I.

The effect of electron-phonon interaction on the transport properties of a two-dimensional electron gas in a strong transverse magnetic field is investigated. Corrections to nondiagonal (Hall) components of electrical and heat conductivities and thermopower tensors are obtained in the context of the diagrammatic technique and are shown to be essential in a wide range of temperatures and magnetic fields. Experimental conditions for the observation of electron-phonon interaction effects in the electronic transport in the quantum-Hall-effect regime are discussed.

We have studied the effect of Joule heating on the dynamics of a single bistable defect in a submicron Bi wire below 1 K. We interpret the ratio of the two defect transition rates as a local thermometer, via the detailed balance relation. As the drive current increases, the defect temperature approaches a power-law dependence with drive, independent of the nominal lattice temperature. The data are consistent with a simple model of electron heating, and strong thermal coupling between the defect and the electron bath below 1 K. A second thermometer, based on the amplitude of the defect-induced resistance fluctuations, does not follow the simple heating model.

Experimental work by Shea and Compton suggests that ${\mathrm{Ag}}^{+}$ ions emitted from a roughened Ag surface irradiated by a nanosecond or picosecond laser beam may absorb the full energy of the Ag surface plasmon (SP). We have modeled this process as an inverse bremsstrahlung-type absorption of the SP quantum by an ${\mathrm{Ag}}^{+}$ ion which undergoes a collision with the surface. We estimate the absorption probability and find it to be consistent with the Shea-Compton results.

We show that the relaxation time for dimer formation that is used in the nonequilibrium theory of crystal growth on vicinal surfaces by molecular-beam epitaxy is not ${\mathrm{\tau }}_0$, the characteristic diffusion time, but rather (${\mathrm{\tau }}_0$τ${)}^{1/2}$ where τ is the flux arrival time. This conclusion follows from a consideration of the equilibrium state for which independent results for the adatom and dimer densities are known from statistical mechanics.

We have measured the momentum-dependent inelastic-electron-scattering spectra of ${\mathrm{C}}_{60}$ solid thin films. These data allow the oscillator strength of dipole-forbidden transitions and the dispersion coefficient of the volume plasmon to be evaluated. In addition, we evaluate the complex molecular polarizability from the dielectric function using a point-dipole model for the local-field effects. The results are consistent with the presence and calculated energies of both dipolar and quadrupolar collective molecular excitations in molecular ${\mathrm{C}}_{60}$.

In the subject publication, Mathiot casts doubt upon our Monte Carlo simulations of Au diffusion in Si with the kick-out mechanism. We argue that our simulations were correct. In addition, we observe that Mathiot’s treatment of diffusion lacks information on boundary conditions which is necessary for accurate modeling. Mathiot’s model also ignores the possible existence of vacancy clusters. Mathiot concluded that diffusion of Au in Si is dominated by the kick-out mechanism. We argue that this conclusion is unjustified.