Raman spectra of ${\mathrm{C}}_{60}$ films on gold and silicon substrates have been taken over the frequency range 200–3200 ${\mathrm{cm}}^{\mathrm{-}1}$. Besides two ${\mathit{A}}_{\mathit{g}}$ and eight ${\mathit{H}}_{\mathit{g}}$ fundamental frequency modes, 16 weak peaks are observed. According to the selection rules of overtone and combination bands, most of them are attributed to the second-order Raman spectra of ${\mathrm{C}}_{60}$ molecules. A comparison of experimental results with theoretical predictions shows satisfactory agreement.

The low-temperature behavior of transport properties across an Anderson-type metal-insulator transition is considered. It is shown that previous discussions are flawed; in particular, we demonstrate that the thermopower S is not divergent as the mobility edge ${\mathit{E}}_{\mathit{c}}$ is approached from the metallic side. At ${\mathit{E}}_{\mathit{c}}$, S achieves a temperature-independent value, determined solely by the conductivity index ν. Furthermore, far from being universal in the metallic phase, the Lorentz number changes as the mobility edge is approached and, in addition, depends on ν. We also prove that, contrary to previous assertions, the thermal conductivity varies like ${\mathit{T}}^{\nu +1\mathrm{}}$ as T→0.

A method, using the Lanczos diagonalizatioin technique and random sampling, is introduced to evaluate finite-temperature static and dynamical quantities in small many-body quantum systems. As an example the method is applied to the calculation of the optical conductivity of a single charge-carrier hole in the system of strongly correlated electrons, as described by the t-J model.

Hydrogen passivates shallow dopants as well as the EL2 defect in n-type GaAs. The passivation of the EL2 defect is shown to be thermally less stable than the shallow dopant passivation. Thermal dissociation of the EL2-H complex under an applied reverse bias occurs more rapidly than under zero-bias conditions. It is shown that the reverse bias dissociation is characterized by two first-order processes, one considerably more rapid than the other. The rapid process has an activation energy of 1.33 eV and a dissociation attempt frequency of 1.0×${10}^{14}$ ${\mathrm{s}}^{\mathrm{-}1}$.

Photoluminescence (PL) has been measured in hydrogenated amorphous silicon (a-Si:H) at energies above the exciting light energy, ${\mathit{E}}_{\mathit{x}}$, for ${\mathit{E}}_{\mathit{x}}$=1.33 and 1.38 eV. The shape of the PL at energies above ${\mathit{E}}_{\mathit{x}}$ is essentially identical to the shape of the PL in the same spectral region when excited with ${\mathit{E}}_{\mathit{x}}$ above the optical gap (≥1.9 eV) and is also essentially independent of temperature even though the shape of the PL below ${\mathit{E}}_{\mathit{x}}$ exhibits a marked temperature dependence. Although the dependence of PL intensity above ${\mathit{E}}_{\mathit{x}}$ on excitation power is only slightly greater than unity, we attribute the excitation mechanism to a two-step process that proceeds through the manifold of silicon dangling-bond states even in high-quality a-Si:H.

Quantum dots in GaAs heterostructures display a complex inner structure in a large magnetic field. The distribution of electrons over different Landau levels results in the formation of alternating compressible and incompressible regions. We investigate theoretically the situation when only a few electrons are confined to the uppermost Landau level. We show that these extra electrons form a Wigner molecule. The magnetic field controls the number of electrons in the molecule, and this creates the possibility of manipulating few-electron objects. Moreover, in this way one can manipulate fractional charges. We discuss briefly the stability of the molecule in the presence of a fluctuating donor potential and the slow charge dynamics, which is intrinsic for the system under consideration.

Based upon the similarities in properties between carbon- and BN-based (BN=boron nitride) materials, we propose that BN-based nanotubes can be stable and study their electronic structure. A simple Slater-Koster tight-binding scheme has been applied. All the BN nanotubes are found to be semiconducting materials. The band gaps are larger than 2 eV for most tubes. Depending on the helicity, the calculated band gap can be direct at Γ or indirect. In general, the larger the diameter of the nanotube the larger the band gap, with a saturation value corresponding to the calculated local-density-approximation band gap of hexagonal BN. The higher ionicity of BN is important in explaining the electronic differences between these tubes and similar carbon nanotubes.

We consider the ‘‘fractional quantum Hall atom’’ in the vanishing Zeeman energy limit, and investigate the validity of Hund’s maximum-spin rule for interacting electrons in various Landau levels. While it is not valid for electrons in the lowest Landau level, it generally predicts the ground-state spin correctly provided it is applied to composite fermions. The composite fermion theory also reveals a ‘‘self-similar’’ structure in the filling factor range 4/3>ν> 2) / 3 .

In the hierarchical theory of the fractional quantum Hall effect, the low-energy behavior of a daughter state in the next level of the hierarchy is described by an interacting system of quasiparticles of the parent state. Taking the filled lowest Landau level as the parent state, we examine analytically the quantitative consequences of this approach for electrons interacting via a pseudopotential interaction. It is shown that the ground-state energy per particle in the daughter state at a filling factor 2/3 is exactly equal to that of a system of quasiholes in the parent state with half filling, precisely as predicted by the hierarchical approach. This is achieved with only up to two-particle interactions in the effective Hamiltonian for the quasiholes. Their single-particle energy and two-particle interaction are derived. The results are generalized to the other filling factors attainable from the filled Landau level.

We investigate experimentally hot-electron transport in GaAs/${\mathrm{Ga}}_{0.5}$${\mathrm{Al}}_{0.5}$As superlattice (SL) oscillators designed for high-frequency operation at room temperature. Two effects are considered: intra- miniband heating and intervalley scattering involving L and X well and barrier states. We show that both effects can be sensitively measured by using the hydrostatic-pressure dependence of the SL electrical conduction. Finally we demonstrate the feasibility of wide-miniband ultrafast GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As SL transport, fully devoid of intervalley transfer.

In this paper we show that (1) for graphene tubules an axial magnetic field can induce a semimetal-semiconductor transition periodically at zero bias. This is an Aharonov-Bohm-type effect and is generally true for thin cylindrical conductors; (2) the Landauer approach can be used to estimate the conductance of individual tubules measured by scanning tunneling probes as a function of applied bias, temperature, tubule geometric factors, and magnetic field. Numerical calculations are presented showing that the conductance modulation should be observable even at room temperature.

A system of two parallel constrictions in a two-dimensional hole gas has been studied. At zero magnetic-field conductance quantization is observed through the individual constrictions. At high magnetic fields Aharonov-Bohm oscillations result from resonant tunneling through states encircling the repulsive potential between the constrictions. The oscillations demonstrate directly phase-coherent transport of holes.

We investigate the lateral tunneling properties of electrons through a one-dimensional double-quantum-well system using the recursive Green’s-function technique. The conductance exhibits a number of interesting quantum-interference effects, including a strong resonance at the onset of conductance, a ‘‘beating’’ effect due to competing characteristic times for the system, and a finite conductance at large Fermi energies, where the densities of states for the individual wells is small. It is shown that the last property may be exploited to use this system as a quantum-interference transistor.

We demonstrate a direct surface structural tool with high resolution of ∼1 Å in all directions by direct Fourier transformation of measured Kikuchi patterns using a multiple-energy phase-summing method. In this method, with an integral over continuous energy spectra in each direction, both the forward- and backward-scattering oscillations are selected for Fourier transformation by varying the energy range and size of the grid. High-fidelity and artifact-free three-dimensional images of Ag atoms for (100) and (111) single-crystal surfaces are obtained.

We studied the dissociation of molecular oxygen adsorbed on Ag(110) by electron-energy-loss spectroscopy. We observe that the dissociation of the adsorbed molecules at 150 K is connected to increased surface disorder. The elastic reflectivity is recovered at 200 K independently of coverage when the added row reconstruction of the substrate sets in.

A partially occupied surface state at the Fermi level of a well-ordered La(0001) film grown on W(110) is observed by photoemission and inverse photoemission, which provide direct pictures of the occupied and unoccupied part, respectively. The occupied part of the surface state gives rise to an intense direct recombination line in the ${\mathrm{N}}_{4,5}$${\mathrm{O}}_{2,3}$${\mathrm{O}}_{2,3}$ Auger-electron spectrum. The surface state is found to be sensitive to ${\mathrm{O}}_2$ adsorption and disordering of the surface induced by Ar-ion bombardment. The present results support a view that such surface states occur generally on close-packed surfaces of the rare-earth metals.

The addition of suitable dopants to ${\mathrm{ZrO}}_2$ can induce dramatic phase stabilization, and this dopant-induced phase stabilization is the basis of transformation toughening of zirconia-based structural ceramics. We have determined the electronic structure of three phases of ${\mathrm{ZrO}}_2$, cubic, tetragonal, and monoclinic, using vacuum-ultraviolet and x-ray-photoemission spectroscopies, coupled with ab initio band-structure and optical property calculations using the orthogonalized linear-combination-of-atomic-orbitals method, in an attempt to understand the complex interaction of the stabilizing dopants and associated atomic defects with the crystal structures of ${\mathrm{ZrO}}_2$ and their phase transitions. The experimental samples were single or polycrystalline stabilized materials which contain atomic defects, while the calculations were performed for undoped idealized ${\mathrm{ZrO}}_2$ structures without atomic defects. Reasonable agreement is found between experiment and theory at this level. The primary difference among the three phases of ${\mathrm{ZrO}}_2$ is the hybridization or mixing of the Zr 4d (${\mathit{x}}^2$-${\mathit{y}}^2$ and ${\mathit{z}}^2$) and the Zr 4d (xy, yz, and zx) bands, which form the conduction bands as the symmetry decreases from cubic to monoclinic. This leads to a complex evolution of the O 2p to Zr 4d and the O 2s to Zr 4d interband transitions. In addition, in the real materials, the presence of yttrium stabilizer introduces additional Y 4p valence bands and Y 4d conduction bands. The effective coordination of zirconium by oxygen is reduced from eightfold to sevenfold by the presence of the stabilizing ions and defects and this leads to the introduction of an occupied Zr 4d valence band suggestive of the presence of ${\mathrm{Zr}}^{2+}$.

The thermoelastic effect (TEE) of perfectly grown ${\mathrm{Bi}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ crystals shows a clear periodic behavior as a function of the composition parameter x with a period of 5.5% for 0≤x≤0.19. Melting and subsequent rapid solidification of the samples causes this phenomenon to disappear, leading to the assumption of superlattice incorporation of the Sb constituents into the bismuth matrix in the first case. Furthermore, since the weakest bonds should dominate the TEE, they have to be correlated with the electronic energy extrema at the L points of the pseudocubic Brillouin zone. Insertion of Sb and Bi atoms in the ratio 1:2 into one of the trigonal layers with a trigonal lattice constant over three double layers leads to a 1/3 occupation of the superlattice for x=0.055 with equidistant hexagonal layers. The assumption of two different ${\mathit{sp}}^3$${\mathit{d}}^2$ and ${\mathit{p}}^3$${\mathit{d}}^3$ valence hybrids suggests a relation between the superlattice crystal axes and the main axes of the L-band constant-energy surfaces, which also applies to ${\mathrm{V}}_2$${\mathrm{VI}}_3$, IV-VI, IV-${\mathrm{VI}}_2$, and similar materials.

We study here the stability of soliton and polaron excitations in a single strand of trans-polyacetylene as a function of the strength of the electron-electron correlations as well as the doping level. We proceed by first solving exactly the continuum version of the Su, Schrieffer, and Heeger Hamiltonian for the single-particle states that arise when electrons are added to a single polymer chain. The role of on-site (U), nearest-neighbor (V), and bond repulsion (W) Coulomb interactions are then obtained from a first-order perturbative calculation with the exact single-particle states. By minimizing the total energy, we are able to determine the relative stability of polaron and soliton configurations. We show that, at a fixed doping level, a polaron lattice is favored over a soliton configuration provided that U and V exceed critical values. However, as the doping level is increased, we show that the critical values of U and V at which a soliton lattice converts to a polaron lattice increase significantly beyond experimentally accepted estimates. W is also shown to favor solitons. We estimate that at a doping level corresponding to the insulator-metal transition (5%), U=4 eV and V=0.4 eV, a soliton lattice is 0.6 eV lower in energy than the corresponding polaron lattice. If we argue that the insulator-metal transition in polyacetylene results from the onset of a polaron lattice at 5%, then our work places restrictions on the magnitude of such effects as interchain coupling which have been proposed in the literature as a principal player in the metal transition in polyacetylene.

We present ab initio nonrelativistic and relativistic calculations for fcc Co alloys with 3d and 4d elements. The calculations are based on the local-density approximation of density-functional theory and employ the Korringa-Kohn-Rostoker Green’s-function method. Results are given for the density of states, the local moments and hyperfine fields of the impurities and their nearest neighbors. The calculated magnetic moments and hyperfine fields are in reasonable agreement with the experimental data.