An exact numerical calculation of ensemble-averaged length-scale-dependent conductance for the one-dimensional Anderson model is shown to support an earlier conjecture for a conductance minimum. The numerical results can be understood in terms of the Thouless expression for the conductance and the Wigner level-spacing statistics.

We have studied scattering from locally periodic optical potentials. The band-structure characteristics of periodic potentials along with resonances are observed. These features are solely due to the interference from absorptive potentials. Contrary to assertions in the literature, we show that the absorption along with the enhanced reflection induces coherence in quantum systems. We also discuss some conceptual aspects of absorptive potentials.

The absolute magnitude of the narrow-band noise is estimated within the framework of the Fukuyama-Lee-Rice model. The noise is predicted to vary with sample volume scrV as ${\mathit{scrV}}^{\mathrm{-}1/2}$, as has been previously noted, and with impurity concentration ${\mathit{n}}_{\mathit{i}}$ as ${\mathit{n}}_{\mathit{i}}^{1/2}$. Experiments on the sliding charge-density wave in ${\mathrm{NbSe}}_3$ agree with both of these predictions. The predicted magnitude differs from that observed in ${\mathrm{NbSe}}_3$, but it is in quantitative agreement with data for the charge-density waves in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_3$ and (${\mathrm{TaSe}}_4$${)}_2$I. These results provide additional evidence both for bulk narrow-band-noise generation and for weak pinning.

A general method for describing a second-order phase transition is discussed. It starts with the energy-level statistics and uses finite-size scaling. It is applied to the metal-insulator transition in the Anderson model of localization, evaluating the cumulative level-spacing distribution as well as the Dyson-Metha statistics. The critical disorder ${\mathit{W}}_{\mathit{c}}$=16.5 and the critical exponent ν=1.34 are computed.

The high-frequency dielectric-constant tensor and the Born effective charge of ${\mathrm{TiO}}_2$ rutile are calculated using variational density-functional perturbation theory. The calculated structural properties and high-frequency dielectric constant show agreement with experiments at the level of a few percent. The Born effective charges of ${\mathrm{TiO}}_2$ rutile are found to be much larger than the charges corresponding to ${\mathrm{Ti}}^{4+}$ or ${\mathrm{O}}^{2\mathrm{-}}$ ions. We compare these results with those obtained for ${\mathrm{SiO}}_2$ stishovite, for which the value of the Born effective charges is closer to an ionic picture.

We have studied the origin of the visible photoluminescence (PL) from oxidized porous Si fabricated by rapid-thermal-oxidization processes. At low oxidation temperature (${\mathit{T}}_{\mathrm{ox}}$), the PL spectrum with a peak near 750 nm is observed, and silicon oxyhydrides and silicon oxides are formed at the surface of nanocrystallites. At high ${\mathit{T}}_{\mathrm{ox}}$ above 800 °C, strong blue PL is observed around 400 nm and the surface of crystallites is covered by ${\mathrm{SiO}}_2$. The electronic state of porous Si is very sensitive to the surface structure of crystallites. Spectroscopic data indicate that the red PL originates from the near-surface region in the crystallite, while the blue PL originates from the core region in the crystallite.

The electron tunneling of a GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As superlattice with randomly distributed layer thicknesses is studied within the framework of the effective-mass approximation in the Wannier representation. The transfer-matrix method and Landauer formula are used to calculate the electron transmission coefficient and tunneling conductance, respectively. Analytical and numerical calculations are performed for the tunneling conductance with different values of potential barrier (different x for ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As) and layer thicknesses. The present calculations show that the peak positions of the tunneling conductance are shifted to lower energy as x decreases. The number of peaks can be increased by increasing the layer thicknesses. Therefore, optical and electronic semiconductor devices may be artificially prepared by the proper choice of these parameters for quantum-wire superlattices with random layer thicknesses.

We investigate in the lowest-Landau-level approximation the tunneling between the edges of two planar samples in the integer quantum Hall effect. We establish a mapping from the microscopic Hamiltonian of the system to a (1+1)-dimensional fermonic quantum-field theory and in this framework we show that the phenomenon of fermion-number fractionalization at the interface is expected to occur for a specific class of tunneling barriers. Further, we verify our statements through a direct numerical analysis of the Schro¨dinger equation in two spatial dimensions.

We discuss the influence of elastic reflectivity of the electrons for the analysis of dissociation phenomena on surfaces and adsorbate-induced reconstructions of the substrate. Two situations corresponding to the enhancement of the reflectivity induced by low-energy-electron-diffraction fine structures and by multiple scattering for ${\mathrm{O}}_2$ chemisorbed on Ag(110) are presented.

Adsorption of pure ${\mathrm{C}}_{70}$ and its mixture with ${\mathrm{C}}_{60}$ on the Cu(111)1×1 surface was investigated by using scanning tunneling microscopy. The pure ${\mathrm{C}}_{70}$ monolayer was found to form the 4×4-commensurate superlattice, similar to the case of pure ${\mathrm{C}}_{60}$, after a brief annealing at approximately 320 °C. The individual ${\mathrm{C}}_{70}$ molecules take the upright position with the long axis perpendicular to the surface. Introducing ${\mathrm{C}}_{60}$’s in the ${\mathrm{C}}_{70}$ layer allows some of the ${\mathrm{C}}_{70}$ molecules to take the lying-down position with the long axis parallel to the 〈1¯10〉 direction of the surface. By comparing with theoretical simulations, we propose that ${\mathrm{C}}_{70}$ molecules in the ${\mathrm{C}}_{70}$-${\mathrm{C}}_{60}$ mixture cease their internal rotation and are ratcheted with one of the hexagonal rings around the equator facing down the Cu(111) surface.

Raman spectra of ultrathin ${\mathrm{C}}_{60}$ films on ${\mathrm{SiO}}_{\mathit{x}}$ (x∼2) in ultrahigh vacuum show strong irradiation-induced changes that persist but decay with time (2–3 h). The induced changes are suppressed upon cooling (120 K) and additional changes are introduced with heating (600 K). The observation of induced low-energy vibrational bands between the molecular and lattice vibrational spectrum suggests intermolecular bond formation. Application of the semiempirical interatomic Tersoff potential to estimate the intermolecular coupling reveals that the interatomic potential for ${\mathrm{C}}_{60}$ differs strongly from that between two graphitic sheets and it is suggested that weak intermolecular covalent bonds are formed.

Thermal lattice fluctuations in charge-density-wave (CDW) condensates have been studied by means of optical investigations on the prototype CDW compound ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_3$ and its alloys. The temperature dependence of the CDW gap absorption in the mid-IR frequency range is strongly indicative of the important role played by the thermal lattice fluctuation effects. The latter remove the inverse-square-root singularity, expected for the case of the static distorted lattice. In fact, a considerable broadening (i.e., larger than ${\mathit{k}}_{\mathit{B}}$T) of the subgap tail absorption is found by increasing the temperature towards ${\mathit{T}}_{\mathrm{CDW}}$. Moreover, we find that the phase phonon modes also give and important contribution to the disorder parameter, thus being an essential ingredient for the thermal fluctuation effects.

Four local vibrational modes are reported for Mg-doped wurtzite GaN, which as grown possesses high concentrations of hydrogen. The modes, studied by Raman and infrared absorption spectroscopy appear to form two pairs. Based on the observed selection rules, one pair, with room-temperature frequencies of 2168 and 2219 ${\mathrm{cm}}^{\mathrm{-}1}$ is assigned to inequivalent Mg-H complexes in the c plane and parallel to the c axis, respectively. The origin of the second pair of modes at 2151 and 2185 ${\mathrm{cm}}^{\mathrm{-}1}$, which are IR inactive, is speculatively linked to the presence of diatomic molecules such as ${\mathrm{N}}_2$ or ${\mathrm{H}}_2$.

The interaction between hot electrons, injected into a confined doped GaAs layer, and cold electrons residing in this layer is studied. Utilizing a hot-electron transistor, we found that the output current is larger than the input current, indicating that electron heating is taking place in the layer. Two possible pictures were considered, one assuming single-particle energy and momentum transfer from hot to cold electrons, and the other assuming that thermal equilibrium is reached among the heated cold electrons. By comparing different devices we conclude that the equilibrium picture describes better the heating process, with measured electron temperatures of 10–20 K. Using a power-balance criterion we estimate the energy relaxation time of the hot injected electrons.

We investigate the interaction of hydrogen with dangling bonds in bulk silicon, using first-principles pseudopotential-density-functional calculations. Two geometries are employed: the first is that of a hydrogenated vacancy; the second geometry allows investigation of an isolated dangling bond and minimizes any H-H interactions. The effect of H-H repulsion is quantified. We find that the energy required to remove the hydrogen from the dangling bond and place it in an interstitial position is 2.5 eV. We also investigate the effect of strain in the back bonds on the energy of the Si-H bond. Finally, we propose a low-energy path for the dissociation of the Si-H bond, and discuss the effect of carriers on the dissociation process.

Relaxation of a stepped (111) diamond surface is studied using an accurate, total-energy tight-binding method. We determined that the bare surface will become ${\mathit{sp}}^2$ bonded near a step. This spontaneous graphitization is accompanied by a large increase in spacing between the surface layers. Band structures of the ideal and relaxed surfaces indicate that surface states are removed from the gap upon relaxation, but additional gap states arise from the lower layers. Addition of hydrogen to the surface will force the C atoms to revert back to ${\mathit{sp}}^3$ bonding. These results are significant towards the understanding of the behavior of a stepped surface under conditions of film growth or even the possibility of diamond nucleation from graphite.

There is some experimental evidence that the c(4×2) structure is the low-temperature phase of the Si(001) surface. The critical temperature for the order-disorder transition was experimentally determined as about 200 K. This structural phase transition was studied by using a two-dimensional Ising spin model whose interaction parameters were estimated by first-principles total-energy calculations. Monte Carlo simulations were then performed. The effects of dimer defects on the nature of the phase transition are discussed.

We report the observations of inelastic light scattering by the elementary excitations of the one-dimensional electron gas. The quantum wires are fabricated from a modulation-doped single GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum well electron-beam lithography and shallow electron-cyclotron-resonance reactive-ion etching. In spectra of intersubband excitations we observe a clear separation of spin-density and charge-density collective modes. The splitting associated with the lateral potential of the wires is also seen in transitions to states of the first excited level of the parent quantum well. Determinations of electron-electron interactions in intersubband excitations of the one-dimensional electron gas show large corrections due to the exchange terms (or vertex corrections) that represent the excitonic binding of the particle-hole pairs.

Photoluminescence studies of ${\ln }_{0.13}$${\mathrm{Ga}}_{0.87}$As/GaAsmodulated -barrier quantum wires have been performed with varying orientations. By using high optical excitation, emission from three quantum wire subbands is observed. For fields perpendicular to the wires we observe a distinct transformation of the quantization from quasi-one-dimensional quantum-wire states at low fields to fully quantized quasi-zero-dimensional states at higher fields. This change occurs as the Landau orbit sizes for the several subbands decrease compared to the lateral quantum-wire dimension. The experimental results are consistent with numerical calculations for these quantum wires in magnetic fields.

The spin-splitting factor ${\mathit{g}}^{\mathrm{*}}$ of the electrons at the very bottom of the conduction band in strained ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/InP type-I quantum wells is reported. Experimental proof of quantum confinement-dependent anisotropy of ${\mathit{g}}^{\mathrm{*}}$ is given. Changing the alloy composition at fixed quantization, equivalent to introducing compressive and tensile strain, changes ${\mathit{g}}^{\mathrm{*}}$. The values of ${\mathit{g}}^{\mathrm{*}}$ perpendicular to the quantum-well plane can be explained in a model calculation. Apparently however, no quantitative theory on which to base the calculation of the anisotropic spin splitting is at present available.