We have used angular-dependent magnetoresistance to probe the anomalous low-temperature state in the two-band (quasi-one- and -two-dimensional) organic conductor family α-(BEDT-TTF${)}_2$MHg(SCN${)}_4$, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Our results provide strong evidence for a mixed state of both normal-metallic and Peierls-like domains with correspondingly different electronic structure. The different attributes of these coexisting domains are shown to be responsible for the complex magnetotransport phenomena in these materials, including the strong hysteretic behavior and the quantum-oscillation wave forms.

Local density functional theory is used to show that both α and β dislocations in GaAs are reconstructed. This is done by relaxing large 158-atom H-terminated clusters of GaAs containing 90° partial dislocations. The reconstruction is strongly influenced by impurities: acceptor pairs destroy the reconstruction of β partials but strengthen it for α dislocations. Donors have opposite effects. The implication of these results for the pinning of dislocations in GaAs is discussed.

We have investigated the low-temperature (T=0.43–4.25 K) magnetotransport of quantum dots fabricated in the shape of an open circle and a circle having a central bar. The characteristic magnetic fields for both coherent backscattering and conductance fluctuations are strongly shape dependent: both are larger by a factor ≥3 in the device with the central bar. Comparison of large and small devices of nominally identical shape shows that characteristic trajectory areas are proportional to the device area.

In bilayer electron systems realized in wide single quantum wells with intermediate density, we observe a many-body quantum Hall state at Landau-level filling ν=1 stabilized by interlayer Coulomb interaction. This quantum Hall ground state makes a phase transition to a compressible state as density is increased. The unusual temperature and density dependence of the data as the transition boundary is approached is suggestive of an additional finite-temperature transition from a quantum Hall state to a compressible state, which is unique to bilayer systems.

Photoemission spectroscopy experiments have shown that photons used to determine semiconductor band bending can induce measurable photovoltages. Because of the pulsed-photon source, the charging and discharging processes taking place at the semiconductor surface have been proposed as seriously affecting synchrotron-radiation photoemission measurements [M. H. Hecht, Phys. Rev. B 43, 12102 (1991)]. In this paper we present some illustrative experiments showing the effects of using a pulsed-photon source on the surface potential of n-type GaAs with a semitransparent Schottky diode. Implications for synchrotron-radiation photoemission results are discussed. We conclude that in these experiments the originated photovoltages are practically flat, and that they can be calculated from average photocurrent data, taken as effective restoring currents.

We show that the spin dynamics of the four-channel Kondo model is equivalent to a model with a spin-1/2 impurity coupled to spin-1 conduction electrons. By Abelian bosonization of the latter model we find a kind of Toulouse limit analogous to that recently discussed by Emery and Kivelson for the two-channel case. We analyze the model by exploiting the close analogy to the problem of an impurity in the Luttinger liquid.

We theoretically study electron transport in disordered, quantum-well-based, semiconductor superlattices with structural short-range correlations. Our system consists of equal-width square barriers and quantum wells with two different thicknesses. The two kinds of quantum wells are randomly distributed along the growth direction. Structural correlations are introduced by adding the constraint that one of the wells always appears in pairs. We show that such correlated disordered superlattices exhibit a strong enhancement of their dc conductance as compared to usual random ones, giving rise to quasi-ballistic-electron transport. Interestingly, this phenomenon is also detected in superlattices with random fluctuations of the well thicknesses. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder and, most important, that it should be clearly observed even in the presence of imperfections.

Thin-film multilayer structures with up to 20 repeat layers have been grown in a high-vacuum chamber by sequential deposition of aluminum (Al) and fullerene (${\mathrm{C}}_{60}$) onto room-temperature substrates. The periodicity of the layers is confirmed by x-ray-diffraction and in situ resistance measurements. The presence of underlying layers of ${\mathrm{C}}_{60}$ reduces the critical thickness at which Al becomes conducting from ∼35 to ∼20 Å. In addition, there is a sudden increase in resistance that occurs when each Al layer is covered by a monolayer of ${\mathrm{C}}_{60}$. These observations, together with the measurement of a downward shift in frequency of a considerably broadened Raman-active ${\mathit{A}}_{\mathit{g}}$(2) pentagonal-pinch mode, imply that up to six electrons per ${\mathrm{C}}_{60}$ are transferred from the Al to the ${\mathrm{C}}_{60}$ layer. This demonstration of charge transfer across planar metal-${\mathrm{C}}_{60}$ interfaces suggests that multilayers may be a useful vehicle for forming fullerene interface compounds in two-dimensional structures.

The optical properties of carbon nanotubes are studied within the gradient approximation. The calculated dielectric function exhibits many divergent structures, due to the divergencies in its density of states. As a result, the electron-energy-loss spectrum has many peak structures, including a prominent one at ∼6 eV, which is identified as the collective excitations of the π-band electrons. This plasmon is found to be insensitive to both radius and chiral angle, due to the unique one-dimensional band structure of carbon nanotubes. The result is consistent with the experimental measurements. The reflectance also exhibits many interesting features, including a weak but sharp plasmon edge at ∼6 eV.

Interference between x-ray beams scattered from a nuclear resonance in stainless-steel foils of different thicknesses has been observed. In-phase and out-of-phase oscillations have been measured for yields with three different time windows, which exist because of a sign change of the amplitude describing the time evolution of the nuclear-resonant forward scattering. This confirms the amplitude modulation in subsequent decays brought about by x-ray dispersion in addition to the increase in the decay rate.

The Griffith criterion is rewritten to account for the propagation of a self-affine crack. It is shown that the stress-field singularity in the close vicinity of the crack tip involves an exponent -(${\mathit{d}}_{\mathit{F}}$-1)/2 instead of -1/2. Furthermore, the fracture toughness is shown to be related to the ratio of the maximum height ${\mathit{z}}_{\max }$ to the self-affine correlation length ξ.

The low-energy optical response of few-nanometer-thick ${\mathrm{SiO}}_2$ films is reported and compared with theoretical predictions. The limit where the surface effects become fundamental in determining the optical properties of the film is discussed and very good agreement is found between experimental and theoretical results.

We present an approach to the calculation of absolute deformation potentials (ADP’s) based on ab initio all-electron methods. The ADP of a given single-particle state is obtained from the variation of its energy between a compressed and an expanded region of the same material. Core levels are used to calculate the band offset at the compressed-expanded homojunction. We present results for a simple metal (Al), a semiconductor (Si), and an insulator (NaCl) under uniaxial strain. We find that (i) the ADP of the valence-band maximum is positive in Si, as predicted by a simple tight-binding model, but it is negative in NaCl, in conflict with tight binding; (ii) while most conduction-band states have negative ADP’s, in agreement with the tight-binding picture, some conduction states have positive ADP’s; (iii) core levels have nonvanishing ADP’s, so they cannot be used as ‘‘absolute’’ reference energies in the presence of strain.

The dielectric functions of the intermetallic B2 phases NiAl, CoAl, and FeAl have been determined by differential reflectometry and the Kramers-Kronig analysis in the energy range of 1.2–5 eV for various compositions near stoichiometry. The experimental absorption maxima are correlated to the band structure of these alloys taken from the literature. All transitions could be assigned to the band structure. The shift of absorption maxima could be explained by a shift of the Fermi level according to the Ekman rule for transitions involving the Fermi level, and for other transitions by small changes of the band-structure levels in the range of about 0.01 eV/at % Al.

We discuss the potential impact of N-scaling algorithms on self-consistent density-functional calculations. N-scaling algorithms can increase numerical efficiency in two qualitatively different ways: First, by eliminating the O(${\mathit{N}}^3$) scaling of numerical diagonalizations or orthogonalizations, and second, through the transferability of localized electronic-structure information between chemically related, but globally different systems. We argue that the second aspect is potentially of great practical importance to self-consistent density-functional calculations. We describe how the transferability of electronic-structure information can be exploited and give numerical examples.

The spectra of the K-edge absorption and the magnetic circular dichroism (MCD) for ferromagnetic Ni and Fe are calculated on the basis of a tight-binding approximation including the spin-orbit interaction. The calculated spectra are in good agreement with the experiments and the previous band calculations near the photothreshold. It is shown that the spin-orbit interaction of the 3d states gives rise to a dominant contribution to the MCD while that of the 4p states plays only a minor role. This suggests that the MCD, which probes the orbital moment of p-symmetric (such as 4p) states on the core-hole site, is generated mainly by the 3d orbital moment on the neighboring sites through the p-d hybridization. The effect of the electric-quadrupole transition to the MCD of Ni is found at a close vicinity of the photothreshold. Sum rules for the dipole transition are discussed.

Using an all-electron mixed-basis approach and within the local-density approximation, the electronic structure of ${\mathrm{C}}_{58}$BN heterofullerenes in the solid fcc phase (F3¯m) has been calculated. We study the two cases where the boron and nitrogen atoms are neighbors and where they are located far apart. The effect of this substitutional doping, which is expected to create acceptor and donor levels in the gap, is investigated and the electron dispersion relations are calculated in both cases. We find that when N and B are neighbors the donor and acceptor levels are pushed into the continuum, and we deduce that this configuration is slightly more stable than the one in which they are far apart. Total and partial charge densities of the occupied bands are displayed. From the present study, we have concluded that there is a single bond between N and B when they are neighbors, and the additional two electrons are mostly localized around N, and therefore only a small relaxation of the atoms is expected.

We present a microscopic slave-boson calculation of the three Landau parameters ${\mathit{F}}_0^{\mathit{s}}$, ${\mathit{F}}_0^{\mathit{a}}$, and ${\mathit{F}}_1^{\mathit{s}}$ of the Hubbard model for any strength of the interaction U and any filling δ=1-n. The Landau parameter ${\mathit{F}}_1^{\mathit{a}}$ is then obtained by using the s-p approximation on the forward-scattering sum rule for the Landau parameters. General analytic expressions are given for the four Landau parameters and related observables. Simple asymptotic expressions for these quantities in four interesting regimes on the U,δ manifold are presented. We also show the results of numerical calculations of these quantities for the full range of U and δ for a system with a flat density of states, which are then compared with the experimental results for normal ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. We find good agreement with the experimentally deduced Landau parameters for both the half-filled-band model and the δ-dependent model at reasonable values of U and δ.