The electronic structure of pure solid ${\mathrm{C}}_{84}$ has been studied using high-energy electron energy-loss spectroscopy in transmission and photoemission spectroscopy. Information is obtained on the occupied and unoccupied σ and π bands. The reduced degeneracy of the electronic states and smaller band gap of ${\mathrm{C}}_{84}$ in comparison with ${\mathrm{C}}_{60}$ are evident, as well as the excitonic nature of the transition across the band gap of solid ${\mathrm{C}}_{84}$.

The product of a solid solution reaction of ${\mathrm{C}}_{60}$ and ${\mathrm{AsF}}_5$ in liquid ${\mathrm{SO}}_2$ is reported. The change of weight of the ${\mathrm{C}}_{60}$ by the reaction indicates that there is (${\mathrm{AsF}}_5$${)}_{\mathit{x}}$${\mathrm{C}}_{60}$ with x=1.88±0.04. Mass spectroscopy identifies the ${\mathrm{AsF}}_5$ and ${\mathrm{C}}_{60}$. The x-ray diffraction is distinctly different from that of ${\mathrm{C}}_{60}$ and can be indexed with a body-centered-tetragonal unit cell. The infrared spectrum has an absorption at 702 ${\mathrm{cm}}^{\mathrm{-}1}$ which is not present in ${\mathrm{C}}_{60}$.

Optical second-harmonic generation from a ${\mathrm{C}}_{60}$ thin film was used to monitor the orientational phase transition. A jump in second-harmonic intensity was observed around 245 K. This observation is in good agreement with the results obtained from infrared and Raman spectroscopy from ${\mathrm{C}}_{60}$ films. The different response of ${\mathrm{C}}_{60}$ films in the two phases is analyzed and tentatively explained.

The effects of the Coulomb interaction on persistent currents in disordered one-dimensional rings are investigated numerically. First, the effectiveness of the Hartree-Fock approximation is established on small systems. Then the calculations are performed for systems with 40 electrons in 100 sites. It is found that the amplitude of the average persistent current in the diffusive regime is suppressed as the strength of the Coulomb interaction increases. The suppression of the current is stronger in larger rings than in smaller ones. The enhancement of the current by the electron-electron interaction was not observed in the diffusive regime.

The effects of spin-orbit scattering of conduction electrons in the Kondo regime are investigated theoretically. It is shown that due to time-reversal symmetry, spin-orbit scattering does not suppress the Kondo effect, even though it breaks spin-rotational symmetry, in full agreement with experiment. An orbital magnetic field, which breaks time-reversal symmetry, leads to an effective Zeeman splitting, which can be probed in transport measurements. It is shown that, similar to weak localization, this effect has anomalous magnetic-field and temperature dependence.

When the ensemble average of a disordered system is periodic, its three-dimensional energy spectrum may have multiple mobility edges. Then strong coupling yields narrow metallic bands adjacent on wide localized-state regions. This leads to a linear temperature dependence of the resistivity.

The application of gradient-corrected exchange-correlation functionals in total-energy calculations using a plane-wave basis set is discussed. The usual form of the exchange-correlation potential includes gradients whose calculation requires the use of a high-quality representation of the density which is computationally expensive in both memory and time. These problems may be overcome by defining an exchange-correlation potential for the discrete set of grid points consistent with the discretized form of the exchange-correlation energy that is used in Car-Parrinello-type total-energy calculations. This potential can be calculated exactly on the minimum fast-Fourier-transform grid and gives improved convergence and stability as well as computational efficiency.

Spatial variations of the local density of states near the Fermi level were observed in the charge-density wave of 1T-${\mathrm{TaSe}}_2$ at 77 K by site-specified tunneling spectroscopy. The local density of states was significantly different, depending on whether the charge-density wave was at a maximum or minimum. The overall structures and spatial variations of the local density of states in the presence of a charge-density wave were consistent with band calculations. Characteristic tunneling spectra were also found at the charge-density-wave defect site, at which they showed a strong peak structure in the occupied band.

Total-energy calculations predict that the As vacancy in GaAs adopts a 3+ charge state under p-type conditions, and that the formation energy of ${\mathit{V}}_{\mathrm{As}}^{3+}$ is competitive with that of the gallium interstitial ${\mathrm{Ga}}_{\mathit{i}}^{3+}$. After a breathing-mode relaxation, which reduces the energy by more than 1.5 eV, the nearest-neighbor Ga atoms exhibit a nearly ${\mathrm{sp}}^2$ bonding configuration. On the basis of our results we propose that charged anion vacancies with ${\mathrm{sp}}^2$-bonded cation neighbors should be examined as possible mechanisms for the compensation of p-type doping in III-V and II-VI semiconductors, and as possible facilitators of acceptor diffusion and anion self-diffusion in these materials.

Interesting in-plane low-temperature transport properties are proposed in doped thin double quantum wells in an in-plane magnetic field (B). The density of states diverges at a saddle point (SP). The SP is formed at the lower edge of the partial energy gap at a sufficiently high B due to an anticrossing of the two displaced energy-dispersion parabolas. At high carrier densities, the conductance (G) shows a maximum when the upper branch is emptied and a minimum at a higher B=${\mathit{B}}_{\min }$ when the Fermi level lies at the SP. These features are confirmed by recent data. At low densities (i.e., with only the lower branch populated), only a G minimum is predicted to occur. The electron-diffusion thermopower diverges both above and below ${\mathit{B}}_{\min }$ with opposite signs. The correlation between the recently observed tunneling G and the in-plane G is discussed.

Measurements of the diagonal magnetothermopower (${\mathit{S}}_{\mathit{xx}}$) of a high-quality two-dimensional hole system reveal that in the diffusion regime (T<100 mK), ${\mathit{S}}_{\mathit{xx}}$ at filling factors ν=1/2 and 3/2 depends approximately linearly on temperature and has a much smaller magnitude than expected for noninteracting particles at half-filled Landau levels. We show that both the temperature dependence and the magnitude of ${\mathit{S}}_{\mathit{xx}}$ at these two fillings can be explained remarkably well based on a recently proposed particle-flux composite-fermion picture.

We observed flat, smooth heterointerfaces in GaAs/AlAs superlattices grown on GaAs (311)A by molecular-beam epitaxy. In our investigation of GaAs/AlAs superlattice structures along both the [011¯] and [2¯33] directions by cross-section transmission-electron microscopy, we found that the heterointerfaces formed in the (311)A orientation are sharp and smooth. This indicates that the GaAs (311)A surface is stable for epitaxial growth. Thus, it is clear that the recently reported occurrence of interface corrugation in the GaAs/AlAs superlattice grown on GaAs (311)A substrates, viewed along the [2¯33] direction, is not intrinsic to the (311) surfaces.

We propose a family of nanotubules originating from a hexagonal ${\mathrm{BC}}_2$N sheet. Because of the anisotropic structure of the ${\mathrm{BC}}_2$N sheet, there are a variety of ways to obtain chiral structures by rolling the ${\mathrm{BC}}_2$N sheets into tubules. We performed first-principles and tight-binding band-structure calculations and predict that the chiral ${\mathrm{BC}}_2$N tubules will behave as semiconductors that can be doped with either acceptors or donors. Since the ${\mathrm{BC}}_2$N sheets have anisotropic conductivity in the in-plane directions, helical currents are expected for the chiral tubules. The tubules will resemble nanoscale coils.

We have studied the current I(ω,t)=${\mathit{I}}_0$(ω)+${\mathit{I}}_1$(ω,t) through a double-barrier resonant tunneling system (DBRTS) under a dc-ac bias. With a 2D emitter, the ac shift of the resonant level can be determined from ${\mathit{I}}_0$(ω), and the intrinsic inductance of a DBRTS is unambiguously defined from ${\mathit{I}}_1$(ω,t). For presently available DBRT diodes under maximum operation frequency, the photon replica cannot be observed in the I-V curve. With a 3D emitter, a quantum-equivalent-circuit model can be constructed to calculate the maximum operation frequency of the diode, with good agreement with the measured value.

Using the Kubo formula, we show explicitly that a noninteracting electron system cannot behave like a Hall insulator, i.e., a dc resistivity matrix ${\mathrm{\rho }}_{\mathit{xx}}$→∞ and finite ${\mathrm{\rho }}_{\mathit{xy}}$ in the zero-temperature limit, as has been observed recently in experiment. For a strongly interacting electron system in a magnetic field, we illustrate, by constructing a specific form of correlations between mobile and localized electrons, that the Hall resistivity can approximately equal its classical value. A Hall insulator is realized in this model when the density of mobile electrons becomes vanishingly small. It is shown that in noninteracting electron systems, the zero-temperature frequency-dependent conductance generally does not give the dc conductance.

We report a study of Raman scattering at resonance with the band gap in (ZnSe${)}_{\mathit{d}}$(${\mathrm{ZnS}}_{0.18}$${\mathrm{Se}}_{0.82}$${)}_{\mathit{d}}$ superlattices, 20 Å≤d≤150 Å. Taking advantage of a microprobe, all the independent geometries have been excited in backscattering either from the sample surface or the superlattice edge. The energies of optical phonons with their wave vectors both parallel and normal to the growth axis have been measured, thus showing the expected anisotropic behavior. The comparison between first- and second-order spectra allows us to assess the nature of the phonons dominating the iterated electron-phonon scattering as a function of the well width.

The surface electronic structure of Cu on stepped and flat W(110) is determined with inverse photoemission. An intense, Cu-induced surface state is found at 0.6 eV above the Fermi level on W(110). It is located in the ${\mathrm{\Sigma }}_1$ band gap. On the stepped W(331) surface there is a similar state at 0.7 eV. This terrace state shifts down to 0.3 eV when the coverage of Cu is reduced such that only a single row of Cu atoms remains at a step edge.

A previous ab initio calculation of the Gd(0001) surface found that the surface plane relaxed outward by 6.3% and is antiferromagnetically oriented with respect to the bulk magnetization. Experiment shows it relaxes inward by 3.5% and is either ferromagnetic or canted with in-plane component ferromagnetic with respect to the bulk. After correcting errors inherent in the local-spin-density approximation for exchange and correlation we find that the surface is ferromagnetically oriented and that it relaxes inward by 3.38%.

We have prepared rutile ${\mathrm{TiO}}_2$(110) surfaces that are atomically flat over large areas as shown by scanning tunneling microscopy (STM). We have obtained atomic resolution images of ${\mathrm{TiO}}_2$(110) where the (1×1) structure is found to cover all of the observed areas on the sample. In some cases, we also observe an elongated defect structure oriented along the [001] direction. We discuss geometric and electronic properties of models for the clean and defective surfaces which can be used to explain the observed STM images.

We investigate the anomalous Fermi-level resonance observed in the K(110) spectra. A careful calculation of the spectra is performed in which surface effects, band-structure effects, and self-energy effects are included. The result indicates that a charge-density wave in the system is essential for the formation of the resonance. The close agreement between the calculation and the measurement strongly suggests that further investigation on charge-density waves in K is necessary. It also provides evidence for the need of careful matrix-element calculations to understand photoemission spectra, even for simple metals.