It is shown that linearized elasticity theory fails for nematic polymers in less than four dimensions. Instead, the polymer osmotic elastic modulus E and the elastic moduli ${\mathit{K}}_2$ and ${\mathit{K}}_3$ all become singular functions of wave vector q as ‖q‖→0, with E vanishing like ${\mathit{q}}^{\mathrm{\eta }\mathrm{\perp }}$ and ${\mathit{K}}_{2,3}$ diverging like ${\mathit{q}}_{2,3}^{\mathrm{-}\mathrm{\eta }}$. These exponents satisfy an exact scaling relation ${\mathrm{\eta }}_{\mathrm{\perp }}$+${\mathrm{\eta }}_2$+${\mathrm{\eta }}_3$=1 in three dimensions, and are calculated to second order in ɛ=4-d, yielding ${\mathrm{\eta }}_{\mathrm{\perp }}$=0.46±0.015, ${\mathrm{\eta }}_2$=0.28±0.015, and ${\mathrm{\eta }}_3$=0.21±0.015 in d=3.

We present results of isothermal molecular-dynamics calculations for ${\mathrm{Na}}_8$ microclusters. The Born-Oppenheimer dynamics is calculated using a many-body potential developed by Tománek and co-workers. We analyzed the system in a temperature range from 25 to 1500 K. The ${\mathrm{Na}}_8$ cluster seems to undergo two phase transitions, a solid-glass transition at ≊100 K, and a melting transition at T≊800 K.

We demonstrate that the predictions of continuum elasticity theory fail in the ultimate limit of monolayer films. We directly measure the lattice distortion of ultrathin InAs layers in GaAs by high-resolution electron microscopy. For InAs films of 3 monolayer thickness, the observed tetragonal distortion agrees with the prediction of elasticity theory. For single InAs monolayers, however, the measured strain is much higher than expected. The InAs unit cell in this case is strained such as to conserve the bulk bond length at the interface.

The interaction of atomic hydrogen (deuterium) was studied by nuclear reaction analysis and Rutherford backscattering-channeling analysis. The hydrogen coverage as a function of exposure is found to exhibit a plateau at about 2.0 monolayers. The coverage continues to increase with further exposures of atomic hydrogen (deuterium), a consequence of localized etching of the silicon surface. The channeling data show that the Si(100)-2×1 surface is highly strained. The strains parallel to the (100) surface are mostly removed on the ‘‘saturated’’ bulklike Si(100)-1×1-H surface.

The molecular dynamics and electronic structure of the ${\mathrm{\mu }}^{+}$-${\mathrm{C}}_{60}$ radical in crystalline ${\mathrm{C}}_{60}$ have been studied using muon spin rotation and relaxation. At room temperature ${\mathrm{\mu }}^{+}$-${\mathrm{C}}_{60}$ appears to be in a state of quasifree rotation. At the critical temperature ${\mathit{T}}_{\mathit{S}}$=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on ${\mathrm{C}}_{60}$, suggesting that the molecular dynamics of ${\mathrm{\mu }}^{+}$-${\mathrm{C}}_{60}$ are strongly coupled to those of its ${\mathrm{C}}_{60}$ neighbors.

We have implemented a set of total-energy pseudopotential codes on a parallel computer which allows calculations to be performed for systems containing many hundreds of atoms in the unit cell. Using these codes, we have calculated the total energies and structures of the 3×3, 5×5, and 7×7 Takayanagi reconstructions of the (111) surface of silicon. We find that the 7×7 structure minimizes the surface energy and observe structural trends across the series which can be correlated with the degree of charge transfer between the dangling bonds on the adatoms and rest atoms.

An ab initio investigation of the Si(111)-(7×7) surface reconstruction is undertaken using the state of the art in massively parallel computation. Calculations of the total energy of an ∼700 effective-atom supercell are performed to determine (1) the fully relaxed atomic geometry, (2) the scanning-tunneling-microscope images as a function of bias voltage, and (3) the energy difference between the (7×7) and the (2×1) reconstructions. The (7×7) reconstruction is found to be energetically favorable to the (2×1) surface by 60 meV per (1×1) unit cell.

The popular local-density approximation neglects long-range correlations which, in the presence of the rapid rate of change of the electron density at the surface, lead to observable effects. We evaluate the exchange-correlation potential ${\mathit{V}}_{\mathrm{xc}}$ for the electron gas-vacuum interface from the knowledge of the electron self-energy ${\mathrm{\Sigma }}_{\mathrm{xc}}$ in the GW approximation. The electron-electron correlations built into ${\mathrm{\Sigma }}_{\mathrm{xc}}$ automatically produce an imagelike surface barrier. Our result for ${\mathit{V}}_{\mathrm{xc}}$ is the basis of a nonlocal density-functional calculation of the electronic structure of Al(100) which yields a Rydberg series of image states from first principles.

Electronic structure calculations of GaAs inversion domain boundaries (IDBs) on different planes are reported. The resulting interface energies are analyzed in terms of the number of "wrong" bonds (Ga-Ga and As-As) and their mutual compensation. The compensation energy varies roughly inversely proportionally to the distance between the wrong bonds. This favors local compensation in stoichiometric material. This automatically occurs for {110} planes or by chemical reconstruction for other planes. Ga-rich IDBs are predicted to have low energy in either Ga-rich or $n$-type material.

We show that chaos and nonlinear resonances are clearly reflected in the magnetotransport in lateral surface superlattices and thereby explain a series of magnetoresistance peaks observed recently in ‘‘antidot’’ arrays on semiconductor heterojunctions. We find a mechanism of cyclotron-orbit pinning in an electric field resulting from Kolmogorov-Arnol’d-Moser tori. An experimental verification is suggested in terms of an enhanced cyclotron frequency associated with an anomalously reduced cyclotron radius.

Arrays of field-effect-confined quantum dots with diameters smaller than 100 nm have been prepared starting from ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-GaAs heterostructures. In far-infrared spectroscopy, we induce transitions between the 2-meV-separated quantum levels. We observe discrete steps in the gate-voltage dependence of the integrated absorption strength indicating directly the incremental occupation of each dot with N=1, 2, 3, and 4 electrons. From the gate-voltage dependence, we can estimate a Coulomb charging energy of about 15 meV. On a very fine scale, we also observe a spectral fine structure for the excitation of the quantum-dot atoms.

We have performed numerical calculations for noninteracting electrons in two dimensions in the high-magnetic-field (lowest-Landau-level) limit in the presence of a random potential. By identifying the first Chern character of each eigenstate, we show directly from the data for sizes varying from scrN=8 flux quanta to scrN=128 that the lowest Landau level has extended eigenstates only at a single energy ${\mathit{E}}_{\mathit{c}}$ (the center of the band). The localization length ξ is found to diverage as (E-${\mathit{E}}_{\mathit{c}}$${)}^{\mathrm{-}\nu }$, with ν=2.4±0.1, in agreement with previous calculations.

We report the observation, for the first time, of a fractional quantum Hall state at ν=1/2 Landau-level filling in a low disorder, double-layer electron system realized in a 680-Å-wide GaAs/AlGaAs single quantum well. A nearly vanishing diagonal resistance and a Hall resistance quantized at 2h/${\mathit{e}}^2$ to within 0.3% are observed at ≃15 T and ≃26 mK. The activated temperature dependence of the diagonal resistance minimum yields a quasiparticle excitation energy gap of 230 mK.

Transport studies on bilayer two-dimensional electron systems in GaAs double quantum wells have revealed a new fractional quantum Hall state that has no known counterpart in a single-layer 2D system. At total filling fraction ν=1/2 we observe a deep minimum in the diagonal resistivity and a flat Hall plateau within 1.5% of 2h/${\mathit{e}}^2$. Studies of this new state in several samples with varying densities and layer separations strongly suggest this new state arises from interlayer Coulomb correlations. The data also suggest that the ν=1 quantum Hall effect seen in these samples has a similar origin.

Experimental evidence is presented for the tunneling of polarized electrons from the apex of a ferromagnetic Ni tip into GaAs(110). The polarization is found to be negative and of highest magnitude at very low injection energies, which shows that highly polarized minority 3d electrons are preferentially extracted from the Fermi level of the tip.

Coherent magnetic anisotropy is shown to be induced in vapor-deposited amorphous Tb-Fe by a thermally activated growth process. This process is hypothesized to involve rearrangement of local adatom configurations into energetically favorable orientations which minimize surface energy during the growth, a process analogous to the frequently observed crystallographic texturing of polycrystalline thin films. The anisotropy is found to be independent of the state of stress in the film during the growth, and does not depend on film thickness; these results are inconsistent with recently proposed models.

The Fortuin-Kasteleyn transformation is employed to relate two-dimensional classical ferromagnets to correlated site-bond percolation problems. Modulo some technical assumptions, this approach is used to produce an alternative rigorous proof for the existence of a phase characterized by algebraic decay of correlations in the O(2) model. Certain rigorous results are also derived for some discrete spin models [Z(N), cube, dodecahedron].

(100)-oriented Fe/Mo/Fe sandwiches were grown by molecular-beam epitaxy onto a Mo(100) crystal and were investigated in situ by means of the magneto-optic Kerr effect. The Fe layers are each 14 monolayers (ML) thick, and the intervening Mo is wedge shaped to facilitate the study of the magnetic coupling between the two Fe films as a function of Mo thickness in the range 1–18 ML. Five oscillations between antiferromagnetic and ferromagnetic coupling were observed with a periodicity of ∼3 ML of Mo.

We have measured the magnetoconductance of amorphous ${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ samples in the diffusive, barely, and deeply insulating regimes. Application of a magnetic field decreases the localization length ξ in the barely insulating regime where spin-orbit scattering (SOS) is important. In an insulator without SOS, we see an increase of ξ, indicating that spin-reversal symmetry and time-reversal symmetry control the magnetoconductance in the barely insulating regime, in contrast with predictions obtained by directed-path approximations.

We determine from a two-band Hubbard Hamiltonian the spin susceptibility and the self-energy for the holes in the ${\mathrm{CuO}}_2$ planes. From this, the quasiparticle lifetime τ, the spectral density, the electrical resistivity ρ resulting from spin scattering, and the NMR relaxation rate 1/${\mathit{T}}_1$ at Cu sites are calculated. Our results, which are expected to be valid for larger hole doping, are in reasonable agreement with experiment and indicate that an interplay of narrow-band phenomena and spin fluctuations leads to the observed anomalous normal-state properties. It is shown how to extend the theory to smaller hole doping when local magnetic moment behavior should be more pronounced.