The interaction of chlorine with semiconductor surfaces is investigated using the first-principles pseudopotential method. We determine the adsorption geometries of Cl atoms on both GaAs(001) and Si(001) dimerized surfaces in the high-coverage limit. It is found that Cl atoms can break Ga dimers at Ga-terminated GaAs surfaces without any potential barrier. The Cl adsorption simultaneously weakens the Ga-As back bonds at the surface, which contributes to the etching process. The effect of Cl adsorption on the bonding character of semiconductor surfaces is elucidated.

Surface energy anisotropy, as opposed to surface energy, is shown to influence change of growth mode in Ge/Si by ‘‘surfactant’’ impurities. By annealing thin Ge/Si films, we find the equilibrium island shape, and hence the surface energy anisotropy; radical changes in shape are seen for impurity-terminated surfaces: Ge:Sb enhances (100) facets compared to clean Ge and Ge:In favors {311}. Thus Sb impurities favor large flat islands which would lead to earlier island coalescence and can aid planar (100) growth, while In (though otherwise a good ‘‘surfactant’’) leaves the film faceted. Island suppression by a ‘‘morphactant’’ thus depends on enhanced faceting onto (100), as well as reduced diffusion.

The π-electronic structural changes in a polyene molecule containing seven double bonds, α,ω-diphenyltetradecaheptaene (DP7), have been studied upon gradually doping with sodium, using x-ray and ultraviolet photoelectron spectroscopies. The spectra are interpreted with the help of detailed quantum chemical calculations. Analysis of the evolution of the XPS and UPS spectra as a function of doping with sodium indicates that the extra charges are stored in the form of two charged solitons on the polyene part of the molecules, which results in two new energy levels in the originally forbidden energy gap.

It is shown that, for certain universality classes, logarithmic corrections to scaling and to some asymptotic power laws exist near the metal-insulator transition. These slow transients make it virtually impossible to experimentally probe the asymptotic critical reigon. Upon application of a magnetic field the transients are predicted to vanish, making the asymptotic region accessible. It is shown that, with the slow transients taken into account, the measured critical behavior of the conductivity in Si:P is consistent with a rigorous lower bound on the correlation length exponent.

We obtain the ground-state energy ${\mathit{E}}_0$(Φ) exactly as a function of flux Φ in a one-dimensional ‘‘copper-oxide model’’ on rings of finite circumference L, including on-site interactions and a nearest-neighbor interaction V. For V of the order of the charge-transfer gap or larger, the model extrapolated to large L exhibits flux quantization with charge 2e, and a slow algebraic decay of the singlet superconducting correlation function on oxygen sites. The extrapolated superfluid stiffness appears, however, finite only for not too large V. These results suggest a superconductive state at V of order the charge-transfer gap of the model, but a paired and phase-separated state at larger V.

We discuss the scattering theory of quantum transport, in which properties of a disordered sample connected to ideal, multichannel leads are expressed in terms of its transfer matrix, T. Taking a continuum limit in which the sample is divided into many thin slices, the Lyapunov exponents, characterizing the dependence of T on sample length, are calculated in terms of the probability distribution of the eigenvectors of ${\mathit{TT}}^{\mathrm{°}}$. It is shown that localization of these eigenvectors accompanies the disorder-induced metal-insulator transition, and an approximation scheme is developed to study the transition.

We discuss the voltage dependence of the differential conductance fluctuations in metal samples containing a disordered region much smaller than the inelastic diffusion length at low voltage. Measurements are made for an ensemble of different defect configurations within the same sample. We find that the root mean square amplitude of differential conductance fluctuations within a phase coherent sample is independent of voltage, in conflict with existing theory. The measured voltage correlation scale is at least an order of magnitude smaller than the Thouless energy divided by e.

Tunneling into the correlated electron system in a quantum wire is considered. For the first time, the problem is solved for an arbitrary number of transverse electron modes in the wire. The tunnel density of states has power-law asymptotics at low energies, ν(ε)∝${\mathrm{\epsilon }}^{\mathrm{\alpha }}$. A sample formula expressing α in terms of velocities of soundlike excitations (plasmons) propagating along the wire is obtained and the eigenvalue equation for these velocities is found. The general result for α allows one to extrapolate correctly between the known cases of a one-mode wire and a wide three-dimensional wire.

We have measured the difference between the free energies of an isolated superconducting electrode with odd and even number of electrons using a Coulomb blockade electrometer. The decrease of this energy difference with increasing temperature is in good agreement with theoretical predictions assuming a BCS density of quasiparticle states, except at the lowest temperatures where the results indicate the presence of an extra energy level inside the gap.

The total resistivity (ρ=E/J not dE/dJ) was measured in epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films up to high dissipation levels using a pulsed current source. In the reversible region, ρ(J) has an ssS shape; it is Ohmic at low J (thermally activated free-flux motion), goes through a nonlinear transition region (depinning), and becomes Ohmic again at the highest J (free flux flow, i.e., purely viscous motion). The free-flux-flow resistivity ${\mathrm{\rho }}_{\mathrm{fff}}$ obeys ${\mathrm{\rho }}_{\mathrm{fff}}$/${\mathrm{\rho }}_{\mathit{n}}$≃H/${\mathit{H}}_{\mathit{c}2}$(T), with ${\mathit{dH}}_{\mathit{c}2}$/dT≊2 I/K.

A torsion pendulum experiment employing spin-polarized masses provides an upper limit for the strength of an anomalous interaction of the dipole-monopole form at ranges from 1 to 100 cm.

This Letter derives lower bounds on the energy of braided magnetic fields, based on crossing number techniques pioneered by Freedman and He. Mean crossing numbers are defined for magnetic fields inside a cylinder; for configurations with a uniform axial component the square of the crossing number provides a lower bound for the free magnetic energy. An energy-crossing number relation is also derived for a random field with Gaussian amplitude distribution, in the limit of small correlation lengths. These results provide information on the structure of solar coronal loops and on the problem of coronal heating.

We study low energy consequences of the heavy particle mass spectrum in two specific supersymmetric SU(5) models, the minimal model and the missing doublet model with natural doublet-triplet splitting. The grand unification threshold effects are found to be significant and strongly model dependent: The minimal model favors a high supersymmetry threshold (${\mathrm{\Lambda }}_{\mathrm{SUSY}}$>1 TeV) for ${\mathrm{\alpha }}_{\mathit{s}}$(${\mathit{m}}_{\mathit{Z}}$${)}_{\mathrm{MS}\mathrm{¯}}$=0.113±0.004 (MS¯ denotes the modified minimal subtraction scheme), whereas the missing doublet model favors a low threshold (${\mathrm{\Lambda }}_{\mathrm{SUSY}}$<9 TeV). The X-boson-mediated proton decay lifetime can be shortened by more than one order of magnitude in both models.

We present a measurement of jet shapes in p¯p collisions at √s =1.8 TeV at the Fermilab Tevatron using the Collider Detector at Fermilab (CDF). Qualitative agreement is seen with the predictions of recent next-to-leading [O(${\mathrm{\alpha }}_{\mathit{s}}^3$)] calculations and with leading logarithm QCD based Monte Carlo simulations. The dependence of the jet shape on transverse energy is studied.

Quasielastic e-d cross sections have been measured at forward and backward angles. Rosenbluth separations were done to obtain ${\mathit{R}}_{\mathit{L}}$ and ${\mathit{R}}_{\mathit{T}}$ at ${\mathit{Q}}^2$=1.75, 2.50, 3.25, and 4.00 (GeV/c${)}^2$. The neutron form factors ${\mathit{G}}_{\mathit{E}\mathit{n}}$ and ${\mathit{G}}_{\mathit{M}\mathit{n}}$ have been extracted using a nonrelativistic model. The sensitivity to deuteron wave function, relativistic corrections, and models of the inelastic background are reported. The results for ${\mathit{G}}_{\mathit{M}\mathit{n}}$ are consistent with the dipole form, while ${\mathit{G}}_{\mathit{E}\mathit{n}}$ is consistent with zero. Comparisons are made to theoretical models based on vector meson dominance, perturbative QCD, and QCD sum rules, as well as constituent quarks.

We measure the relative cross sections for D mesons produced in interactions of ${\mathrm{\pi }}^{\mathrm{-}}$ and ${\mathrm{\pi }}^{+}$ beams with targets of Be, Cu, Al, and W. The measurement is based on 1400 fully reconstructed decays of the types ${\mathit{D}}^0$→${\mathit{K}}^{\mathrm{-}}$${\mathrm{\pi }}^{+}$, ${\mathit{D}}^{+}$→${\mathit{K}}^{\mathrm{-}}$${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{+}$, and charge conjugates. We find that the cross section for the production of both neutral and charged D’s by either ${\mathrm{\pi }}^{\mathrm{-}}$ or ${\mathrm{\pi }}^{+}$ is well fitted by the form ${\mathit{A}}^{\mathrm{\alpha }}$ where A is the atomic mass and α=1.00±0.05±0.02, where the errors are statistical and systematic, respectively. There is no significant dependence of α on the transverse or longitudinal momentum of the D meson or on the charge of either the incident pion or the produced D mesons.

The α spectrum following the β decay of ${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$ from the isotope separator TISOL has been measured by detecting ${10}^6$ α particles in coincidence with ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ nuclei. These data, which show a low-energy interference anomaly accompanying the main α peak, permit a more precise determination of the p-wave amplitude of the astrophysically important reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(α,γ${)}^{16}$O. The α spectrum and previous γ-ray data have been fitted simultaneously by a K-matrix parametrization; a value of S(E=0.3 MeV)=57±13 keV b has been obtained for the E1 part of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(α,γ${)}^{16}$O reaction.

Kinematically complete measurements for Coulomb dissociation of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ into ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$+2n were made at 28 MeV/nucleon. The n-n correlation function suggests a large source size for the two-neutron emission. The electromagnetic excitation spectrum of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ has a peak, as anticipated in low-energy dipole resonance models, but a large post-breakup Coulomb acceleration of the ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ fragment is observed, indicating a very short lifetime of the excited state and favoring direct breakup as the dissociation mechanism.

A complete set of diagonal spin-transfer observables has been measured at excitation energies between 30 and 40 MeV for inelastic proton scattering from ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ at 497 and 581 MeV at a momentum transfer of 0.56 ${\mathrm{fm}}^{\mathrm{-}1}$. The longitudinal and transverse spin-flip probabilities are deduced. Their unequal enhancements relative to free nucleon-nucleon scattering values are explained by a distorted-wave impulse approximation calculation in which the finite nucleus random-phase approximation is used to describe the nuclear respose.

We report the first measurement of analyzing powers and spin correlation parameters using a storage ring with both beam and internal target polarized. Spin observables were measured for elastic scattering of 45 and 198 MeV protons from polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei in a new laser-pumped internal gas target at the Indiana University Cyclotron Facility Cooler Ring. Scattered protons and recoil ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei were detected in coincidence with large acceptance plastic scintillators and silicon detectors. The internal-target technique demonstrated in this experiment has broad applicability to the measurement of spin-dependent scattering in nuclear and particle physics.