We have investigated the effect of stress upon the Rydberg series of infrared absorption bands due to oxygen donors in silicon. Our results establish that the donor has an effective-mass-like $1s$ ground state that is constructed from conduction-band valleys selected by the $C_{2v}$ distortions associated with an extended "central cell."

It is known that the thermal radiation from an uncharged, nonrotating black hole into a vacuum carries an entropy about $\frac{4}{3}$ times the entropy lost by the hole. For a black hole placed in a radiation bath with a temperature infinitesimally close to that of the hole, the entropy of the radiation is equal to the hole's entropy loss. This previously known result is here extended to the case of a charged black hole, which gives further support to the statistical interpretation of black hole entropy.

Measurements of the position $x$ of a free mass $m$ are thought to be governed by the standard quantum limit (SQL): In two successive measurements of $x$ spaced a time $\tau$ apart, the result of the second measurement cannot be predicted with uncertainty smaller than ${\left(\frac{\hslash \tau }{m}\right)}^{\frac{1}{2}}$. Yuen has suggested that there might be ways to beat the SQL. Here I give an improved formulation of the SQL, and I argue for, but do not prove, its validity.

The electrostatic field distribution due to the contact potential difference in a bimetallic wire introduces a quantum phase difference which can be detected in Fresnel and Fraunhofer diffraction experiments by means of an electron microscope.

A new Monte Carlo method is introduced which generates configurations according to any desired probability distribution. Unlike previous techniques, which require the relative probability of any two configurations to be computed exactly, this method allows the prescence of large but unbiased noise in this computation. The method has important applications in including the effects of dynamical fermions in Monte Carlo calculations, amongst other problems.

A search for monojet production in $e^{+}{e}^{-}$ annihilation has been performed at $\sqrt{s}=29\mathrm{}$ GeV with the MAC detector at PEP. No events beyond those expected from tau decays have been found. The result has been interpreted as an upper limit for $Z^0$ decays into monojets, giving an upper limit for the branching ratio of 0.5% at the 90% confidence level. This limit excludes an interpretation that the monojets discovered by Arnison et al. are due to $Z^0$ decays into light Higgs pairs.

This paper is a continuation of an earlier study of the ground-state energy density $E_0$ in two-dimensional supersymmetric field theories. Lattice strong-coupling techniques are used to calculate $E_0$ to eleventh order, extending the previous calculation by four additional orders. Our results continue to suggest that supersymmetry invariance is restored in the continuum limit. Moreover, new and unexpected large-order behavior in the lattice perturbation-expansion coefficients is revealed in this high order of perturbation theory.

The behavior of ten-dimensional superstrings at high temperatures is studied by use of the microcanonical ensemble. We focus on closed superstring theories. The massive string excitations alone have negative specific heat. Such a system can reach equilibrium with the zero modes of the string if the energy carried by zero modes is less than a given fraction of the total energy. The temperature can then exceed the Hagedorn temperature. Some consequences for the evolution of the universe are discussed.

The branching fraction for ${\tau }^{-}\to 5{\pi }^{\pm }\left({\pi }^0\right){\nu }_{\tau }$ is measured from data accumulated with the Mark II detector at PEP. Four events are observed with an estimated hadronic background of (${0.03}_{-0.02\mathrm{}}^{+0.04\mathrm{}}$) events and a background from ${\tau }^{-}\to 3{\pi }^{\pm }{\pi }^0{\nu }_{\tau }$ of (${0.05}_{-0.04\mathrm{}}^{+0.07\mathrm{}}$) events. This results in a measured branching fraction of (0.16±0.08±0.04)%. The candidate events are used to establish an upper limit of 125 MeV/${\mathit{c}}^2$ on the ${\nu }_{\tau }$ mass at the 95% confidence level.

The laboratory and cosmological bounds on the masses of the scalar leptons and the photino are used to put constraints on the supersymmetric origin of the CERN monojets. The latest MAC-detector data at PEP exclude the scalar quarks, of masses up to 45 GeV, as the origin of these monojets; the cosmological bounds, for a stable photino, exclude the mass range necessary for the $\tilde{g}\tilde{q}$-production interpretation.

Large-angle cross sections for $\gamma d\to {\pi }^{0}d$ are systematically measured in the photon energy range between 500 and 1000 MeV. A good fit is obtained by use of a Glauber-model calculation which includes the dibaryon resonances ${}_{}{}^3{}_{}{}^{}F_3^{}{}_{}{}^{}\mathrm{}\left(2.26\right)\mathrm{}$ and ${}_{}{}^1{}_{}{}^{}G_4^{}{}_{}{}^{}\mathrm{}\left(2.51\right)\mathrm{}$, but the fit has an unusual nature in the role of resonance and nonresonance contributions.

We present observations and measurements of convection between two moderately conducting plates. A staionary square pattern is observed just at the convective threshold. But at high Rayleigh number the preferred convective pattern becomes the usual roll structure. The transition from squares to rolls involves an unexpected Hopf bifurcation.

Direct numerical simulation of the Navier-Stokes equations is used for the investigation of local helicity fluctuations in plane Poiseuille (channel) and Taylor-Green vortex flows. It is shown that in regions of high dissipation, the cosine of the angle between velocity and vorticity is evenly distributed; in regions of low dissipation, the velocity and vorticity vectors have a tendency to align. It is also shown that near the central part of the channel, velocity and vorticity vectors have a strong tendency to be aligned, while in the buffer region, all angles are nearly equally probable.

Hot-electron fractions measured in Nd-laser-plasma interactions at ${10}^{14}$ W/${\mathrm{cm}}^2$ increase from <0.02% to 0.3% as the plasma scale length at quarter-critical density ($\frac{n_c}{4}$) is systematically increased from 140 to 320 μm. Correlated x-ray and $\frac{3\omega }{2}$ emissions indicate that the hot electrons are produced by an $\frac{n_c}{4}$ instability.

We have used a high-resolution, high-stability, graphite-fiber oscillator to measure properties of oxygen films adsorbed on graphite. We show that the bulk triple temperature is the wetting temperature. The observation of a transition from solid to liquid in the saturated film at a well-defined temperature below the bulk triple temperature is in accord with the prediction of a Potts lattice-gas model of adsorbed systems.

The energetics of the early growth stages of epitaxial Ni${\mathrm{Si}}_2$ on Si have been investigated by use of local-density-functional theory implemented as the linear-augmented-plane-wave method. We find that a proposed diffusion region of interstitial Ni several atomic layers thick is highly unfavorable energetically. The energy difference between interstitial and reacted Ni at monolayer coverage calculated for a (111) surface is less, but still large enough to argue strongly against a significant intersitial concentration for any surface.

Stage purity and staging transitions of potassium-graphite intercalation compounds have been investigated in situ by high-resolution x-ray scattering. The positions and profiles of ($00L$) Bragg reflections indicate staging disorder in thermal equilibrium for all compounds larger than stage 2, which increases with increasing stage number. The transition region between stages exhibits a miscibility gap for stage mixing whose size decreases with higher stages. The results are compared with recent theories on the staging of graphite intercalation compounds.

We report parameter-free, self-consistent, electronic-structure and total-energy calculations for chalcogen point defects in crystalline silicon. Both possible defect sites of tetrahedral symmetry are investigated. The calculated total energies are discussed in terms of dissolution and reaction energies. From these, the stable defect position is identified as substitutional. The comparison of our theoretical results (spin and ground-state wave function) with experimental data such as infraredabsorption, EPR, and ENDOR spectra further supports this identification.

$\frac{1}{f}$ noise near the percolation threshold $X_c$ of carbon-wax mixtures was carefully studied. The noise spectral density was found to diverge with a power-law dependence on $X-X_c$. Our results might be qualitatively explained by carrier-number fluctuations, produced by tunneling conduction, in infinite clusters.

Scattering data are presented for a monoenergetic beam of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ incident on both commensurate and incommensurate Kr films adsorbed on graphite. For the commensurate layer, four bound-state resonance levels are observed, at binding energies 4.67, 1.86, 0.58, and 0.14 meV, which agree with a calculation incorporating three-body interactions. For the incommensurate layer, only the lowest level is identified; its binding energy of 5.2 meV also agrees with the calculation.