The frequency dependence of second-harmonic generation at simple metal surfaces is calculated by use of the time-dependent density-functional approach. The perpendicular surface contribution to the second-harmonic current is found to be significantly larger than had been assumed in previous work. For $2\omega$ close to the threshold for electron emission, the self-consistently screened nonlinear electronic response becomes resonantly enhanced in analogy to the well-known local field enhancement seen in linear response near $0.8{\omega }_p$, where ${\omega }_p$ is the bulk plasma frequency.

We report the discovery of a density, $n$, and electric field, $E_p$, dependent critical temperature for electrons trapped on the surface of liquid helium for densities $n>\mathrm{}\frac{1.2E_p}{2\pi e}$. Above this critical temperature a finite fraction of the electrons escape rapidly from the surface into the vacuum. The critical temperature increases with decreasing density, for a given electric field.

Be measuring the degree of linear polarization we identify the orbital and spin contributions to the x-ray magnetic scattering in holmium. When the incident x-ray energy is tuned through the $L_{\mathrm{III}}$ absorption edge, we observe a fiftyfold resonant enhancement of the magnetic signal, and resonant integer harmonics. The line shapes of the two linear components scattered parallel and perpendicular to the diffraction plane are distinct in energy with a 6-eV splitting.

Large resonant magnetization-sensitive x-ray scattering is predicted to occur in the vicinity of $L_{\mathrm{II}}$, $L_{\mathrm{III}}$, and $M_{\mathrm{II}}-M_{\mathrm{V}}$ absorption edges in the rare-earth and actinide elements, and at the $K$ and $L$ edges in the transition elements. These "magnetic" resonances result from electric multipole transitions, with the sensitivity to the magnetization arising from exchange. For some transitions, the magnetic scattering will be comparable to the charge scattering. The general features of the observed $L_{\mathrm{III}}$ resonance in Ho are discussed.

Tc and Ru ions in alkali-metal hosts exhibit local magnetic $4d$ moments with large orbital contributions which imply small crystal fields and spin-orbit coupling for the $4d^6$ shell of Tc in Rb. Probably we have found mixed valence for a $d$ system. The results for Tc in Rb can be well reproduced by a mixture of the ionic ${\mathrm{Tc}}^{2+}$ and ${\mathrm{Tc}}^{1+}$ levels. The magnetism of Ru in Rb and Cs is consistent with a predominant $4d^7$ state along with spin fluctuations of ∼ ${10}^2$ K. The systems were produced and investigated by the perturbed $\gamma$-ray distribution method following heavy-ion reactions.

Spin-polarized tunneling into the side of an Al film covered with a submonolayer of Gd reveals the presence of the localized Ruderman-Kittel-Kasuya-Yosida spin polarization in the normal state and its absence in the superconducting state. This result is attributed to the fact that the long-range part of the spin susceptibility in the superconductor vanishes at low temperature. The exchange constant is derived from the Zeeman splitting of the superconductor density of states, and its value agrees with that obtained from the Abrikosov-Gor'kov theory.

If light superconducting strings were formed in the early Universe, then it is very likely that now they exist in abundance in the interstellar plasma and in stars. The dynamics of such strings can be dominated by friction, so that they are "frozen" into the plasma. Turbulence of the plasma twists and stretches the strings, forming a stochastic string network. Such networks must generate particles and magnetic fields, and may play an important role in the physics of stars and of the Galaxy.

The possibility is explored that moderately massive neutrinos (1-10 keV) with a lifetime shorter than the age of the Universe, decaying predominantly without emitting radiation but with a small branching ratio to the photon channel, have reheated the Universe after recombination. For a reasonable parameter range, it is possible to explain the recently observed spectral distortion of the cosmic microwave background radiation (CBR). Implications are also discussed for the interpretation of the anisotropy measurement of the CBR.

In certain string models for $\mathrm{pp}$ scattering the multiplicity distribution $P\left(n\right)\mathrm{}$ is given as a superposition $\int \mathrm{dm}W\left(m\right)P(m,n)\mathrm{}$ of contributions for a given string mass $m$. We demonstrate that Koba-Nielsen-Olesen scaling follows as a consequence of a scaling property for the string-mass distribution $W\left(m\right)\mathrm{}$, thus relating the Koba-Nielsen-Olesen scaling function $\psi$ to the quark structure functions, which determine $W\left(m\right)\mathrm{}$.

Pairs of like-charged $W$ bosons, $W^{+}{W}^{+}$ and $W^{-}{W}^{-}$, are a unique probe of the electroweak-symmetry-breaking sector since they have no background from $q\overline{q}$ annihilation. We compute the observable signal in several models and the irreducible QCD background. We find that the signal will be observable at the Superconducting Super Collider (SSC) if and only if the symmetry-breaking sector is strongly interacting, provided the SSC operates at full energy and luminosity.

$\eta$ production has been investigated by the Mark II collaboration at the SLAC $e^{+}{e}^{-}$ storage ring PEP. $\eta$ particles are reconstructed by their $\gamma \gamma$ decay mode. The $\eta$ fragmentation function has been measured and found to be in good agreement with the Lund-model prediction. ${\eta }^{\prime }$ production has been measured for the first time in high-energy $e^{+}{e}^{-}$ annihilation. There is evidence at the $3\sigma$ level for $D_s^{}{}_{}{}^{\pm }$ decay into $\eta {\pi }^{\pm }$ and ${\eta }^{\prime }{\pi }^{\pm }$.

We report new results on a precision measurement of the ratio $R=\frac{{\sigma }_L}{{\sigma }_T}$ and the structure function $F_2$ for deep-inelastic electron-nucleon scattering in the kinematic range $0.2\le x\le 0.5$ and $1\le Q^2\le 10$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. Our results show, for the first time, a clear falloff of $R$ with increasing $Q^2$. Our $R$ and $F_2$ results are in good agreement with QCD predictions only when corrections for target-mass effects are included.

At the HELIOS neutron source in Harwell, the neutron transmission of Pb and C was very precisely measured within the $50 \mathrm{eV}<E<\mathrm{}50\mathrm{} \mathrm{keV}$ energy range, with use of a 150-m flight path, newly developed time-of-flight electronics, and a ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}-\mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}\mathrm{e}\mathrm{d}$ liquid scintillator. After correction for neutron-electron, Schwinger, and resonance scattering, the electric polarizability of the neutron was determined to be ${\alpha }_n=(1.2\mathrm{}\pm 1.0)\times {10}^{-3\mathrm{}}$ ${\mathrm{fm}}^3$ from the energy dependence of the total scattering cross section.

We have begun a program to measure dielectron production in $p$-nucleus and nucleus-nucleus collisions at the LBL Bevalac. Results are presented for the reaction $p+\mathrm{Be}$ at 4.9 GeV. For the first time, direct dilepton production is observed below 10 GeV incident energy. The cross sections are discussed and compared to previous data at higher energies. The observation of a structure at a mass of about 275 MeV suggests that pion annihilation may be the dominant production mechanism in this mass range.

We present results of our systematic studies of charged-shower-particle multiplicities and their dependence on pseudorapidity intervals for nearly central events produced by ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ at 200 GeV/nucleon and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ at 200 and 60 GeV/nucleon in nuclear emulsion. An increase in the particle density with the increase of particle energy and mass is observed. We find an energy-independent linear relation between the maximum particle density (in a given pseudorapidity interval) and shower-particle multiplicity.

Modified Faddeev-type three-nucleon equations that also include the Coulomb potential between the two protons are solved in momentum space. With a separable representation of the Paris potential for the $N-N {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$, the coupled ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}$, and the $P$ states, we have calculated for the first time $p-d$ polarization observables. Our $p-d$ results at $E_p=2.5\mathrm{}$ MeV are compatible with the measured $p-d$ data and demonstrate that a proper treatment of the $p-d$ Coulomb corrections is required.

The isoscalar monopole and quadrupole giant multipole resonances of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ have been studied in the excitation region of 9 to 16 MeV by coincident electron scattering. Concentrations of strength have been found at 10.4 and 14.2 MeV, in reasonable agreement with the predictions of mean-field theory. The measurements reported are the first nuclear-structure investigation carried out by use of the ($e,e^{\prime }n$) reaction.

We show that the usually neglected interaction between the electric dipole of the water molecule and the quantized electromagnetic radiation field can be treated in the context of a recent quantum field theoretical formulation of collective dynamics. We find the emergence of collective modes and the appearance of permanent electric polarization around any electrically polarized impurity.

Accurate $S$-wave phase shifts for low-energy ($0.1\le k\le 0.7$) positron-hydrogen scattering have been calculated by use of the finite-element method. Simple criteria are developed for which the truncation of the relevant configuration space and its subsequent discretization appear to be optimized. These criteria are expected to be independent of the details of the collision system.

A laser with squeezed-pump fluctuations is found to oscillate with one of two macroscopically distinct phases. The phase diffusion rate is reduced below that of the usual laser and the output light may have amplitude fluctuations reduced below the vacuum level.