Superlattices of disjoint bilayer atomic microdomains stabilized by epitaxial misfit strain energies are proposed to account for $n\times n$ ($n=5\mathrm{}$ at lower $T$, $n=7\mathrm{}$ at higher $T$) surface structures of both Si on Si(111) and Sn on Ge(111). The microdomain model accounts well for the relative stability of Si(111) 7×7 against hydrogenation and chlorination and is energetically more favorable than the original Lander model of 2×2 building blocks centered on point vacancies.

Time-differential perturbed angular correlations have been determined with use of the dynamical critical exponent $z$ for Fe and Ni. In both systems, crossover from $z=2.5\mathrm{}$ to $z=2.0\mathrm{}$ is observed as $T\to T_C$. The value $z=2.5\mathrm{}$ ($d=3\mathrm{}$ Heisenberg model) has been previously observed in neutron-scattering experiments; the value $z=2.0\mathrm{}$ (order-parameter-nonconserving models), in hyperfine-interaction experiments. Thus, the existence of two distinct kinds of dynamic critical behavior, each corresponding to a different universality subclass, is confirmed.

This Letter describes the physical behavior of superfluid dispersions created by pressure quenching ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures into the miscibility gap. The two types of dispersions, namely, superfluid droplets within a normal-fluid continuous phase and the inverse, evolve differently. Under certain conditions near the tricritical point, light scattering during the early stages of separation displays a halo at a time-dependent wave vector $k_m\propto t^{-\varphi }$, where $\varphi$ shows a crossover from slow ($\varphi \approx \frac{1}{3}$) to fast ($\varphi =1\mathrm{}$) growth within 0.1 s.

The energy of interacting spin-polarized atomic hydrogen (${\mathrm{H}}_{\mathrm{\downarrow }}$) on a helium surface and the density dependence of the adsorption energy have been calculated, permitting a realistic determination of the adsorption isotherms. Surface interactions are shown to allow Bose-Einstein condensation in the gas. At the Bose-Einstein condensation the surface coverage attains a maximum which is essentially independent of the isotherm temperature. Also calculated is the stability condition for surfaces in magnetic fields. Easily met requirements are found.

Thermal convection in dilute solutions of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been studied in a novel cell of unity aspect ratio which permits inhomogeneity in the heat flux to be measured. At least three different convecting states have been observed, including a time-dependent state with diminished thermal conductance which can exist very close to the onset of convection.

A Landau theory of the martensitic phase transition in $\beta$-phase alloys was constructed, starting with the instability which drives the "$\omega$" distortion in some body-centered-cubic metallic structures, and succeeds in accounting for the wide range of precursor phenomena and low-temperature structures observed in $\beta$-phase alloys. The theory is similar to that proposed for the $A15$ alloys, but with the simpler electronic and vibrational properties here, critical experiments to test the theory can be suggested.

Theoretical studies of the vibrational excitations of charged solitons in polyacetylene are reported. Two strongly infrared active modes, derived from bulk optical phonons, are associated with these defects and we suggest the presence of a third, as yet unobserved, infrared-active mode in the acoustic phonon regime.

With application of the notion of phase-slip centers, the quasiparticle charge-diffusion length in amorphous W-Re wires of cross-sectional dimension of 250 Å wide by 100 Å thick has been measured. The normal electron-diffusion length deduced from this data is in good agreement with values obtained from the application of localization theory on the same materials. This result provides the first quantitative support for the theory of localization in one-dimensional metallic wires.

The metallization lattice constant for fcc Xe has been calculated in three different, independent ways. The result is relatively insensitive to basis set, details of local exchange-correlation potential, or use of the muffin-tin potential. The calculated metallization lattice constant is 7.9 a.u. or $P=1.28\mathrm{}$ Mbar, confirming Ross and McMahan's calculation but disagreeing with Nelson and Ruoff's experimental value.

The observation of a frequency-dependent conductivity ($\sigma$) and dielectric constant ($\epsilon$) in Nb${\mathrm{Se}}_3$ is reported. In both charge-density-wave phases a strong frequency dependence and huge dielectric constant are observed below 100 MHz, with greatest effects observed at 42 K. The conductivity $\sigma$ increases smoothly from the dc value to the high-frequency ($f=100\mathrm{}$ MHz) limit; this increase is accompanied by the reduction of $\epsilon$. A resistance-capacitance network model is suggested to account for the observed frequency dependence.

Neutron diffraction experiments under perturbations, such as uniaxial stress and hydrostatic pressure, suggest strongly that Ce${\mathrm{Al}}_2$ exhibits two magnetic phases: a single-$\overrightarrow{\mathrm{q}}$ collinear modulated structure and a type-II antiferromagnetic structure.

Slowly moving massive neutrinos may be responsible for the invisible mass in galactic halos and the missing mass of the universe. Massive neutrinos are expected to decay into lighter neutrinos and uv photons, with lifetimes long on the Hubble scale. The possible detection of these neutrino-decay photons is discussed.

The results of new calculations of solar-neutrino fluxes are presented; the fluxes are obtained from detailed solar models that make use of improved opacities and nuclear-physics cross sections. By evaluating known uncertainties in the predicted capture rate for the ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$ solar-neutrino experiment, we find that the ratio of theoretical to (best-estimate) observed capture rate lies in the range 4.0 to 2.6. These results constitute a strong constraint on models of neutrino oscillations if the entire discrepancy is ascribed to neutrino oscillations.

The production of ${K_s}^0$, ${\Lambda }^0$, and ${\overline{\Lambda }}^0$ has been measured in deep-inelastic muon scattering at 225 GeV; decays into two charged hadrons were detected. Momentum distributions are compared to the inclusive charged-hadron distributions measured in the same experiment. The range of virtual-photon parameters covered is $0.4<\mathrm{}{Q}^2<50\mathrm{}$ ${\mathrm{GeV}}^2$ and $20<\mathrm{}\nu <210\mathrm{}$ GeV.

A measurement of charged-particle production is reported for the forward region in events triggered by high-transverse-momentum ($p_{\perp }$) jets and single particles. The momentum distributions of forward-going particles are observed to scale in a simple $p_{\perp }$-dependent longitudinal variable. Forward-going (beam) jets are observed to be tilted away from the original direction by an amount which agrees with muon-pair data when interpreted in a parton (quantum-chromodynamics) model.

The divergences in lowest-order electroweak contributions to quark masses are discussed. The example model of U${\mathrm{}\left(1\right)\mathrm{}}_{\mathrm{em}}$ ⊗ SU${\mathrm{}\left(3\right)\mathrm{}}_{\mathrm{color}}$ is worked out. If the bare mass and color coupling constant are constrained to be independent of the electromagnetic coupling, as is natural, then the lowest-order electromagnetic shifts of the renormalized mass and strong coupling will be infinite. This is the expected result and is true even though we treat the strong interactions to all orders and thereby incorporate the relatively soft ultraviolet behavior of quantum chromodynamics.

High-$p_{\perp }$ elastic $\mathrm{NN}$ scattering is analyzed in terms of two quark-interchange amplitudes, which conserve and depend on quark helicities. Resulting relations between spin observables are compared with polarization data. Predictions are made for $\mathrm{np}$ scattering at $p_L=12\mathrm{}$ GeV/c, and for $\mathrm{pp}$ scattering at higher momenta, where the rise of ${A_{\mathrm{NN}}}^{\mathrm{pp}}$ (${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}=90\mathrm{}°$) changes into a turn over. A sharp transition marks the onset of hard scattering.

Pion-deuteron elastic scattering in the region of the (3,3) resonance is calculated with two covariant reductions of the relativistic Faddeev equations. The two $S$-wave nucleon-nucleon channels and the six $S$- and $P$-wave pion-nucleon channels are included by means of separable $T$ matrices. Both of the space and the spin variables are treated relativistically, by use of the three-body helicity formalism of Wick. The results agree very well with the data at 142 and 182 MeV, but do not resolve the discrepancy at 256 MeV.

This Letter presents the $x$ and $y$ distributions of events with visible strange particles produced in charged-current $\overline{\nu }p$ interactions. It is shown that charmed-particle production via the process ${\overline{\nu }}_{\mu }\overline{s}\to {\mu }^{+}\overline{c}$ is required. Using a simple model, we obtain, for the proton, $\int x\overline{s}\mathrm{}\left(x\right)\mathrm{dx}=0.007\mathrm{}\pm 0.002$ and $\frac{\int x\overline{s}\mathrm{}\left(x\right)\mathrm{dx}}{\int \mathrm{xu}\left(x\right)\mathrm{dx}}=0.019\mathrm{}\pm 0.006$ in the kinematic region $0.05<x<~0.8$ and $0.1<y<~0.8$. Moreover, these observations require a significant left-handed component for the $\left(\left\{c\right\}\left\{S\right\}\right)$ doublet.

With use of an equation-of-motion approach, a new microscopic theory of the nuclear optical-model potential which is explicitly energy independent has been derived. This theory suggests the possibility of replacing a wide range of conventional energy-dependent optical-model potentials by a single, energy-independent but nonlocal, optical-model potential.