The order-disorder transition at the surface of binary alloys with fcc structure is studied within the Landau theory. The interplay of spatial order and segregation leads to results that provide a different mechanism for the bulk first-order phase transition characteristic of these systems. The surface transition may be of first or second order. A new instability temperature towards phase separation, regulated by the surface, is obtained.

Ab initio calculations show that a spherically symmetric charge relaxation of ions in a crystal, in response to the long-range electrostatic potential, is important for understanding the splitting between longitudinal- and transverse-optic-mode frequencies, and the violation of the Cauchy relations among elastic constants.

We develop a model for the recrystallization of Si films that are traversed by a molten zone. The model simulates the branching behavior of low-angle grain-boundary defects in these films to a remarkable degree. The simulated subboundary patterns scale approximately as the square root of the scan velocity, in excellent agreement with experiment.

Application of the evanescent-wave-induced fluorescence technique to the study of the interfacial depletion layer between a polymer solution and a solid barrier has yielded the first measurement of the monomer concentration as a function of the distance to the wall. The results have been obtained on a dilute aqueous solution of xanthan, a stiff polysaccharide of high molecular weight. The data are described quantitatively by an empirical model for semiflexible chains. Good agreement is also found with the Auvray model for fully rigid rods, for an equivalent rod length of $L=600\mathrm{}\pm 10$ nm.

The first steady-state enhanced nuclear polarization of liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has been produced by dynamic nuclear polarization. The ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is in contact with a high-surface-area fluorocarbon substrate containing electronic paramagnetic centers. Slightly off-resonant irradiation of the electronic spins cools the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclear spins in the substrate through a dipolar interaction. The ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ subsequently transfers magnetization to the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ through a ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$-${}_{}{}^3{}_{}{}^{}\mathrm{He}$ interaction. Our results are in good agreement with recent models which explain this coupling.

We report the first systematic study of above—band-gap optical anisotropies in cubic semiconductors. The anisotropies are large, of the order of 1%. The dominant intrinsic contributions for (110) Si and Ge are due to surface many-body screening and bulk spatial dispersion. Extrinsic contributions from chemisorbed and physisorbed species also play important roles.

A well-resolved core exciton at the bulk diamond C-$1s$ absorption edge has been observed using high-resolution partial-yield spectroscopy with synchrotron radiation. The obtained excitonic binding energy, 0.19±0.015 eV, agrees well with a first-principles effective-mass approximation (EMA). This is in sharp contrast to other semiconductors (Si, Ge, and GaAs) where reported excitonic shifts far exceed EMA estimates. In light of these results, one must question whether previous measurements overestimate the core-hole interaction or if they indicate a breakdown of the EMA for core excitons.

It is shown that the criterion for smoothness of invariant tori in certain dissipative dynamical systems approaching chaos can be formulated in terms of an eigenvalue problem—a discrete Schrödinger equation in a potential generated by the map. By this method it is shown that there exists a continuous line through parameter space—the "critical line"—on which the invariant circles lose smoothness. It is explained why this line should approach smoothness for the very-high-order mode-locking regions and this corroborates earlier numerical results relating global scaling properties of dynamical systems to those of circle maps. Further, this method yields effective approximation schemes for obtaining the critical line.

Traditional WKB theory yields wave functions which have divergences and are not invariant under any reasonable class of transformations. This Letter presents alternative WKB wave functions which have no divergences and are symplectic invariants.

The use of nonunitary discrete, irreducible representations of Lie algebras is proposed as a method for the determination of widths and energies of resonances. The method is illustrated by explicit application to three kinds of resonances: in transmission over a well, in transmission over a barrier, and in barrier penetration for a family of one-dimensional potentials. The same procedure is also useful in an algebraic approach to scattering by a complex (optical) potential, and for parametrization of the resonant contribution to the scattering matrix.

Using the Mark-J detector at the high-energy $e^{+}{e}^{-}$ collider PETRA, we compare the data from hadron production with the complete second-order QCD calculation over the energy region 22 to 46.78 GeV. We determine the QCD parameter $\Lambda =100\mathrm{}\pm {30}_{-45\mathrm{}}^{+60\mathrm{}}$ MeV which yields the strong-coupling constant ${\alpha }_s=0.12\mathrm{}\pm 0.02$ for $\sqrt{s}=44\mathrm{}$ GeV.

Cross sections for the production of secondaries with large transverse momentum can become comparable to the total cross section in the teraelectronvolt energy range. We argue that the onset of this effect is observed at subteraelectronvolt energies via (i) an increase in the rapidity distribution near $y=0\mathrm{}$, (ii) an increase of $〈p_T〉$ with energy, and (iii) a correlation between $〈p_T〉$ and multiplicity. We discuss the implications for future hadron colliders and cosmic-ray experiments in the corresponding energy range.

The measured ratio of production cross sections of $W$ to $Z$, $R=\frac{{\sigma }^{W^{+}+W^{-}}B(W\to e\nu )}{{\sigma }^{Z}B(Z\to e^{+}{e}^{-})}$, is barely consistent with the standard model with three families, and puts stringent limits on the number of neutrinos $N_{\nu }$. The effect of supersymmetric extension of the standard model is to strengthen these limits further. Improvement in the measured value of $R$ could not only limit $N_{\nu }$, but also remove from contention some extensions of the standard model.

Narrow positron peaks are observed in five supercritical collision systems with combined nuclear charge $180<~Z_u<~188$. The peaks do not originate from nuclear internal pair conversion and their production appears to occur in a narrow projectile-energy interval near the Coulomb barrier. The line shapes are consistent with emission by a source moving with the c.m. velocity. Particularly notable is an apparent independence of the peak energies on $Z_u$. These observations are discussed in the context of the spontaneous decay of the QED vacuum and other new potential sources of line positron spectra.

We study the $\beta$ function of SU(2) lattice gauge theory in the presence and absence of dynamical fermions with the (improved) "ratio method." The pseudofermion algorithm used to simulate the fermion action seems to be capable of giving the right contribution to the $\beta$ function. For two flavors of fermions we find agreement with asymptotic scaling to within ∼ 15% for $\beta \gtrsim 2.0$.

The mixing and the $\mathrm{CP}$-nonconserving effects are studied for $K^0$, ${\overline{K}}^0\to 2\gamma$. The Kobayashi-Mashawa scheme can give observable $\mathrm{CP}$-nonconserving effects different from those of the superweak interaction. Experimental ways to detect such effects are pointed out.

Using microcanonical simulations we determine the dependence of the finite-temperature QCD effective-spin model coupling $J$ on $g^2$, the gauge-field coupling. The results are compared with the theoretical prediction.

The production of $\tau$-lepton pairs in $e^{+}{e}^{-}$ collisions at 29 GeV has been used to study the inclusive $\tau$ decay into five charged particles. Ten events are observed in the high-resolution spectrometer giving a branching ratio of $B(\tau \to 5{\pi }^{\pm }+n\gamma +{\nu }_{\tau })=(1.3\mathrm{}\pm 0.4)\times {10}^{-3\mathrm{}}$. Five of the events contain photons which are consistent with coming from a single ${\pi }^0$. With this interpretation we measure $B(\tau \to 5{\pi }^{\pm }+{\nu }_{\tau })$, $B(\tau \to 5{\pi }^{\pm }+{\pi }^0+{\nu }_{\tau })=(6.7\mathrm{}\pm 3.0)\times {10}^{-4\mathrm{}}$. The branching-ratio upper limit for the decay mode $\tau \to K^0+3\mathrm{}{\pi }^{\pm }+n\gamma +{\nu }_{\tau }$ is found, at 90% confidence level, to be 2.7×${10}^{-3\mathrm{}}$.

The elastic-scattering angular distribution for the reaction of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ has been measured at $E_{\mathrm{lab}}=1503\mathrm{}$ MeV. Optical-model analysis shows that the data determine the nuclear potential between 4 and 6 fm, i.e., well inside the strong-absorption radius. Above ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}=3\mathrm{}°$, the main contribution to the cross section comes from negative-angle scattering; however, the measured angular distribution is not sensitive to the position of the nuclear rainbow.

The $\alpha$ decay of mass-separated Bi isotopes is studied to characterize the shell-model intruder states in the odd-mass Tl and Bi isotopes. Allowed $\alpha$ decay is observed between the ${}_{}{}^{189-195}{}_{}{}^{}\mathrm{Bi}$ $\pi h_{\frac{9}{2}}$ ground states and the ${}_{}{}^{185-191}{}_{}{}^{}\mathrm{Tl}$ $\pi h_{\frac{9}{2}}$ intruder states and between the ${}_{}{}^{191-197}{}_{}{}^{}\mathrm{Bi}$ $\pi s_{\frac{1}{2}}$ intruder states and the ${}_{}{}^{187-193}{}_{}{}^{}\mathrm{Tl}$ $\pi s_{\frac{1}{2}}$ ground states. The observation of forbidden $\alpha$ branches provides excitation energies for the intruder states in ${}_{}{}^{189,191}{}_{}{}^{}\mathrm{Tl}$ and ${}_{}{}^{189,191,193,195}{}_{}{}^{}\mathrm{Bi}$ and confirms the intruder-state excitation energies in ${}_{}{}^{185,187}{}_{}{}^{}\mathrm{Tl}$.