The fully directed compact lattice-animal model on the square lattice is solved exactly. An explicit expression is obtained for the cluster-number generating function which has a complicated complex-plane singularity pattern including a natural boundary of essential singularities. However, the cluster numbers are controlled by a simple pole singularity and grow proportionally to ${\lambda }^{\mathrm{N}}$, where N is the number of sites, and λ≃2.66.

We searched for ultrahigh-energy γ rays emitted by the supernova 1987A with a new cosmic-ray facility installed at the Black Birch Range in New Zealand. The observations from 13 October to 3 December 1987 suggest no clear clustering of events around the direction of the supernova. We conclude that an upper limit on the flux γ rays of energies greater than 100 TeV is 1.1×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{s}}^{\mathrm{-}1}$ (95% confidence limit) for a differential spectral index α=2.0 and source distance d=50 kpc. This value gives an upper bound on the γ-ray luminosity of the supernova of 5.5×${10}^{38}$ erg ${\mathrm{s}}^{\mathrm{-}1}$ for ${10}^{14}$–${10}^{17}$ eV.

The results of a systematic numerical study of spontaneous chiral-symmetry breaking in gauge theories with slowly running couplings are summarized. For a range of such theories, the chiral condensate can be considerably enhanced with potentially important physical consequences in technicolor theories.

Cross sections or upper limits are reported for 12 meson-baryon and two baryon-baryon reactions for an incident momentum of 9.9 GeV/c, near 90° c.m.: ${\mathrm{\pi }}^{\pm }$p→p${\mathrm{\pi }}^{\pm }$,${\mathit{pp}}^{\pm }$,${\mathrm{\pi }}^{+}$${\mathrm{°}}^{\pm }$,${\mathit{K}}^{+}$${\mathrm{\Sigma }}^{\pm }$, (${\mathrm{\Lambda }}^0$/${\mathrm{\Sigma }}^0$)${\mathit{K}}^0$; ${\mathit{K}}^{\pm }$p→${\mathit{pK}}^{\pm }$; ${\mathit{p}}^{\pm }$p→pp. By studying the flavor dependence of the different reactions, we have been able to isolate the quark-interchange mechanism as dominant over gluon exchange and quark-antiquark annihilation.

We have observed changing Einstein coefficients of spontaneous emission as a function of electron density in ${\mathrm{CO}}_2$ laser-produced plasmas. These measurements are based on the intensity branching ratio of O iv lines 5801–5812 and 312.41–312.46 Å which share a common upper level. Similar observations for O iii lines are also discussed.

The gain-feedback approach to lasing and optical instabilities has been applied to sodium vapor driven by a nearly resonant intense field. The observed lasing frequencies agree with the two-beam-coupling gain curve calculated for a Doppler-broadened two-level medium. Rayleigh-gain lasing is seen for no external cavity with use of counterpropagating beams, and Raman-gain lasing is seen in a ring cavity.

We find experimentally that in the forward direction the length scale of quasiballistic photons dominates the dynamic intensity-intensity correlation function of light multiply scattered from randomly diffusing spheres. This implies a breakdown of the photon-diffusion approximation, which is characterized by a single length, the transport mean free path. We suggest why this breakdown occurs, introduce phenomenologically a dynamic correlation length as an additional length scale, and with this extension reconcile the theory with experiment.

We obtain useful information from the intensity autocorrelations of light scattered from systems which exhibit strong multiple scattering. A phenomenological model, which exploits the diffusive nature of the transport of light, is shown to be in excellent agreement with experimental data for several different scattering geometries. The dependence on geometry provides an important experimental control over the time scale probed. We call this technique diffusing wave spectroscopy, and illustrate its utility by studying diffusion in a strongly interacting colloidal glass.

High-resolution x-ray scattering measurements on a liquid-crystalline smectic-${\mathrm{A}}_1$ phase show two collinear diffuse maxima at incommensurate wave vectors in addition to a fundamental quasi-Bragg peak. These diffuse peaks are shown to agree qualitatively with calculated profiles; they indicate the close proximity of an incommensurate phase with two strongly coupled order parameters.

We have studied the wetting behavior of ${\mathrm{K}}_{\mathrm{x}}$${\mathrm{KCl}}_{1\mathrm{-}\mathrm{x}}$ solutions at a fluid-solid interface by optical reflectivity and ellipsometric measurements as a function of temperature and composition. In metal-rich solutions a wetting transition is observed along the phase boundary near 480 °C. Analysis by the Drude slab model yields that the intruding wetting film corresponds to roughly ${\mathrm{K}}_{0.1}$${\mathrm{KCl}}_{0.9}$ and has a thickness of the order of 200 nm for conditions well away from the critical point.

Surface premelting of lead crystals has been examined by the measurement of integrated LEED beam intensity as a function of temperature. In agreement with previous results, surface premelting on the (110) surface begins well below the bulk melting temperature. Surface premelting is anisotropic in that complete disorder is achieved at lower temperature in the [11¯0] direction than in the [001] direction. The interface between the ordered and fully disordered phase is atomically sharp for disorder along [11¯0] but extended along [001], where the substrate acts to preserve partial translational order.

Water adsorption to substrate-deposited, thin multilayers of phospholipids has been probed by high-resolution x-ray scattering. A model of pure displacement disorder in a one-dimensional crystal is applied to analyze the data, and reveals the existence in these films of at least three distinct pure states of hydration. These are characterized by an integer number of monolayers of water associated with each lipid bilayer. Transitions between pure states are similar in nature to staging transitions in intercalation compounds.

A model of classical relaxation in a one-dimensional potential with a hierarchical distribution of barriers is studied. The vanishing of an effective diffusion constant results in a transition of the low-lying states of the master equation from extended to localized, producing a transition from exponential to anomalous decay.

In situ transmission electron microscopy during the growth of ${\mathrm{NiSi}}_2$ on Si by molecular beam epitaxy shows that the metastable phase θ-${\mathrm{Ni}}_2$Si grows at ∼300 °C upon the reaction of certain deposited Ni films on (111) but never on (100) Si. Its existence correlates with the subsequent growth of type-A ${\mathrm{NiSi}}_2$ at 450 °C. The low energy of epitaxial interfaces can influence phase formation in very thin films, leading to unusual thickness-dependent behavior such as the type-B–type-A ${\mathrm{NiSi}}_2$ enigma.

Can commensurate states exist on incommensurate lattices? We introduce and study by simulated annealing a simple 1D model related to flux quantization on superconducting networks or charged particles on a substrate to answer this question. For periodic and quasicrystalline patterns we find a set of low-energy states related to both decimation symmetry and periodicity.

An ordered monolayer of chemisorbed sulfur adatoms on a Mo(001) surface is imaged in air, by means of scanning tunneling microscopy. The p(2×1) overlayer is observed, in agreement with low-energy electron-diffraction studies. The adsorption sites of the sulfur atoms on the molybdenum substrate are clarified, based on the measured corrugation amplitudes. Atomic resolution has also been obtained in the barrier height mode, by use of an ac synchronous detection, and with a better signal-to-noise ratio than in the constant current (topographic) mode.

It is suggested that the ‘‘supermodulus effect’’ observed for composition-modulated superlattices arises from the presence of the structurally disordered solid interfaces and not necessarily from electronic structure effects.

Four phases are observed in superconducting Bi-Ca-Sr-Cu-O samples. The superconducting phase, with onset temperature near 120 K, is a 15.4-Å-layered compound with composition near ${\mathrm{Bi}}_2$${\mathrm{Ca}}_1$${\mathrm{Sr}}_2$${\mathrm{O}}_9$ and an A-centered orthorhombic unit subcell 5.41×5.44×30.78 A${̊}^3$. X-ray diffraction and electron microscopy data are consistent with a structure of alternating perovskite and ${\mathrm{Bi}}_2$${\mathrm{O}}_2$ layers. High-resolution transmission electron microscopy images reveal a b-axis superstructure of 27.2 Å, numerous (001) stacking faults, and other defects.

Gaussian fluctuations have been observed in the specific heat of two single crystals ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Deviations from a BCS-type step are well accounted for by 3D Gaussian fluctuations whose amplitude ratio, in the cleaner sample, implies that the number of order-parameter components is 7±2. This result is inconsistent with an O(2) Ginzburg-Landau theory.

Ultrasound propagation in both known classes of high-$T_c$ ceramic superconductors reveals the presence of an anomaly above $T_c$, at $T_s$≃95 K in ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ and ≃120 K in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, suggesting that the state between $T_s$ and $T_c$ is the ‘‘normal’’ state preceding superconductivity. The anomalous elastic behavior at and below $T_c$ is dominated by the shear modulus and not by the bulk modulus, implying an unusually strong coupling of the order parameter to shear distortions. Implications of these results for superconductivity are discussed.