Within the framework of boson parquet-diagram summations in perturbation theory, we show analytically that several simple approximations lead inevitably to the radial distribution function $g\left(r\right)\mathrm{}$ which would be obtained with the Jastrow hypernetted-chain variational method. This is the first derivation of the Jastrow result from perturbation theory. Without mentioning pair-correlation functions, we have a clear interpretation of $g\left(r\right)\mathrm{}$ and the structure function, $S\left(k\right)\mathrm{}$, in terms of diagram sums.

We have searched in niobium and tungsten for fractionally charged particles with effective nuclear charge $Z=N+\frac{1}{3}$ (where $N=0, 1, 2, 3, \dots$). In addition, we have looked for fractionally charged particles with $Z=\frac{2}{3} \mathrm{and} \frac{5}{3}$ in the same materials. No positive signal was observed, and concentration limits are reported.

Simulation data of SU(2) and SU(3) gauge theories with a variable number of flavors ranging from zero to eight are gathered together and new data for the eight-flavor case are presented. The finite-temperature chiral transition is studied on 4×$8^3$ lattices and the critical coupling is recorded as a function of the number of flavors $N_f$. The data exhibit stronger fermion screening as $N_f$ increases and the SU(3) data display a new first-order phase transition at $N_f=8\mathrm{}$.

We consider the massive Euclidean Gross-Neveu model. Thanks to the Pauli principle the bare perturbation expansion for the model with an ultraviolet cutoff is convergent in a disk whose radius corresponds by asymptotic freedom to a small finite renormalized coupling constant. The theory constructed in this way is physical (it satisfies Osterwalder and Schrader's axioms) in contrast with the planar theories obtained by similar perturbative expansions.

Differential cross sections for the double-isobaric-analog-state transition in the reaction ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}({\pi }^{+}, {\pi }^{-}){}_{}{}^{14}{}_{}{}^{}\mathrm{O}$ were measured over the angular range 20° to 130°. The cross section extrapolated to 0° is 3.9±0.5 μb/sr and the angle-integrated cross section is 15.3±1.6 μb. These values are much larger than expected from the trends of data at higher energies and may indicate the presence of new phenomena. The results are compared to recent six-quark bag-model calculations which predict such trends.

Splitting of the giant dipole resonance built on excited states of Er nuclei, observed in continuum $\gamma$-ray spectra from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{148}{}_{}{}^{}\mathrm{Sm}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$ reactions, reveals the persistence of prolate, ground-state-like deformation at elevated temperature $T\sim 1$ MeV and spins $I=0\mathrm{} \mathrm{to} 25\hslash$.

Outer-shell electrons coherently driven by intense radiation can transfer energy in a direct intraatomic process to inner-shell excitations. Provided that the effective momentum transfer $\Delta q$ is sufficiently low ($\Delta q\lesssim \frac{\hslash }{a_0}$), the amplitudes governing the coupling of the outer electrons to the atomic core constructively sum. The effective cross section, which can be related to fast atom-atom collisions (≳10 MeV/u), is evaluated in a limiting form closely resembling the Bethe result for inelastic electron scattering from atoms.

New experiments in a fast-flow reactor have uncovered a strong correlation between the reactivity of free iron clusters and cluster ionization thresholds: Clusters with low thresholds efficiently add molecular hydrogen, and the relative rates of this reaction closely follow variations in cluster-electron binding energy. This correspondence can be understood in terms of a requirement for metal-to-hydrogen charge transfer in the activation of the ${\mathrm{H}}_2$ bond.

The development of viscous fingering patterns has been observed from very early times to deep in the nonlinear regime for the flow of immiscible liquids in a Hele-Shaw cell. If the dimensionless viscosity contrast, $A$, is large, the number of fingers decreases rapidly with time after the initial pattern is established. If $A\simeq 0$, all fingers grow through the entire time range of this experiment. For all values of $A$ there is a power-law relation between the length of the longest finger and the time.

Numerical simulation of a three-dimensional interaction between colliding high-$\beta$ and low-$\beta$ plasmas, notably, the interaction of the solar wind with the magnetosphere, shows that reconnection occurs along a line on the contact surface for $\theta >90\mathrm{}°$ and pointwise for $\theta <90\mathrm{}°$, where $\theta$ is the angle between the colliding field vectors; accordingly, the flux transport is very efficient for $\theta >90\mathrm{}°$. An interesting finding is that reconnection is driven by nonlinearly generated diamagnetic currents for almost all field angles $\theta$.

The perfect screening of charge fluctuations in an equilibrium plasma is extended to the time-displaced structure function of a general inhomogeneous one-component plasma. We find that the long-wavelength modes oscillate undamped with a single frequency $\overline{\omega }$, ${\overline{\omega }}^2$ being an angular average of squares of plasma frequencies ${\omega }_p^2=\frac{4\pi e^2\rho }{m}$ in uniform systems with density $\rho$. Our results are derived rigorously from the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy under some reasonable assumptions on the spatial decay of correlations and contain as special cases previously obtained results of this kind.

Molecular-dynamics results on isothermally compressed liquids in the metastable region indicate a broad transition where an increasingly slowly relaxing component (nondecaying on the time scale of simulation) appears in the time-dependent structure factor and the diffusivity gradually vanishes in an Arrhenius-type manner. The observed dynamical behavior, clearly signifying the onset of structural arrest, appears to be the manifestation of nonlinear density fluctuations which have been shown recently to lead to a liquid-glass transition.

Soft-sphere scaling laws and nonequilibrium particle-dynamics simulations show that time-dependent shear-thinning behavior involving structural ordering (thixotropy), typically seen in complex colloidal dispersions, derives from a first-order thermodynamic phase transition between a normal shearing fluid and a partially ordered smectic phase. The transition stems from a perturbation of the equilibrium fluid freezing point by the applied strain rate.

The crystal structure with icosahedral point-group symmetry and long-range orientational order observed by Shechtman et al. in Mn-Al alloys can be explained as a multiple-q density-wave structure with only one length scale. Its existence and stability can be understood from a simple Landau theory. The diffraction spots can be indexed by six- (or more-) dimensional space-group symmetries. Icosahedral incommensurate structures constitute natural extensions of smectic, rodlike, and cubic crystal structures.

Theories for elasticity and dislocation defects in two-dimensional pentagonal and three-dimensional icosahedral quasicrystals are presented.

A simple natural Landau theory of two- or three-component systems is described, which appears to give a region of the phase diagram in which quasicrystalline ordering is the state of lowest free energy. The quasicrystals are stabilized by special geometric relations between the length scales characterizing the components. Three components are required to stabilize a two-dimensional quasicrystal (a Penrose tiling) but two components suffice to stabilize an icosahedral three-dimensional quasicrystal.

Measurements of the velocity of third sound $C_3$ in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on Nuclepore filter material with and without pores have been made and reveal striking structure as a function of film thickness for the porous case. In the absence of capillary condensation the data are understood in terms of a model which predicts the porous index of refraction as a function of coverage. It is likely that the techniques employed will be a useful alternative to the Brunauer-Emmett-Teller method for the determination of surface areas. For helium film coverages above those necessary for capillary condensation a fast propagating mode with speed $v\sim C_3^{\frac{1}{2}}$ is observed.

Over a narrow range of hydrostatic pressure, using high electric fields, we have observed microwave transitions between the on-center and off-center configurations in RbCl: ${\mathrm{Ag}}^{+}$. These are the first direct observations of the on-center configuration and provide a measurement of the separation between the on-center and off-center states.

The structure of C${\mathrm{D}}_4$ films adsorbed on a highly homogeneous MgO(100) surface has been probed by neutron diffraction. Both the monolayer and the bilayer crystallize in a commensurate square $c(2\mathrm{}\times 2)$ superstructure. This is the first clear evidence of a two-dimensional condensation of a solid van der Waals film with square symmetry. An order-disorder transition is observed and appears to be a first-order melting.

We have used electron diffraction to study submonolayers of Se adsorbed on Ni(100); $p(2\mathrm{}\times 2)$, $c(2\mathrm{}\times 2)$, and disordered phases were observed and the boundaries between them located. Symmetry arguments indicate that the phase diagram belongs to the universality class of the Ashkin-Teller model and allow us to predict the critical behavior near the phase boundaries and at the multicritical point where they meet. Our lattice-gas-model calculations support these results. The predicted critical behavior, and thus the universality arguments, should be tested by additional experiments, in particular synchrotron-x-ray diffraction.