Localized baglike solutions in the pairing theory of superconductivity are studied. Starting from the Bogoliubov–de Gennes equations on a two-dimensional square lattice for half-filled negative-U Hubbard model, cigar- and star-shaped bags are numerically obtained, inside of which the order parameter is reduced, self-consistently trapping an added quasiparticle. These nonlinear excitations are important when the coherence length is small as for the new high-temperature superconductors. Several experimental consequences are discussed.

About fifteen elements in the periodic table skip certain valences in all the compounds they form. This is understood to arise from a negative effective intra-atomic interaction. A model with such interactions as well as interatomic repulsions gives insulating behavior near half-filling and superconductivity farther from it, in agreement with the properties of ${\mathrm{BaPb}}_{\mathrm{x}}$${\mathrm{Bi}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{O}}_3$ and ${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{K}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{BiO}}_3$.

A small amount of bismuth was added to granular aluminum in order to enhance spin-orbit scattering. The metal-insulator transition was found to be shifted in accord with the antilocalization effect on an Anderson transition. The threshold for superconductivity is also shifted, to the same extent, remaining, as in the undoped system, separated from the metal-insulator transition by a narrow range of composition in which the samples are metallic but not fully superconducting. The correlation of the shifts demonstrates that superconductivity is suppressed by the vicinity of the metal-insulator transition.

Shubnikov–de Haas oscillations are observed in the low-temperature magnetoresistance of the organic metal β’’(BEDT-TTF${)}_2$${\mathrm{AuBr}}_2$, which indicate the presence of small carrier pockets in the Fermi surface. The nonoscillatory part of the magnetoresistance is highly anisotropic, and in one current direction an anomalously large negative magnetoresistance is observed. Some possible explanations for the negative magnetoresistance are considered, including an explanation based on magnetic-field–induced alignment of the carrier motion in the presence of scattering from internal boundaries.

Direct photolysis and electron-induced fragmentation of ${\mathrm{CH}}_3$Cl on a Ni(111) surface was observed in time-of-flight studies using 193-nm radiation. With 248-nm radiation, only electron-induced fragmentation was observed. With 351-nm radiation no fragments were observed. The electron-induced process was surface specific, occurring within a limited adsorbate thickness determined by the penetration depth of the excited substrate electrons into the adsorbate. Cross sections are reported as a function of coverage.

We define a Lyapunov exponent for a class of quantum dissipative systems which in the classical limit can undergo a cascade of period-doubling bifurcations into chaos. We do so by computing the average of a functional over a semiclassical trajectory for a dynamical system whose Poincaré section corresponds to the Hénon map. In the strongly dissipative limit we establish a scaling law which determines the way in which chaos can set in for finite values of Planck's constant.

We consider in two dimensions $L (L\ge 2)$ independent Brownian paths of common lengths $S$, all starting at the origin, and the probability $P_L$ that their trajectories do not intersect. For $S$ large, $P_L\sim S^{{-\zeta }_L}$ where ${\zeta }_L$ is universal. In 2D the ${\zeta }_L\text{'}\mathrm{s}$ are identified as Kac conformal dimensions with $c=0\mathrm{}$ central charge ${\zeta }_L=h_{0,L}=\frac{({4L}^2-1)}{24, L\ge 2}$. This is generalized to $L$ walks in a half-plane, with a common origin on the boundary.

The relationship between the maximum neutron-star mass and observable parameters of the equation of state is explored. In particular, the roles of the nuclear incompressibility and the symmetry energy are considered. It is concluded that, for realistic symmetry energies, the compression modulus cannot, by itself, be severely limited by observed neutron-star masses. Several directions for further study are suggested.

We report new limits on the nucleon lifetime obtained with the Irvine-Michigan-Brookhaven water Cherenkov detector. Techniques in track fitting, particle identification, and invariant-mass reconstruction provide information about multitrack events which leads to a significant reduction in background for most decay modes. Data obtained during the initial 417-day run and during the subsequent 69-day run with enhanced light collection are discussed. A background subtraction is made, yielding 90%-confidence-level lower bounds on the nucleon partial lifetime per branching fraction in the range (0.6-46.0)×${10}^{31}$ yr for 23 decay channels.

Timelike scalar and vector form factors are recalculated using chiral perturbation theory and dispersion theory. It is shown that chiral perturbation theory at the one-loop level violates the final-state theorem (i.e., unitarity). In order to satisfy this theorem, chiral perturbation theory should be applied to the inverse of the form factor whose result is shown to be equivalent to the Padé-approximant method.

We have measured the differential cross section for two-body deuteron photodisintegration at ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}=90\mathrm{}°$ and for photon energies between 0.8 and 1.6 GeV. At energies above ≃ 1.2 GeV, the data appear to obey a simple scaling law predicted by constituent-counting rules assuming parton degrees of freedom for the deuteron and nucleons. Agreement with model calculations based on meson exchange or "reduced nuclear amplitudes" is discussed.

We perform an analysis of exclusive heavy-ion reactions around 100 MeV/nucleon in terms of multipole moments of the momentum distribution of nucleons. Numerical calculations based on the Boltzmann-Uehling-Uhlenbeck theory are presented and compared to an approximate scaling function. We find that nuclei completely stop each other even at 100 MeV/nucleon, if $A_P+A_T>200\mathrm{}$ in symmetrical central collisions. We propose that the mass dependence of the quadrupole momentum tensor can be used to determine the nuclear compressibility.

The application of generalized quantum-defect theory to photodissociation processes is described. The theory is of particular value when sharp resonance behavior is present in the photodissociation cross section. We show how the theory is capable of correctly predicting very sharp resonance features without the necessity of repeatedly solving systems of coupled differential equations at a very closely spaced sequence of energies. All the essential quantities occurring in the theory are smoothly varying functions of the energy despite the sharp resonances they predict.

A monoenergetic positronium (Ps) beam of 0-60-eV energy and an angular width of ±5° is created by charge-exchange collisions of a slow positron beam passing through an Ar gas cell. The Ps beam is directed at a Lif(100) crystal, and reflected Ps atoms are detected at an annihilation target. With angles of incidence of 50° to 60° we observe a specularly reflected beam with a maximum reflected fraction $R=(30\mathrm{}\pm 5)\%$ at a Ps energy of 7 eV. At higher energies (20-60 eV) the reflectivity ($R=0.5\%$ at 60 eV) can be ascribed principally to a short Ps mean free path $\lambda =(0.75\mathrm{}\pm 0.15)$ Å, and to a lesser extent a Ps inner potential of about 4 eV.

Inhibited spontaneous emission in atomic physics has been intensively investigated recently. In solid-state physics these effects are no less important. We have studied the spontaneous emission of light from electron-hole recombination in optically thin GaAs double heterostructures. The electron-hole radiative recombination rate coefficient $B$ is not purely a property of the GaAs itself, but depends strongly on the optical-mode density and refractive index of the medium in which it is immersed. The spontaneous-emission rate can be markedly increased or decreased depending on whether the surrounding refractive index is higher or lower than that of GaAs.

We compare a microscopic simulation of a fluid made up of 5000 hard disks and maintained at a supercritical Rayleigh number to the corresponding macroscopic hydrodynamics. Very good quantitative agreement is demonstrated.

Forced Rayleigh scattering is used to measure the self-diffusion of charged polyball suspensions driven far from equilibrium by an oscillating shear flow. Aside from the usual enhancement of the diffusion due to Taylor dispersion, we observe an additional isotropic contribution which increases linearly with the shear rate. This anomalous contribution is absent in noninteracting samples and increases monotonically with the interparticle interaction strength.

We report experimental evidence indicating that two-dimensional zinc electrodeposition clusters in the limit of small ionic concentration and small voltage are self-similar with generalized dimensions $D_q=1.66\mathrm{}\pm 0.08$. Small-mass clusters obtained in a numerical simulation of diffusion-limited aggregation in a strip geometry are also found to be self-similar, with $D_q=1.60\mathrm{}\pm 0.02$. These results suggest that electrodeposition and diffusion-limited clusters have a similar geometrical structure.

Studies by small-angle neutron scattering have been conducted on a main-chain thermotropic nematic polymer composed of alternating mesogens and flexible spacers. The anisotropy of chain conformation in the nematic phase was measured in the melt and in a dilute nematic solution. The variation as a function of degree of polymerization and spacer length is discussed. The results agree favorably with a mean-field theory of wormlike chains.

The phase diagram for liquid-crystal models of hard molecules is studied by use of a nonlocal density functional for the free energy. Nematic, smectic-$A$, and smectic-$C$ phases can be obtained from purely packing effects without any attractive interaction. A system of parallel hard spherocylinders shows a continuous phase transition from the nematic to a smectic-$A$ phase, in good agreement with existing computer simulations. In a system of parallel oblique cylinders we predict first- or second-order phase transitions between the smectic-$A$ and the smectic-$C$ phases.