A simple model is studied describing low-temperature free-carrier dynamics in extrinsic photoconductors due to capture by impurity atoms and impact ionization. Numerical results are presented which exhibit period doubling, chaotic behavior, and elevated effective noise temperatures. Photoconductor properties necessary for chaotic behavior are discussed.

A condensed-matter analog of (2+1)-dimensional electrodynamics is constructed, and the consequences of a recently discovered anomaly in such systems are discussed.

We consider a grand canonical ensemble of self-avoiding walks and study its properties on a Bethe lattice of coordination number $q=3\mathrm{}$ and in one dimension, without using the $n\to 0$ trick. The study enables us to identify the ordering field as the activity for the walk ends. We also identify the order parameter of the problem for the first time: The order parameter is the probability of obtaining an infinite self-avoiding walk (reaching the "boundary"), having started at the origin in some direction.

Dynamic light-scattering measurements on fractal aggregates of colloidal silica show that the mean decay rate (Rayleigh linewidth) of the intensity autocorrelation function scales like $q^{2.7}$, where $q$ is the momentum transfer. This result is compared to the $q^3$ prediction for percolation clusters and the more complex results derived here for cluster-cluster aggregation.

A new technique is described for direct measurement of mass changes of a conducting polymer electrode during electrochemical cycling. A quartz microbalance is used, with one of its contacts serving as the working electrode. Its high sensitivity permits resolution of mass changes less than 100 ng/${\mathrm{cm}}^2$. The results demonstrate that on reduction of poly(pyrrole perchlorate), charge compensation is achieved by diffusion of the lithium cation into the polymer and not the perchlorate anion into solution.

Evidence for a narrow state decaying into an $F$ meson and a photon has been obtained in $e^{+}{e}^{-}$ annihilation events at 29-GeV c.m. energy. This state lies 139.5 ± 8.3(stat.) ± 9.7(syst.) MeV above the $F$-meson mass and is consistent with the expected $F^{*}$ meson. The $F$ mesons are identified by a peak in the $K^{+}{K}^{-}K{\pi }^{\pm }$ mass at 1.948±0.028±0.010 GeV.

The $\gamma$ decay from the $2_3^{+}$ state at about 2-MeV excitation in the nuclei ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ba}$, ${}_{}{}^{142}{}_{}{}^{}\mathrm{Ce}$, and ${}_{}{}^{144}{}_{}{}^{}\mathrm{Nd}$, with 84 neutrons, is shown to be consistent with its identification as the lowest state of mixed symmetry in the U(5) limit of the neutron-proton version of the interacting-boson model.

Oblate shapes of four high-spin isomers in Gd isotopes (${10}^{+}$ in ${}_{}{}^{144}{}_{}{}^{}\mathrm{Gd}$, ${\frac{13}{2}}^{+}$, ${\frac{27}{2}}^{-}$, and ${\frac{49}{2}}^{+}$ in ${}_{}{}^{147}{}_{}{}^{}\mathrm{Gd}$) were established in measurements of the signs of the respective quadrupole moments. A combined tilted-multifoil and time-differential perturbed-angular-distribution technique was employed. The present results are direct evidence for the presumed oblate nature of high-spin states in the $A\approx 150$ region.

A quasicrystal is the natural extension of the notion of a crystal to structures with quasiperiodic, rather than periodic, translational order. We classify two- and three-dimensional quasicrystals by their symmetry under rotation and show that many disallowed crystal symmetries are allowed quasicrystal symmetries. We analytically compute the diffraction pattern of an ideal quasicrystal and show that the recently observed electron-diffraction pattern of an Al-Mn alloy is closely related to that of an icosahedral quasicrystal.

A unique CO chemisorption mode (${\alpha }_1\mathrm{CO}$), with the lowest reported CO stretching frequencies (1150-1330 ${\mathrm{cm}}^{-1\mathrm{}}$) on any clean or promoted metal surface, has been identified on Cr(110) by means of EELS, ESDIAD, LEED, AES, and oxygen coadsorption experiments. A "lying down" binding configuration in twofold-symmetric hollow sites is proposed for ${\alpha }_1\mathrm{CO}$ molecules. For ${\theta }_{\mathrm{CO}}>0.25\mathrm{}$, a second mode (${\alpha }_2\mathrm{CO}$) bound perpendicular to Cr(110) is populated.

Nematic order induced near a solid boundary in an otherwise isotropic liquid crystal is studied theoretically, at temperatures just above the bulk nematic-isotropic phase transition. Three distinct regimes are found, depending on the strength of orientational torques at the boundary: (i) strong orientational order, (ii) strong orientational order followed by a first-order transition to a state of weak orientational order as temperature is raised, and (iii) a state of weak orientational order.

We show that the results of Lüty and Ortiz-Lopez relating the cyanide reorientation rates to the high-temperature phase diagrams of alkali-halide-alkali-cyanide mixed crystals can be understood within simple mean-field theory.

The Kohn-Sham scheme is used to calculate the interaction energy of an ${\mathrm{H}}_2$O monomer with clusters simulating different adsorption sites on an Al(100) surface. A well-defined on-top ground state with the molecular plane tilted away from the normal is found. The adsorption is accompanied by a substantial donation of charge to the metal. The electron energy-loss spectrum is dominated by a frustrated rotation of the molecule.

In the Landau-Lifshitz equation which describes the evolution of spin fields in nonequilibrium continuum ferromagnets, by stereographic projection of the unit sphere of spin onto a complex plane, it is shown that the effect of the Landau-Lifshitz-Gilbert damping term is a mere rescaling of time by a complex constant. Consequently, for any given undamped motion of spatially regular and/or irregular spin structures, the nature of the damping can be analyzed exactly in a simplified manner.

The discovery of a "two-layer" behavior of the surface resulting from the concomitant action of sputtering and annealing at high temperatures is reported. As a result the intensity of thermal He beams scattered from a surface under ion bombardment exhibits pronounced oscillations. The use of this new feature for the measurement of sputtering yields of virgin surfaces is proposed.

Core excitonic effects at the $L_{2,3}$ absorption edge were investigated with a combination of optical and synchrotron-radiation photoemission techniques in crystalline, amorphous, and amorphous-hydrogenated silicon and silicon-carbon alloys. Our results support a Frenkel-type nature of these excitons.

We report on a new electron-spin-resonance (ESR) effect occurring in one-dimensional systems at very low frequencies. The ESR line is narrowed and enhanced when the chain axis is directed along the linearly oscillating field (perpendicular to the static one). In the ideal case the line becomes singular for $\omega \to 0$. The phenomenon has been observed by measuring ESR absorption in a quasi-one-dimensional ${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$NMn${\mathrm{Cl}}_3$ sample at frequencies 25 and 225 MHz. This effect is described by Bloch equations with damping on a single spin component, in perfect agreement with a microscopic theory.

We present an extension of the density-functional theory to the lowest states compatible with arbitrary constraints. A broad class of interesting excitation problems connected with charge and magnetization fluctuations can be treated by such a method. We demonstrate the usefulness in ab initio calculations for Ce impurities.

The Dirac propagator ${(\gamma p-m)}^{-1\mathrm{}}$ has been calculated as a path integral over a recently proposed classical action. Thus a new formulation is given to the longstanding problem of extending the path integration to discrete quantum spin as an integral over a continuous variable.

The Hamiltonian formulation is analyzed in a basis of gauge-invariant loop states. A natural geometric asymptotic approximation for the eigenstates of the Hamiltonian is developed to identify the phase diagram of the theory. Satisfactory explicit calculations are performed in the case of $Z_2$.