A nonlinear optical response of conjugated polymers is calculated using an electron-hole (exciton) representation. Equations of motion are derived which map the calculation of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ onto the dynamics of N(2N-1) nonlocal coupled anharmonic oscillators representing electrons and holes, N being the number of double bonds. The scaling of the static ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ with size and Coulomb interactions is shown to be directly correlated with the exciton coherence size associated with the relative electron-hole motion.

We present the results of a first-principles calculation for the geometrical and electronic structure of four ${\mathrm{C}}_{84}$ fullerene isomers. The method we use is based on the all-electron local-density functional approach for molecules with complete geometry optimization. Our study confirms that two isomers with ${\mathit{D}}_2$ and ${\mathit{D}}_{2\mathit{d}}$ symmetries are the most energetically favorable. Results from the present calculation are compared with recent experimental observations.

A class of exact large-amplitude traveling-wave solutions to the nonlinear Vlasov-Maxwell equations describing a one-dimensional collisionless magnetized plasma is obtained. These waves are complementary to the electrostatic Bernstein-Greene-Kruskal modes and can be classified as nonlinear fast electromagnetic waves and (slow) electromagnetic whistler waves. The wave characteristics are discussed for the case of a trapped-particle distribution function.

Nonlinear bound-free x-ray-absorption processes in an intense monochromatic x-ray radiation field such as an x-ray laser are investigated by using a collisional-radiative model. Above a threshold of the x-ray intensity, gain of stimulated free-bound emission is induced by strong photoionization. The wavelength of the stimulated free-bound emission is longer than that of the incident x rays, leading to a down-conversion of the frequency.

A large transient surge of shear viscosity is observed in the course of spinodal demixing caused by off-critical quenching of a polymer solution. A characteristic length is derived from light scattering measurements along with a related relaxation time ${\mathrm{\tau }}_{\mathit{M}}$ whose value agrees with previous independent experiments. Viscosity peak values plotted versus S${\mathrm{\tau }}_{\mathit{M}}$ (where S is the shear rate) lie on a master curve of the type (S${\mathrm{\tau }}_{\mathit{M}}$${)}^{\mathrm{-}\mathit{p}}$, where p≊2. This scaling procedure helps bring into focus the mechanism responsible for the novel observation.

The modified Simon-Hoh instability is observed in a collisionless cylindrical plasma, produced by a weak electron beam, in which electrons are strongly magnetized and the ions are essentially unmagnetized. The nonlinear evolution of this instability occurs through a sequence of sideband instabilities, thought to be induced by trapped ions, and can lead to a chaotic state.

Neutron diffraction demonstrates that ethanol physisorbed on the basal planes of graphite forms a commensurate unit cell with a melting temperature of approximately 205 K for coverages from 0.7 to 1.6 monolayers. This temperature is 46 K above the melting temperature of bulk ethanol, making this system unique among species physisorbed on graphite. A microscopic model of the system has been developed using a molecular dynamics simulation. It predicts a two-step mechanism for melting and demonstrates that hydrogen bonding plays a dominant role in stabilizing the two-dimensional solid.

We present a method which allows calculation of whole universal finite-size-scaling functions from Monte Carlo data. The crux of the new method is a technique of isolating the singular part from the total free energy. We apply this method to the 3-state Potts model on a square lattice and find the normalized scaling function Y(x) in the form 1+x+5.31${\mathit{x}}^2$-1.2${\mathit{x}}^3$-0.67${\mathit{x}}^4$ . . . near the bulk critical point x=0, together with the normalizing universal amplitude E=1.053 and a nonuniversal metric factor D=-0.387. It also behaves as Y(x)≊(±x${)}^{\mathit{d}\nu }$${\mathit{Q}}_{\pm }$ for large x in agreement with the hyperuniversality hypothesis with ${\mathit{Q}}_{+}$/${\mathit{Q}}_{\mathrm{-}}$≊1.

Brushes of grafted polystyrene have been studied using the classical small-angle neutron scattering techniques. The results show unambigously that the interface, in good solvent, has essentially a parabolic shape, with a small exponential tail. Furthermore, the local structure is similar to that of a semidulute polymer solution with a typical blob size equal to the distance between grafting sites. Mean-field and scaling theories therefore account for these results, within their own range of validity.

The motion of interacting point defects in a uniaxial nematic is considered in the one-constant approximation. It is shown that the dynamics depends in a decisive way on the reduction in nematic order near the defect core. The computed velocities of interacting defects are inversely proportional to their separation when defects approach each other from afar.

It is shown that by including both the spin-up and spin-down components of a spin-1/2 field, a parity conserving Chern-Simons theory can be constructed. For a single fermion field the model contains (aside from the mass and a coupling constant) a parameter ε which can have the values ±1. It is shown that only the positive value allows an interpretation in terms of anyons. This result thus serves to sharpen the frequently overlooked distinction between the Chern-Simons interaction and the anyon. While the former can be introduced into any (2+1)-dimensional field theory, the latter is seen to be a special limiting case obtained only in those instances for which relativistic and spin considerations can be ignored.

We consider a kicked system on the cylinder obtained upon quantization of a chaotic area-preserving map. We use the thermodynamic formalism to investigate the scaling properties of the fractal spectrum. In time evolution we observe anomalous diffusion with an exponent closely related to the Hausdorff dimension of the spectrum, and dependent upon the parameters of the system.

Dynamics involving a discrete second-order time derivative captures many essential, previously unexplained features of the collective behavior of complex, spatially extended systems such as cellular automata and coupled-map lattices. We argue that irrational frequencies can emerge from discrete-time dynamics, and that the second-order dynamics makes previous droplet-type arguments irrelevant for these models. These findings are supported by numerical examples.

The wrong-helicity states of a Dirac neutrino can provide an important cooling mechanism for young neutron stars. Based on numerical models of the early cooling of the neutron start associated with SN 1987A which self-consistently incorporate wrong-helicity neutrino emission, we argue that a Dirac neutrino of mass greater than 30 keV (25 keV if it is degenerate) leads to shortening of the neutrino burst that is inconsistent with the Irvine-Michigan-Brookhaven and Kamiokande II data. If pions are as abundant as nucleons in the cores of neutron stars, our limit improves to 15 keV.

In a four-fermion interaction model of the type suggested by Nambu’s BCS model the highest-mass particles, the τ lepton and t quark, are treated separately from the lower-mass members whose masses are assumed to be generated by self-interactions among degenerate fermions. If, furthermore, one assumes that the contributions of the lower-mass states cancel the logarithmic divergences in τ and t masses, excellent results for the mass spectrum are obtained for cutoff parameter Λ≊${10}^2$ GeV for leptons, and ≊${10}^4$ GeV for quarks, which are very reasonable compared to Λ≊${10}^{15}$ GeV obtained in previous schemes.

The proton elastic electric and magnetic form factors, ${\mathit{G}}_{\mathit{E}\mathit{p}}$(${\mathit{Q}}^2$) and ${\mathit{G}}_{\mathit{M}\mathit{p}}$(${\mathit{Q}}^2$), have been separately measured in the range ${\mathit{Q}}^2$=1.75 to 8.83 (GeV/c${)}^2$, more than doubling the ${\mathit{Q}}^2$ range of previous data. Scaled by the dipole fit, ${\mathit{G}}_{\mathit{D}}$(${\mathit{Q}}^2$), the results for ${\mathit{G}}_{\mathit{M}\mathit{p}}$(${\mathit{Q}}^2$)/${\mathrm{\mu }}_{\mathit{p}}$${\mathit{G}}_{\mathit{D}}$(${\mathit{Q}}^2$) decrease smoothly from 1.05 to 0.91, while ${\mathit{G}}_{\mathit{E}\mathit{p}}$(${\mathit{Q}}^2$)/${\mathit{G}}_{\mathit{D}}$(${\mathit{Q}}^2$) is consistent with unity. Comparisons are made to QCD sum rule, diquark, constitutent quark, and vector meson dominance models, none of which agree with all of the new data. The ratio ${\mathit{Q}}^2$${\mathit{F}}_2$/${\mathit{F}}_1$ approaches a constant value for ${\mathit{Q}}^2$>3 (GeV/c${)}^2$.

We present the result of a search for weakly interacting neutral bosons (X) produced in ${\mathrm{\pi }}^{\mathrm{-}}$p interactions at rest and decaying into ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pairs. The data sample searched contains 98 400 ${\mathrm{\pi }}^0$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$γ decays and 27 200 ${\mathrm{\pi }}^{\mathrm{-}}$p→${\mathit{ne}}^{+}$${\mathit{e}}^{\mathrm{-}}$ events, each with an ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ invariant mass between 25 and 139 MeV/${\mathit{c}}^2$. They were observed in the SINDRUM I magnetic spectrometer at the Paul Scherrer Institute (Switzerland). We measure upper limits for the branching ratios Γ(${\mathrm{\pi }}^0$→Xγ, X→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$)/Γ(${\mathrm{\pi }}^0$→all) and Γ(${\mathrm{\pi }}^{\mathrm{-}}$p→Xn, X→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$)/Γ(${\mathrm{\pi }}^{\mathrm{-}}$p→all) for X lifetimes between ${10}^{\mathrm{-}23}$ and ${10}^{\mathrm{-}11}$ s.

The derivation of the nucleon-nucleon force from the Skyrme model is reexamined. Starting from previous results for the potential energy of quasistatic solutions, we show that a calculation using the Born-Oppenheimer approximation, properly taking into account the mixing of nucleon resonances, leads to substantial central attraction. We obtain a potential that is in qualitative agreement with phenomenological potentials.

Levels in ${}_{}{}^{184}{}_{}{}^{}\mathrm{Pt}$ were studied from the decay of mass-separated ${}_{}{}^{184}{}_{}{}^{}\mathrm{Au}$ by gamma-ray and conversion-electron spectroscopy and by gamma-ray angular distributions from low-temperature nuclear orientation. In addition to coexisting ${\mathit{K}}^{\mathrm{\pi }}$=$0^{+}$ bands, coexisting ${\mathit{K}}^{\mathrm{\pi }}$=$2^{+}$ bands are observed at low energy. String interband electric monopole transitions are observed between states of the two ${\mathit{K}}^{\mathrm{\pi }}$=$0^{+}$ bands and also between states of the two ${\mathit{K}}^{\mathrm{\pi }}$=$2^{+}$ bands. The coexistence of ${\mathit{K}}^{\mathrm{\pi }}$=$2^{+}$ bands at low energy represents a new type of collective structure.

The behavior of many-electron wave functions near points where the positions of the some of the electrons coincide or where they coincide with positions of nuclei is investigated. A representation theorem for many-electron functions near such points is given extending previous results of Kato (cusp conditions) and Hoffmann-Ostenhof and Seiler. Furthermore, the influence of the Pauli principle on the local behavior of fermionic wave functions near many-particle coalescence points is studied.