Alt-Grassberger-Sandhas theory together with the energy-dependent-pole-expansion method is used for the first time in the domain of molecular and chemical physics to calculate the scattering phase shifts and cross sections for three-dimensional elastic, inelastic, and rearrangement collisions in a model atom-triatom and diatom-diatom system at energies below triatomic dissociation. Results indicate that vibrational excitation is conducive to the reactive process which itself is also favored over inelastic collision. The Born approximation poorly represents these scattering phenomena.

In the three-level medium a ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ may be generated by atomic coherence effects which involve negligible saturation and, hence, are essentially "spontaneous-emission-free." This offers considerable experimental advantages for the generation of squeezed states of light via four-wave mixing or optical bistability.

It is demonstrated that a nonlinear Schrödinger equation with certain nonlinearities allows for an existence of multistable singular solitons (i.e., singular solitons with the same carried power but different propagation parameters). In nonlinear optics, these solitons may exist in the form of either short bistable pulses, or bistable self-trapping (both two- and three-dimensional).

Recent lower-hybrid current-drive experiments have clearly demonstrated that the current in a tokamak discharge can be maintained by rf drive alone. We have extended the operating regime of such plasmas to include ramping up of the current. We find that at densities of ∼2×${10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$ approximately 20% of the launched rf power is converted to magnetic field energy.

We outline a theory which suggests that the dynamics of point defects in crystals can be studied by coherent quasielastic neutron scattering. The theory assumes that the surrounding lattice distortion follows each defect instantaneously, and that the distortion fields of different defects can be linearly superposed. The energy width of the scattered intensity yields the hopping rate and jump vectors of the defects. We discuss systems for which the predicted effects for ionic defects are observable, pointing out that the detection of small-polaron hopping should also be possible.

Scanning tunneling microscopy has been used to determine the atomic structure of the clean Si(001) surface. The basic structural unit of the reconstruction has been resolved with a lateral resolution of ∼ 3 Å. Buckled and nonbuckled dimers appear to be present in roughly equal amounts, indicating that they have nearly the same energy. The presence of atomic-scale defects is discussed.

High-resolution Brillouin scattering has revealed the presence of numerous low-energy excitations in $\mathrm{K}{\mathrm{Br}}_{\mathrm{}(1-x)\mathrm{}}{\left(\mathrm{CN}\right)}_x$ alloys where $x<\mathrm{}0.5\mathrm{}$. They arise from the modification of the tunneling levels of the CN, and appear to have the uniform density of states necessary to explain some of the thermal properties of these alloys which are similar to those of amorphous materials.

Adsorption energies ${\epsilon }_a$ and recombination rate constants $K^s$ have been measured for spin-polarized hydrogen on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films as a function of thickness down to a few monolayers. The value of $\frac{{\epsilon }_a}{k}$ at saturation is 0.96(2) K and increase for very thin films; $K^s{B}^2{T}^{-\frac{1}{2}}=5.9\mathrm{}\left(5\right)\mathrm{}\times {10}^{-8\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{T}}^2$ ${\mathrm{s}}^{-1\mathrm{}}$ ${\mathrm{K}}^{\mathrm{½}}$.

We impose anisotropy in modified Hele-Shaw experiments by engraving a grid on one of the plates. Without anisotropy, the dynamics is dominated by tip bifurcations leading to a branched ramified structure. With anisotropy, a dendritic pattern forms, in qualitative agreement with the prediction of recent studies of local growth models.

We report measurements of the ultrasound attenuation and the changes in the sound velocity of the heavy-fermion superconductor (U,Th)${\mathrm{Be}}_{13}$. This material has shown evidence of two transitions, a bulk superconducting transition at $T_{c1\mathrm{}}$ and a lower one at $T_{c2\mathrm{}}$. Near $T_{c2\mathrm{}}$ we find a $\lambda$-shaped attenuation peak two orders of magnitude larger than the total contribution from particle-hole scattering, and a dip in the $c_{11}$ elastic constant. These results are consistent with those expected near a magnetic transition. A state of coexisting anisotropic superconductivity and (antiferro-) magnetic order is suggested below $T_{c2\mathrm{}}$.

The observation of a large piezoelectric contribution to the electroreflectance signal from the (111) $A$ face of CdTe is reported. This contribution, which is absent from the $B$-face signal, is associated with two-dimensional interfacial strains between the $A$ face and a thin native oxide film. Line-shape analysis is used to verify the nature of this contribution and to obtain values for the transition energy and other quantities which characterize the electronic band structure at the interface.

The electronic structure and total energies of eight elementary isolated-$T_d$-site point defects in GaAs have been calculated with use of the self-consistent Green's-function technique. We evaluate reaction energies and from these, using simple thermodynamic considerations, we predict which native defects should be abundant in As-rich or Ga-rich and in $n$-type or $p$-type material. Comparison with available experiments confirms our findings.

The first kinetic data displaying a time-dependent singlet-exciton-annihilation rate have been observed. Subpicosecond optical excitation of polycrystalline thin-film samples of $\beta$-hydrogen phthalocyanine resulted in excitonic saturation of the crystal lattice. The initial exciton decay yields the only unambiguous measurement of a characteristic annihilation interaction rate for molecular excitons on adjacent lattice sites.

This Letter presents for the first time the observation of the resonant optical Stark effect in a semiconductor. The excitonic system of ${\mathrm{Cu}}_2$O is well suited to the investigation of this effect because of its rather large binding energy and well-resolved resonances at low temperatures. The mixing between the $1S$ and $2P$ excitons by the electric field of a tunable high-power C${\mathrm{O}}_2$ laser leads to drastic changes in the one-photon spectrum around the $2P$ absorption line. Even details of the line shape are consistently described in terms of the frequency-dependent nonlinear susceptibility.

The ground-state energies of atoms and of rhombohedral and body-centered tetragonal atomic lattices have been calculated by density-functional theory for magnetic flux densities of ${10}^{12}$-${10}^{13}$ G. The cohesive energies derived are too small to support a finite-electric-field boundary condition at the polar cap in a pulsar magnetosphere.

Commensurate-incommensurate transitions in uniaxial two-dimensional systems are examined in the presence of impurities that interact with domain walls. Exact Bethe-Ansatz calculations, tested by numerical studies, indicate that randomness is relevant and leads to new critical behavior—a discontinuity in the specific heat and divergence of domain size with an exponent of 1. The results are interpreted in terms of the behavior of a single interface. Global phase diagrams are presented and experimental implications are discussed.

The analogy between self-avoiding walks and the zero-component magnetic system on a lattice is considered. The magnetic system possesses a natural boundary in the ($T-H$) plane below which we cannot continue it analytically. It is found, however, that we do not have to cross this boundary to obtain the semidilute regime. The scaling relation of des Cloizeaux is confirmed. There is also discovered a new phase below the boundary which is identified as a collapsed phase. This is quite unexpected because no attractive interactions are allowed.

We show how a generalization of continued fractions can be used to develop a renormalization-group formalism to study the behavior of maps of the two-torus. Such maps may mimic the universal behavior of dynamical systems with three mutually incommensurate frequencies. Numerical evidence indicates that "chaos" may occur in maps which are invertible. While we do not see scaling at chaos onset, subcritical scaling is observed and explained by the number theory.

We predict a novel family of baryons with or without the charm quantum number by quantizing the "maximal solitons" in the SU(4) Skyrme model. The baryon number $B$ of these solitons can take any integer value. The low-lying states with $B=1\mathrm{}$ belong to $4^{*}$ with spin $\frac{3}{2}$, ${20}^{*}$ with spin ½, $\frac{3}{2}$, $\frac{5}{2}$, or $\frac{7}{2}$, and ${20}^{*}$″ with spin $\frac{3}{2}$, $\frac{5}{2}$, or $\frac{9}{2}$. The charm-zero states among them could correspond to some of the observed resonances in meson-baryon scattering between 1.5 and 2 GeV. The lowest among the dibaryon states is an SU(3) singlet contained in the ${10}^{*}$ of SU(4) with spin 1, with mass in the range 2.5-3 GeV.

Hadronic production of charmed particles in association with muons from their semileptonic decay has been observed in a high-resolution streamer-chamber experiment performed at Fermilab. A miss-distance analysis of the pictures gives a signal of 17.3±4.7 events. Depending on the production model this corresponds to a range of neutron-nucleon cross sections from 8 to 50 μb, assuming an $A^{\frac{2}{3}}$ dependence.