The first detailed physical explanation of the dynamics of short pulses in a dispersive system that has loss is presented. The result provides a simple algorithm for predicting pulse behavior without the need for asymptotic analysis. It was derived specifically for electromagnetic pulses in Lorentz media, making it applicable to low-energy pulses in resonant media, pulses in plasmas and dielectrics, remote ionospheric measurements, and many pulsed laser systems. It appears likely, however, that the result applies to even more general wave systems.

Transverse waves at the fundamental ($F$) and second-harmonic whistler sidebands of ${\omega }_p$ can be excited via the nonlinear electron current ${\overrightarrow{\mathrm{j}}}_{\mathrm{nl}}=-en\overrightarrow{\mathrm{v}}$ generated by whistler solitons interacting with Langmuir waves. The intensity of the emission is higher than in the weak-turbulence three-wave interaction regime. Radiation at $F$ dominates. For oblique propagation angles it is in the extraordinary as well as in the ordinary mode.

Megaelectronvolt ion scattering is applied to a study of the initial stage of heteroepitaxy of the Ge/Si(111) system and it is shown that the interface is uniform and pseudomorphic at first, after which island formation takes place and strains are introduced at the interface. This change occurs after deposition of a threshold film thickness of ∼ 3 monolayers. More than 10 monolayers are distorted at the interface after deposition of about 30 monolayers of germanium.

Dynamic light-scattering techniques have been employed to study the nature of phase transition in a single-phase microemulsion system near the cloud-point temperature. Critical opalescence and the critical slowing down of the composition fluctuations were observed near the transition temperature. It is further observed that there exists a close parallelism between the critical phenomena and the cloud-point transition in microemulsions.

With reasonable values of the parameters characterizing a statistically rough solid-helium interface, a first-order perturbation calculation correctly predicts the magnitude, temperature dependence, and pressure dependence of the Kapitza conductance above 0.2 K.

A mechanism, based on the competition between short-range positional order and longrange antiferroelectric order, is presented for reentrant liquid crystals. Antiferroelectric frustration is inherent to the triangular close packing of layers normal to the molecular axis, but is lifted at intermediate temperatures by positional disorder. Phase diagrams evaluated by using prefacing transformations include the doubly reentrant case exhibiting, as temperature is lowered, nematic, bilayer smectic, reentrant nematic, and monolayer smectic phases.

The electrical resistivity of spin-glasses is discussed in terms of the elementary excitations of Walker and Walstedt, and excellent quantitative agreement with experimental data is obtained. With the same model an explanation for the local spin relaxation time behavior in spin-glasses is also given.

Photoemission spectra of ${\mathrm{O}}_2$ physisorbed at 10 K on graphite exhibit vibrational structure in the $2\pi$ band. With the assumption of a Morse potential the potential for the physisorbed ion can be calculated, which is slightly steeper than in the gas phase resulting in a higher vibrational frequency. The difference in equilibrium distance between the ion and the neutral molecule is reduced by about 20% compared to the free molecule.

The effect of thermal and structural disorder on the electronic structure of hydrogenated amorphous silicon is investigated by measurement of the shape of the optical absorption edge as a function of temperature and thermal evolution of hydrogen. The data are consistent with the idea that the thermal and structural disorder are additive, and suggest that the disorder, rather than the hydrogen content, is the fundamental determining factor in the optical band gap.

The spatial distribution of the electric field within a dielectric sample is shown to be obtainable unambiguously from the time dependence of the open-circuit voltage or short-circuit current during the propagation of a stress wave across the sample. Experiments in which the pressure wave is generated by the impact of a pulsed laser beam on a metal target bonded to the dielectric plate under investigation have led to the first straightforward visualization of electric field distributions in solid dielectrics.

Light scattering spectra of photoexcited electron-hole plasmas in GaAs show features due to the multicomponent nature of the plasma, and also specific to the presence of holes. At low energies, ∼5 meV, we observe a Landau-damped acoustic plasma mode. At higher energies inter-valence-band transitions of the photoexcited holes modify the spectra of coupled optic-plasmon-LO-phonon modes.

A driven anharmonic oscillator is described which exhibits period doubling and chaotic behavior. The measured behavior of the oscillator under successive period doublings is in quantitative agreement with a recent theory which describes the behavior of nonlinear systems. Both the scaling and the convergence rate predicted by the theory are verified by the experiment. The oscillator also exhibits period tripling and quintupling.

For a given statistical state at time $t_0$, the expectation value of some observable at a later time $t_1$ is expressed in a variational form. Different trial choices for the quantities to be varied (state and observable) generate different approximations, in which the evolution of the state is optimally fitted to the measured quantity. Examples are given in the context of mean-field theories.

The production of states of fractional charge in the early universe and by cosmic rays is calculated when quantum chromodynamics is broken at a scale less than that of the quantum chromodynamics scale parameter $\Lambda$. It is shown that it is possible to produce a detectable abundance of fractional charge states by both mechanisms.

A model-independent upper limit on the time-reversal-invariance (TRI) violating amplitude in elastic $\mathrm{pp}$ scattering based on a direct experimental reconstruction of the scattering matrix is presented. This analysis is made possible by a set of sixteen polarization parameters measured at the Swiss Institute for Nuclear Research for 579-MeV elastic $\mathrm{pp}$ scattering at six center-of-mass angles between 66° and 86°. Less than 1% is found for the fraction of TRI-violating cross section.

The emission of light charged fermions from a one-dimensional potential step and from a Julia-Zee dyon is analyzed. Even for the case of massless fermions, this emission process violates chirality and provides a simple illustration of the axial anomaly.

As an extension of a four-body investigation by Haberzettl and Sandhas, a partial decoupling of the $N$-body equations by Alt, Grassberger, and Sandhas is performed in such a way that only transitions into a given type of two-cluster configuration remain coupled. Taking into account particle identity, one then has to solve only a single-channel integral equation. All other reaction channels are obtained by quadratures. The resulting expressions resemble optical potential models.

The infinite parameter Kac-Moody algebra $\mathsf{C}\mathrm{}\left[t\right]\mathrm{}\otimes G$, whose elements are loops in $G$ and which is related to the vertex operator for the string model when $G=\mathrm{sl}\mathrm{}\left(2c\right)\mathrm{}$, is identified as the hidden-symmetry algebra of the two-dimensional chiral models. These observations suggest that a Kac-Moody Lie algebra is the hidden symmetry of Yang-Mills fields, a phenomenon which, if true, might lead to complete integrability and nonperturbative information. This algebra, also relevant to integrable soliton theory, may elucidate the classical and quantum inverse method for the chiral theory.

The origin of the large higher-order effects in radiative corrections is traced to charged-lepton scattering. The long radiative tail of the one-photon inclusive cross section, which does not arise in lowest order, is responsible for these large effects.

The $I=0\mathrm{}$ $S$-wave $\pi -\pi$ phase shifts have been determined in the $\pi -\pi$ mass range from threshold to 0.96 GeV by use of data from the reaction ${\pi }^{+}p\to p{\pi }^{+}{\pi }^0{\pi }^0$ at 8 GeV/c. The resulting phase shifts confirm the phase shifts determined from previous ${\pi }^{+}{\pi }^{-}\to {\pi }^{+}{\pi }^{-}$ data above 0.7 GeV, but are significantly below the accepted phase shifts below 0.6 GeV where our phase shifts are now consistent with current-algebra predictions.