A unified microscopic theory of the observed optical bistabilities in narrow-gap semiconductors, such as InSb and GaAs, is developed by calculating the complex, nonlinear dielectric function for arbitrary free-carrier concentrations from an integral equation for the polarization function.

By the observation of critical neutron scattering in the system Co${\mathrm{Br}}_2$·6[0.48${\mathrm{D}}_2$O, 0.52${\mathrm{H}}_2$O] it has been shown unambiguously that an intermediate phase does exist and that the two phase boundaries between the intermediate phase and the antiferromagnetic and spin-flop phases are of second order. The field dependence of the nuclear scattering in the magnetically ordered phases strongly indicates the presence of magnetoelastic coupling, which is possibly related to the existence of the intermediate phase.

A non-Hermitian eigenvalue equation is proposed to determine binding energies and widths of core excitons in semiconductors, taking into account the time dependence of screening effects through the dielectric matrix ${\epsilon }^{-1\mathrm{}}(\overrightarrow{\mathrm{r}}, {\overrightarrow{\mathrm{r}}}^{\prime };\omega )$. Deviations from static screening contribute both an increase of the binding energy and a narrowing of the Auger width. Numerical estimates of both effects for the Si $2p$ transition give qualitative agreement with experimental data when the exciton size is reduced by band-structure effects.

The first quantitative evidence of critical quantum fluctuations and superlocalization of the small polaron model in one, two, and three dimensions is presented. Starting from a discrete version of the Feynman path-integral representation of the partition function, the boson field is eliminated analytically and the polaron contribution is calculated by means of the standard Monte Carlo Method.

It is shown how the Fermi surface of metals and intermetallic compounds can be obtained from the two-dimensional angular correlation of positron-annihilation radiation. Results are given for both vanadium and ${\mathrm{V}}_3$Si. The Fermi surfaces are compared with the results of band-structure calculations.

This paper reports the first absolute measurement of the density dependence of the enhancement factor $g_{\mathrm{eh}}\mathrm{}\left(0\right)\mathrm{}$ for the electron-hole liquid in Ge. This factor $g_{\mathrm{eh}}\mathrm{}\left(0\right)\mathrm{}$ is a measure of the electron-hole spatial correlation function, and provides a valuable and sensitive test for the predictions of various many-body approximations. A strain-confined system of electron-hole liquid and free excitons is used. The data reported here agree quantitatively with the results of two different many-body-theory approximations.

The solitary-wave model of ferroelectric switching (poling) in $\beta$-phase poly(vinylidene fluoride) is developed to allow comparison with recent measurements of the detailed time dependence of this process. This model is found to be consistent with these measurements.

A classical analysis is presented of the microwave perturbation of electrons bound to the surface of liquid helium by their image charge. Since the classical dynamics can exhibit chaotic behavior, this one-dimensional quantum system provides a novel system for the experimental investigation of quantum stochasticity. Analytic estimates for the classical thresholds and rates for stochastic excitation and ionization are determined as functions of microwave field amplitude and frequency. The frequencies and powers required to study stochasticity in the quantum regime are readily available.

The charged-particle multiplicity distribution in $\tau$ decays is determined from data collected at the $e^{+}{e}^{-}$ storage ring PEP. The one-, three-, and five-charged-particle inclusive branching fractions are (86 ± 2)%, (14 ± 2)%, and < 0.5%, respectively.

From observed limits on the x-ray flux from neutron stars, a bound is placed on the product of the galactic flux of massive monopoles and the cross section for monopole-catalyzed nucleon decay: $F_M{\sigma }_{\Delta B}<~5\times {10}^{-49\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$.

The pion single-charge-exchange reactions ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}({\pi }^{\pm }, {\pi }^0)$ were measured at 164 MeV in the angular range 0° to 30°. The spectra show pronounced peaks at energies corresponding to the analog states of the isovector electric dipole resonance in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. The maximum (${\pi }^{-}, {\pi }^0$) cross section is 1.69±0.24 times the (${\pi }^{+}, {\pi }^0$) cross section. The measured angular distribution shapes and magnitudes are quite well described by distorted-wave impulse-approximation calculations based on the Goldhaber-Teller transition density.

Data for electron scattering from $d$, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the region between the elastic and quasielastic peaks are found to be unified by a nuclear scaling function. A simple model is used to understand the general features of the data, to understand the approach to scaling with increasing momentum transfer, and to extract the deuteron wave function from the deuteron data.

The role of mean free paths of product particles in high-energy nuclear collisions has been studied. In inclusive energy spectra the observed slope difference among $p$, $\pi$, and $K^{+}$ can be interpreted as due to the difference in mean free paths of these particles, suggesting that particles with longer mean free paths probe most sensitively the early, highly excited, hot phase of the collision. With use of the data of $\mathrm{pp}$ and $\pi \pi$ interferometries further discussions on the space-time evolution of the system are developed.

The population and the decay properties of the entry states in ${}_{}{}^{160}{}_{}{}^{}\mathrm{Yb}$ have been investigated via the reactions of 136- and 149-MeV ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ with ${}_{}{}^{146}{}_{}{}^{}\mathrm{Nd}$ with use of a $4\pi$ $\gamma$-ray multidetector system gated by a Ge counter for exit channel selection. A sudden decrease in the slope of the entry line accompanied by a change in the structure of the $\gamma$-ray spectrum is observed at multiplicity $M_{\gamma }=25\mathrm{}$. These changes indicate that a sudden transition from a prolate to an oblate shape occurs over a narrow range of spin above $45\hslash$.

A systematic measurement of a series of magnetic moments, using the method of low-temperature nuclear orientation of on-line separated isotopes, is presented for the first time. Strong arguments for fixing the spins of the odd-odd indium nuclei are derived from the gamma anisotropies. The characteristics of the ground states are discussed.

Collision kernels describing elastic collision effects in saturation spectroscopy are completely calculated for the metastable ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ state of neon, starting from true experiment-deduced potentials. Collision processes with and without metastability exchange are respectively considered. A direct observation of these two kernels is provided by a two-laser time-resolved saturated absorption experiment. They are in good agreement with theory.

The crossover behavior (temperature dependence) of universal ratios containing the intrinsic viscosity, the mean square radius of gyration, and the osmotic second virial coefficients is studied with use of the renormalization-group theory. The agreement with experimental results is satisfactory in some cases, but the wild scattering of existing experimental results calls for more accurate and systematic experiments.

The evolution of packets of water waves with wave amplitudes modulated in horizontal directions is investigated. The analysis is based on the Davey-Stewartson equations which are valid for water of finite depth. It is shown that for finite wave numbers $l$ of the transverse disturbances an instability occurs: The maximum growth rates are calculated and their physical implications are discussed.

Experiments on an underdense plasma irradiated with high-intensity 1.064-μm laser light have demonstrated that instabilities operating near ¼ the critical electron density can produce high-energy electrons. Experimental conditions were as follows: 7000-Å CH foil targets, 2.5×${10}^{15}$ W/${\mathrm{cm}}^2$, 900 psec, 400-μm spot diameter. The $\frac{3{\omega }_0}{2}$ light and 40-keV x rays occurred simultaneously at $t=-120\mathrm{}$ psec (laser peak at $t=0\mathrm{}$) and lasted only 300 psec full width at half maximum. Ten percent of the laser energy appeared as Raman light and 0.04% as $\frac{3{\omega }_0}{2}$ light.

A new operational regime has been observed in neutral-injection-heated ASDEX divertor discharges. This regime is characterized by high ${\beta }_p$ values comparable to the aspect ratio $A({\beta }_p<~0.65A)$ and by confinement times close to those of Ohmic discharges. The high-${\beta }_p$ regime develops at an injection power ≥1.9 MW, a mean density ${\overline{n}}_e>~3\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$, and a $q\left(a\right)\mathrm{}$ value ≥2.6. Beyond these limits or in discharges with material limiter, low ${\beta }_p$ values and reduced particle and energy confinement times are obtained compared to the Ohmic heating phase.