In the effective-mass approximation, the mass $M^{*}$ of an exciton is just the sum of the electron and hole masses, ${m_e}^{*}+{m_h}^{*}$. However, the effective-mass approximation is invalid if the forces are strong. Here we derive a plausible formula for the $n$ th bound state of the exciton: ${M_n}^{*}=\frac{({m_e}^{*}+{m_h}^{*})}{(1-\frac{K_n}{W})}$, where $K_n$ is the kinetic energy in the bound state and $W$ is one-half the sum of the electron and hole bandwidths. For Wannier excitons, $\frac{K_n}{W}\ll 1$, while for Frenkel excitons, $\frac{K_{1s}}{W}\to 1$, and the composite particle is effectively localized.

Large reversible changes have been observed in the orientation of the $4p^{\prime }{}_{}{}^2{}_{}{}^{}F_{\frac{7}{2}}^{}{}_{}{}^{}$ level of fast ${\mathrm{Ar}}^{+}$ ions excited by scattering from surfaces with chemisorbed ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, and ${\mathrm{H}}_2$ gases. Equilibrium values of the circular polarization fraction $\frac{S}{I}$ of the decay radiation at 461 nm are related to surface coverage, to the adsorbed species, and to the adsorbant metal. An interpretation in terms of multiple collisions ending with charge transfer is discussed.

This paper reports the discovery of short-term variability in the planetary-scale-size vertical electric field measured in the stratosphere. Measurements were made on superpressure balloons at 26-km altitude separated by up to 3000 km. Data are presented which show that the large-scale current system is variable, with twice the amplitude of the average diurnal variations, on time scales of tens of minutes to hours.

We investigate the classical stability of Freund-Rubin-type solutions $M^{\mathrm{pqr}} \left[\mathrm{SU}\mathrm{}\right(3)\mathrm{}\otimes \mathrm{SU}\mathrm{}(2)\mathrm{}\otimes \frac{\mathrm{U}\mathrm{}\left(1\right)\mathrm{}}{\mathrm{SU}\mathrm{}\left(2\right)\mathrm{}}\otimes \mathrm{U}\mathrm{}(1)\mathrm{}\otimes \mathrm{U}\mathrm{}(1\left)\right]$, $Q^{\mathrm{pqr}} \left[\mathrm{SU}\mathrm{}\right(2)\mathrm{}\otimes \mathrm{SU}\mathrm{}(2)\mathrm{}\otimes \frac{\mathrm{SU}\mathrm{}\left(2\right)\mathrm{}}{\mathrm{U}\mathrm{}\left(1\right)\mathrm{}}\otimes \mathrm{U}\mathrm{}(1\left)\right]$, and $N^{\mathrm{pqr}} \left[\mathrm{SU}\mathrm{}\right(3)\mathrm{}\otimes \frac{\mathrm{U}\mathrm{}\left(1\right)\mathrm{}}{\mathrm{U}\mathrm{}\left(1\right)\mathrm{}}\otimes \mathrm{U}\mathrm{}(1\left)\right]$ against relative dilatations between the coset directions. It is shown that $M^{\mathrm{pqr}}$ is stable only for $\frac{98}{243}<~\frac{p^2}{q^2}<~\frac{6358}{4563}$, $Q^{\mathrm{pqr}}$ is stable only for a certain region of $\frac{p^2}{r^2}$ and $\frac{q^2}{r^2}$, while $N^{\mathrm{pqr}}$ is stable for any $\frac{p^2}{q^2}$ against these small fluctuations.

Quantum effects of the transition to chaos of an (almost) classical dynamical system are studied by renormalization-group methods. Planck's constant is a relevant variable which determines the time $t_h\propto {\hslash }^{-\frac{1}{\gamma }}$ at which the system crosses over to quantal behavior. The exponent $\gamma$ is 6.039 for the limit of period-doubling bifurcations and is 3.04 for the disappearance of the final Kolmogorov-Arnold-Moser trajectory in the standard map. For the standard map the scaling of the energy diffusion coefficient is also calculated.

To test the formation of a deconfined QCD plasma in relativistic nuclear collisions, we propose measuring the charge anticorrelations of pions of similar momenta. These anticorrelations will be greatly weakened by plasma formation, and their kinematic characteristics will also change in a predicted way.

A formula for the two-pion correlation function is derived for an arbitrary chaotic source when the emission spectrum from each point in space-time is known. The experimental fact that pions with high momentum in the center-of-mass frame are more correlated than lowmomentum pions is explained by a collective expansion of the source. A simple model illustrates how the pion correlations can be used to measure the expansion velocity of a nuclear fireball.

A new self-consistent model of the incompressible Euler equations in two dimensions is presented. The vorticity is assumed to be distributed in well separated disjoint piecewise-constant elliptical finite-area vortex regions (FAVORs) $D_k$ with area $A_k$. The evolution equations for four variables that describe each FAVOR are derived by truncating a physical-space moment description by omitting terms $O\left({\left(\frac{A_k}{R_{k\alpha }^2}\right)}^2\right)$. ($R_{k\alpha }$ is the inter-FAVOR centroid distance.) The model is validated by comparing steady-state configurations and dynamical evolutions with contour dynamical results.

The existence of an interchange instability strictly associated with electron inertia is demonstrated. This is characterized by a growth rate significantly larger than the usual ioninertial Rayleigh-Taylor rate and by self-generated magnetic fields localized around the accelerating plasma surface. This novel instability may be partially responsible for the observed magnetic fields in ablatively accelerated laser plasmas.

The effectiveness of plasma heating by electron Landau interaction in the lower hybrid range of frequencies in tokamak plasmas is demonstrated. Upon injection of 850 kW of rf power at a density of ${\bar{n}}_e\simeq 1.4\times {10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$, an electron temperature increase of 1.0 keV and an ion temperature increase of 0.8 keV was achieved. These results are compared with transport and ray-tracing code predictions.

We present laboratory evidence of modulationally unstable electron wave packets which can be described by a nonlinear geometrical optics theory. Growth times for self-contraction are found to be much faster than ion response times and the bursts do not appear to be related to Zakharov Langmuir-wave collapse.

A theory of metal-hydrogen systems is described which predicts how the phase diagram will change when low concentrations of substitutional impurities are added to the system. Realistic models for ${\mathrm{Nb}}_{0.95}$${\mathrm{Mo}}_{0.05}$-H and ${\mathrm{Nb}}_{0.85}$${\mathrm{Mo}}_{0.15}$-H systems are developed which are lattice-gas analogs of the random-field Ising model. By Monte Carlo simulation the phase diagrams are obtained and compared with recent experiments.

The effects of a periodic excitation on the transition to chaos via the period-doubling cascade are considered. Numerical experiments on discrete systems suggest that the Lyapunov characteristic exponent satisfies scaling in the vicinity of the transition. A renormalization-group approach is used to predict the dependence of the critical exponent on the ratio of the driving to the internal frequencies. When this ratio is equal to the reciprocal of the golden mean, it is shown that this critical exponent is the same as for an external random noise.

A graph theory of single-spin-flip kinetic Ising models is developed and applied to a class of spin models with strongly cooperative dynamics. Self-consistent approximations for the spin time correlation function are presented. One of the dynamical models exhibits a glass transition with no underlying thermodynamic singularity. The approximation for the time correlation function predicts a critical temperature, below which small fluctuations from equilibrium in the thermodynamic limit cannot relax in a finite amount of time.

The first shock-compression experiments on liquid helium are reported. With a two-stage light-gas gun, liquid He at 4.3 K and 1 atm was shocked to 16 GPa and 12 000 K and double shocked to 56 GPa and 21 000 K. Liquid perturbation theory has been used to determine an effective interatomic potential from which the equation of state of He can be obtained over a wide range of densities and temperatures.

An atomically abrupt Al-GaAs interface was observed for Si-doped GaAs at 200 K by atom-probe mass spectroscopy. At 300 K an Al-As mixed layer having a low As concentration and containing no Ga was formed. Formation of the mixed layer at 200 K for Zn-doped GaAs could be attributed to a promoted reaction by Zn. A decrease in the thickness of the unreacted Al layer, an increase in the As concentration in the mixed layer with temperature, and the abrupt AlAs-GaAs interface indicate Al diffusion into GaAs and stable Al-As bonding.

The apparent dichotomy between the covalently delocalized nature of Si: Fe (as suggested by its reduced hyperfine field, its extended spin density, and the occurrence of two closely spaced stable charge states within 0.4 eV) and the atomically localized picture (suggested, among other reasons, by the stability of a high-spin ground-state configuration) is resolved through a self-consistent Green's-function calculation with the self-interaction-corrected local-spin-density formalism.

The lifetime $\tau$ of the zero-voltage state of a current-biased Josephson junction in the thermal limit has been measured in the presence of a weak microwave perturbation. When the microwave frequency is close to the plasma frequency of the junction, the junction is "resonantly activated" out of the zero-voltage state, with a corresponding reduction in $\tau$. The results are well explained by numerical simulations.

Measurements of the real and imaginary parts of the complex index of refraction $\hat{n}[\hat{n}=(1-\delta )+ik]$ of gold have been performed in the 280- to 640-eV range by a novel technique based on diffraction in transmission gratings. Uncertainties of approximately ±8% for $\delta$ and approximately ±18% for $k$ have been attained in this first experimental determination. A theoretically predicted absorption resonance at 600 eV has been verified for the first time, and a significant disagreement with previously reported values of $\delta$, derived from Kramers-Kronig analysis, has been established.

We investigate the dissipative motion of a particle coupled to a normal Fermi fluid, where particle-hole pairs are the relevant low-energy excitations. It is shown how to derive an effective action by use of linear-response theory and integrating out of the fermions' degrees of freedom. Its properties are discussed in detail, with particular emphasis on the similarities and connections with other descriptions of friction widely used in the literature.