Zeeman shifts in one of the $J=2\mathrm{}$ order-parameter collective-mode multiplets in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ have been measured in magnetic fields up to 0.16 T. The observed shifts are extremely nonlinear at higher fields. The extent of nonlinearity decreases as $\frac{T}{T_c}\to 0$ and for a given $\frac{T}{T_c}$ is more predominant at lower pressures and/or frequencies. The observed effects can be attributed to the distortion of the $B$-phase energy gap in the presence of a magnetic field as suggested by Schopohl, Warnke, and Tcwordt.

The $\alpha \rightleftarrows \beta$ phase transition in AgI is studied with use of the new molecular-dynamics technique which allows for a dynamical variation of the shape and size of the cell. In the present model, upon heating of $\beta$-AgI, the iodine ions undergo a hep → bee transformation and silver ions become mobile, whereas the reverse transformation is observed on cooling of $\alpha$-AgI. The calculated $\alpha \rightleftarrows \beta$ transition temperature and structural and dynamical properties are in good agreement with experiments.

It has been suggested that quantum fluctuations would cause the interface of quantum crystals such as ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ to be rough even at $T=0\mathrm{}$. It is shown quite generally that this is incorrect: All stable faces must have nonzero roughening temperatures. This implies that the equilibrium shape of all crystals (classical or quantum) is faceted at sufficiently low temperatures. A criterion for estimating the roughening temperature of various crystal faces is discussed.

It is shown that the $P-T$ diagram for the $\gamma -\alpha$ phase transition in ${\mathrm{Ce}}_{0.8}$${\mathrm{La}}_{0.1}$${\mathrm{Th}}_{0.1}$ has two critical points with first-order transitions between them. This phase diagram follows from the Fermi-liquid properties of the $4f$ electronic system, the experimentally known volume dependence of $k_{\mathrm{B}}{T}_{\mathrm{FL}}$, and the dilution effect of alloying.

The magnetic excitations in the well isolated ground-state doublet ${\Gamma }_7$ of CeAs have been studied by inelastic neutron scattering. The spin-wave excitation is split into two modes of transverse polarization as a result of exchange anisotropy. One of these modes exhibits nearly zero energy gap and quadratic dispersion which have not previously been observed in antiferromagnets. A random-phase-approximation calculation based on a single-$\overrightarrow{\mathrm{q}}$ type-I antiferromagnetic structure is in good agreement with the observed spectrum.

The surface response function $d_{\perp }\left(\omega \right)$ (the frequency-dependent centroid of the induced charge density) determines the influence of a metal surface on all dynamical processes occurring well outside of it. It is shown that this function can be measured directly by use of electron-energy-loss spectroscopy.

Energy transfer from ${}_{}{}^3{}_{}{}^{}n{\pi }^{*}$ pyrazine to GaAs(110) has been studied. Within experimental error, a classical dielectric model quantitatively reproduces measurements of the distance-dependent lifetime for emitter-surface separations from 430 to 20 Å. Analysis of the energy transfer shows that the molecular electronic excitation is dissipated through the creation of electron-hole pairs in the solid by the high-wave-vector components of the dipole near field.

The magnetic hyperfine field for isolated Fe impurities in Ca metal has been measured as a function of temperature ($86 \mathrm{K}<~T<~300 \mathrm{K}$) by use of the technique of perturbed angular distributions. The data indicate the presence of an unquenched orbital local moment. This is the first such observation for Fe impurities in any metal host. The results could not be explained by the assumption of a purely ionic configuration for the impurity and are interpreted in terms of the Friedel-Anderson model.

The author derives a set of two-dimensional band structures arising from spin- and/or lattice-induced commensurate symmetry breaking of the high-temperature, ambient-pressure phase of ditetramethyltetraselenafulvalene-$X$. These band structures are proposed as the framework for many of the low-temperature transport properties of these compounds and are shown to be consistent with experiment in those cases where the broken symmetry conditions have been well established.

If one requires as an equilibrium condition that the creation rate for gravitons or minimally coupled massless scalar particles vanishcs in the radiation-dominated early universe, then the expansion of the universe must be isotropic and the cosmological constant must be zero. A similar equilibrium condition involving massive particles is also discussed.

A new type of bifurcation to chaos is pointed out and discussed. In this bifurcation two unstable fixed points or periodic orbits are created simultaneously with a strange attractor which has a fractal basin boundary. Chaotic transients associated with the coalescence of the unstable-unstable pair are shown to be extraordinarily long-lived.

The author reinterprets Witten's analysis of the dyons in the SU(2) Georgi-Glashow model, showing that his peculiar formula for their electric charge is due to a symmetry of the semiclassical configuration space under the noncompact group of real numbers. This method of analysis extends easily to the SU(5) dyons and shows that their color hypercharge is also arbitrary when the vacuum angle $\theta$ is nonzero.

Far away from a fundamental magnetic monopole of the SU(5) grand unified theory, the full theory reduces to one whose gauge group is unbroken SU(3) ⊗ U(1). The algebra of generators of smooth gauge transformations in this unbroken theory, however, will not in general contain any subalgebra isomorphic to su(3). This means that global color rotations are not always defined. The resulting inability to classify semiclassical dyon states by their color climinates an apparent paradox related to the vacuum angle $\theta$.

In the Glashow-Salam-Weinberg model with single Higgs doublet, upper bounds on the Higgs-boson mass and the coupling constant are obtained: $m_{\mathrm{H}}<~455$ GeV, $\lambda <~32.8$ based on one-loop radiative corrections and the existence of the spontaneously broken absolute vacuum. Under two-loop radiative corrections, these bounds are improved to $m_{\mathrm{H}}<~320$ GeV, $\lambda <~9.35$.

The electron-positron pair production for a beam of very energetic photons (or ultra-relativistic charged particles) is predicted to be significantly enhanced when the beam is aligned along crystallographic rows of atoms. Channeling in crystals lowers the total energy of the created electron thus enabling the photon to decay in a one-step process. The new mechanism yields a rapidly increasing flux of electrons and positrons of comparable energies above a threshold near 30 GeV.

Hydrogen atoms are excited into high Rydberg states following fast proton impact on a thin carbon foil. The effects of various electromagnetic fields on these atoms are observed by measuring the integral ionization yield in an additional constant electric field. These experiments reveal oscillations in that yield, regularly spaced as a function of electric field. A model is outlined that quantitatively describes the observed field dependence. This new phenomenon indicates selective excitation of the Stark substates.

Strongly inverted ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ spins have been used to investigate the transient response of a low-$Q$ Ca${\mathrm{F}}_2$ NMR laser after $Q$ switching. The experiments are in fair agreement with the numerical results from a mean-field approach to superfluorescence. The model is based on a set of Bloch-type dynamic order-parameter equations for a distribution of spin packets. It takes into account such well known phenomena as spin fanning, hole burning, and spectral diffusion and, therefore, links basic concepts of nonlinear optics with conventional NMR in solids.

Large-scale quantum fluctuations have been observed in the energy of light pulses produced by transient stimulated Raman scattering of laser pulses in hydrogen gas. This experiment verifies recent theoretical work on quantum initiation of light scattering, and provides an example of a macroscopic system which begins in a pure state and, through essentially pure-state quantum evolution, ends up having highly unpredictable values of its physical observables.

The problem of a collection of two-level atoms interacting with a single-mode classical electromagnetic field is considered. It is found that the model with an initial population inversion exhibits chaotic behavior that becomes stronger for higher atomic number density. The chaos is characterized by Fourier analysis and the maximal Lyapunov exponent. In the rotating-wave approximation, however, there is no prediction of chaos.

An approach to sustaining steady-state toroidal currents in tokamaks by use of two separate electron cyclotron heating sources is described. One source creates a minority group of collisionless, barely trapped electrons. The second source uses frequency modulation to detrap some of these with a large degree of toroidal asymmetry. The power dissipated in sustaining the steady-state toroidal current can be less than 1 W/A for typical tokamak conditions.