Doppler-free two-photon dispersion has been observed in rubidium vapor near the $5S_{\frac{1}{2}}-5\mathrm{}{D}_{\frac{5}{2}}$ two-photon transition. The experiment was performed by looking at the shape of the transmission peaks of a Fabry-Perot cavity filled with rubidium. When the nonlinear susceptibility is large, a bistable behavior occurs. A nonperturbative treatment of the susceptibility is required for the interpretation of these results.

A novel coherent transient effect is observed following a rapid shift in the relative phase of an exciting laser and sample polarization. An analytical expression is derived for the signal which is a sum of exponentially decaying terms with characteristic times involving both $T_1$ and $T_2$. Phase switching is a powerful approach to the study of optical relaxation and is applied here to measure dephasing in La${\mathrm{Cl}}_3$: ${\mathrm{Pr}}^{3+}$.

It is shown that the four-wave interaction of a powerful stationary pump field and a weak signal field in a dielectric modeled with an assembly of anharmonically interacting particles exhibits a behavior radically different from that expected from the usual perturbation theory assuming small oscillation amplitudes. A light-induced critical behavior in the intensity of the reflected phase-conjugated field is obtained as a consequence of the interaction of the pump with the matter.

The first far-infrared laser scattering measurements from a tokamak plasma are reported. Spatially and temporally resolved power spectra of density fluctuations in the Microtor plasma have been studied with an absolutely calibrated far-infrared scattering system which allowed measurement of the wide range of wave numbers from $k=3\mathrm{} \mathrm{to} 50$ ${\mathrm{cm}}^{-1\mathrm{}}$. The wave-number dependence of the scattered power of $0.6\lesssim k_{\perp }{\rho }_i\lesssim 2$ was found to satisfy a $k^{-3.5\mathrm{}}$ form.

The magnetic equation of state of spin-polarized atomic hydrogen is reported, relating density to magnetic field ($B\lesssim 10.5$ T), temperature ($T\gtrsim 300$ mK), and atomic flux used to fill the sample cell. Densities greater than ${10}^{16}$ atoms/${\mathrm{cm}}^3$, more than two orders of magnitude greater than initial observations, have been achieved. Results are also presented on the buildup and decay of the density as a function of external conditions. Preliminary observations on the Fermi fluid, atomic deuterium, are reported.

High-resolution magnetic-resonance studies are reported for an atomic hydrogen gas confined in a closed glass bulb with superfluid-${}_{}{}^4{}_{}{}^{}\mathrm{He}$-coated walls in zero magnetic field and for $1.0<T<\mathrm{}1.3\mathrm{}$ K. H atoms at low density, ${10}^{11}<n_{\mathrm{H}}<5\mathrm{}\times {10}^{12}$/${\mathrm{cm}}^3$, in the equilibrium helium vapor, are found to recombine into molecules in times or order ${10}^3$ s. Results are presented for the rate constant for recombination, the diffusion constant and pressure shift for H in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, and for $T_1$ due to H-H spin exchange.

The time-dependent specific heat of amorphous Si${\mathrm{O}}_2$ has been measured between 0.1 and 1 K over the range ∼ 1 sec to 10 μsec. The decoupling of the roughly $T$-dependent contribution to specific heat is observed in submillisecond time scales, in qualitative agreement with the predictions of the tunneling model. However, the excess $T^3$-dependent specific heat is not decoupled on the time scales involved.

Monte Carlo simulations of the structure of ${\mathrm{Eu}}^{3+}$-doped Be${\mathrm{F}}_2$ glass show that in contrast to the almost perfect fourfold coordination of Be by F, the rare-earth sites are disordered and no fixed coordination number can be defined. The calculated distribution of ${\mathrm{Eu}}^{3+}$ electronic energy levels is in general agreement with the inhomogeneous broadening observed in optical spectra and measured using laser-induced fluorescence line-narrowing techniques.

The experimental data which we have gathered on the superfluid mass content in films of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures yield a characteristic length which varies markedly both with temperature and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentration. The data are analyzed to present their implications on the energy gap and on other parameters characterizing the two-dimensional helium-boundary-interface excitation spectrum.

All previous theoretical predictions of the decay of superflow in thin ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films are in conflict with the recent data obtained by Ekholm and Hallock. For the first time an analysis is presented of two effects: vortex creation and annihilation at the edges of a film of finite width, and the effect of vortex density relaxation. The latter result is used to discuss the above-mentioned discrepancy between theory and experiment.

A theoretical calculation is presented of the excitation spectra of bound excitons and bound multiexciton complexes associated with donors in Si. The energy separations between the one-electron excited states and the ground states obtained in this calculation are in good agreement with the experimental data and indicate that the assignment of electrons to the one-electron donor states forms a good basis for these states.

Inelastic-neutron-scattering studies on ${\mathrm{Fe}}_x{\mathrm{Cr}}_{1-x}$ for $x=0.34\mathrm{} \mathrm{and} 0.26$ reveal well-defined spin-wave excitations in the ferromagnetic regime and show a significant decrease of the spin-wave stiffness as the temperature is lowered towards the mictomagnetic or spin-glass regime. Within the spin-glass phase, no well-defined excitations are seen, but an intense quasielastic peak is observed.

The observation of flux exclusion approaching Meissner proportions (${\chi }_V=-\frac{1}{4\pi }$ cgs units) in pressure-quenched CdS at 77 K is reported. The results can be naturally and simply interpreted on the basis of superconductivity at 77 K. If the superconductive state is not the appropriate description, then a new high-temperature collective quantum state must be invoked.

The valence-band satellite in nickel (and in palladium) is described as due to a local excitation of a $d$ electron. The calculated excitation energies show a good quantitative agreement with experiment. Also the core-level satellites are treated and, where possible, compared with appropriately selected Auger spectra.

The first evidence of nuclear spin-lattice relaxation (NSLR) by magnetic soliton fluctuations is presented. The NSLR rate ${T_1}^{-1\mathrm{}}$ of ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ in an enriched crystal of ${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$NMn${\mathrm{Cl}}_3$ is measured versus field ($20\lesssim H\lesssim 80$ kOe) and temperature ($2\lesssim T\lesssim 4.2$ K). At a given temperature an exponential divergence of ${T_1}^{-1\mathrm{}}$ is observed as a function of $H$. This drastic behavior is in complete agreement with the soliton model for planar antiferromagnetic chains.

Detailed conductivity measurements on cis-${\left(\mathrm{CH}\right)}_x$ doped with As${\mathrm{F}}_5$ as a function of electric field for various temperatures are reported. For concentrations well below the semiconductor to metal transition, an exponential dependence of the resistance on the reciprocal of the electric field is observed. This result is consistent with a model of metallic domains embedded in a dielectric. It is their charging energy which determines both the temperature dependence and the field dependence of the conductivity. Non-Ohmic effects for concentrations above the transition are also discussed.

Measurements of the Hall voltage of a two-dimensional electron gas, realized with a silicon metal-oxide-semiconductor field-effect transistor, show that the Hall resistance at particular, experimentally well-defined surface carrier concentrations has fixed values which depend only on the fine-structure constant and speed of light, and is insensitive to the geometry of the device. Preliminary data are reported.

A four-parametric set of solutions to $G_{\mu \nu }=0\mathrm{}$ is presented; for discrete values of three of the parameters, the metric is constructed from rational functions. The set has the structure of an Abelian group, with flat space-time as group identity, and contains many solutions of physical interest.

The Boltzmann equation considered by Bobylev, and Krook and Wu (BKW) is rewritten in the form of a stochastic equation, similar in form to the kinetic equations of Tjon and Wu, and Ernst. A new class of models, which reflects the transformation of the BKW model to other dimensionalities, is constructed, and its equilibrium distributions, non-equilibrium solutions (corresponding to the BKW mode), and $H$ theorems are found.

Conservation of $B-L$ and nonconservation of $B$ are shown to follow simply from the weak isospin properties of quarks and leptons; namely that left-handed particles and right-handed antiparticles have weak isospin ½, that all other fermions are singlets, and that particles with half-integral isospin can only be created or annihilated in pairs.