It is shown that published third-sound measurements are in very good agreement with the theoretical result by Nelson and Kosterlitz that in the limit as $T\to {T_c}^{-}$ the ratio of the density per unit area of superfluid component to the critical temperature, $T_c$, is the universal constant $\left(\frac{2}{\pi }\right){\left(\frac{m}{\hslash }\right)}^2{k}_{\mathrm{B}}$.

Measurements of the sound velocity in vitreous silica and in amorphous metallic Pd-Si between 0.4 and 20 K and in the frequency range 10-450 MHz are reported. Above 4 K, the sound velocity varies linearly versus temperature and logarithmically versus frequency. From the analysis of the experimental results it is shown that the linear temperature dependence of the sound velocity cannot be explained with relaxation processes.

Inversion, through direct absorption of far-infrared laser light, of a three-level electronic system of ${\mathrm{V}}^{4+}$ ions in dilute solution in ${\mathrm{Al}}_2$${\mathrm{O}}_3$ leads to stimulated emission of 24.7-${\mathrm{cm}}^{-1\mathrm{}}$ phonons. Stimulated phonon emission manifests itself in the presence of ballistic flow of longitudinal phonons along the $c$ axis of ${\mathrm{Al}}_2$${\mathrm{O}}_3$, but this flow is not observed under spontaneous phonon emission.

High-resolution inelastic electron spectra have been measured for a saturated coverage of atomic hydrogen adsorbed on W(100). The fundamental modes corresponding to H occupying a $C_{2v}$ point-group-symmetry bridge site plus an overtone ($v=0\mathrm{}\to 2$) of the intense symmetric stretch are resolved at angles off the specular direction. The intensity of these modes (normalized to the elastic peak) exhibits resonantlike behavior as a function of the impact energy.

A low-energy electron-diffraction intensity analysis of the Fe{001} $c(2\mathrm{}\times 2)$-CO structure confirms the results of photoemission studies by other workers suggesting dissociation of the CO molecule. The data are fitted well by a model with random occupation of fourfold symmetrical sites by carbon and oxygen atoms in a $c(2\mathrm{}\times 2)$ configuration, similar to that proposed earlier by Felter and Estrup and by Riwan, Guillot, and Paigne for Mo{001} $c(2\mathrm{}\times 2)$-CO.

The reflection coefficient of 290-GHz phonon pulses between LiF or NaF and liquid helium was measured with surfaces prepared in situ by cleaving at 1 K. Nearly 100% reflection was found, thus demonstrating the absence of the anomalous Kaptiza conductance in contrast to all previous experiments.

We compare the results of precision electron $g$-factor experiments at low energy and at 110 keV. The agreement between these measurements constitutes the most precise laboratory confirmation to date of the predictions of special relativity. Relativistic electromagnetic theory and Thomas precession are verified in this test. We also consider limits on possible effects of acceleration.

The mass formula for the $\pi$, $\eta$, $A$ (axion) system is analyzed taking into account explicit instanton effects. It is argued that small-size instantons serve as an independent source for the axion's mass which may render the axion more massive than the mixing of the bare axion with $\pi$ and $\eta$.

In collisions with N${\mathrm{H}}_3$ molecules it is observed that $\mathrm{Xe}\mathrm{}\left(\mathrm{nf}\right)\mathrm{}$ Rydberg atoms are selectively excited to discrete, more highly excited states. These states are identified by the technique of field ionization and are found to be displaced in energy from the initial Rydberg states by amounts equal to the rotational spacings of the N${\mathrm{H}}_3$ molecule. These measurements demonstrate, for the first time, the conversion of rotational energy to electronic energy in collisions.

The positions of quasi Landau resonances as a function of magnetic field strength are reported for the first time. These resonances were observed near the ionization limit in the even-parity channels of barium and strontium with use of two-photon laser spectroscopy. The change in the resonance positions as the field strength is varied and the energy spacings of the resonances agree well with hydrogenic semiclassical theory. Resonance profiles, but not positions, are found to be species dependent.

We report the direct observation of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}$, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{Li}_2^{+}{}_{}{}^{}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{Li}_2^{+}{}_{}{}^{}$ arising from single collisions between beams of lithium atoms in the presence of a laser field. Although the relative intensity of atomic ions reflects the natural isotopic abundance, a pronounced enrichment of ${}_{}{}^{13}{}_{}{}^{}\mathrm{Li}_2^{+}{}_{}{}^{}$ relative to ${}_{}{}^{14}{}_{}{}^{}\mathrm{Li}_2^{+}{}_{}{}^{}$ is observed.

A novel technique is described for high-resolution spectroscopy. The time evolution of a coherent superposition of atomic substates created by a short resonant laser pulse is monitored via changes in polarization of a delayed probe pulse due to stimulated emission. As a demonstration the method was applied to the Zeeman splitting of the $6{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ state of ytterbium.

The observed anisotropy of hot electrons in a turbulently heated plasma is consistent with a Maxwellian electron distribution shifted in the direction of the applied electric field.

Wave trajectories propagating obliquely to the magnetic field in toroidal plasmas are studied theoretically. Results show that the ordinary wave at the appropriate incident angle is mode converted to the extraordinary wave at the first turning point and is further converted to the electron Bernstein wave while passing a loop or a folded curve near the second turning point and is cyclotron-damped away, resulting in local electron heating, before arriving at the cyclotron resonance layer.

Thr turbulent state initiated by the electron-beam-ion two-stream instability is investigated. The spectrum of fluctuations is concentrated on a Cherenkov cone with a single modulus of phase velocity, azimuthally symmetric around the beam axis. The associated diffusion in velocity space occurs mainly on the surface of a double cone, complementary to the Cherenkov emission cone. This model is supported by an experiment in which the three-dimensional diffusion coeffient is measured with use of space-time correlation functions.

We show how many mesomorphic states illustrate the following general scheme: The symmetry group of an equilibrium state of Euclidean-invariant quantum statistical mechanics is a subgroup $H$ of the Euclidean group $E$ such that the orbit $\frac{E}{H}$ is compact. Moreover, the homotopy groups of $\frac{E}{H}$ yield a classification of the topologically stable defects and configurations of these ordered media. This suggests a predictive value of this scheme for yet unobserved media and for defects.

We have observed the structure and velocity of laser-driven shock waves in aluminum foils. We have measured shock velocities as high as 13 km/s and shock luminosity rise-times less than 50 ps, and we have inferred pressures of 200 GPa and shock-front thicknesses <0.7 μm. These results suggest that such techniques may be used for measuring equation-of-state parameters and studying the detailed structure of shock fronts.

The dynamics of the freezing process is studied at the surface of a growing ice single crystal. Light ($\lambda =488\mathrm{}$ nm) is scattered by fluctuations of the nonequilibrium interface. The linewidth of the scattered light is proportional to the square of the scattering vector. For a scattering angle of 90° one measured about 2 krad/s. The intensity depends on the growth rate and shows a hysteresis in that dependence.

The spin-glass transition of a magnetic material with quenched impurities is studied with use of perturbative mode-mode coupling theory, without the use of the replica method. In mean-field theory we find that the transport coefficient $\tilde{\Gamma }$ diverges just above the spin-glass freezing temperature $T_f$ like ${(T-T_f)}^{-\frac{1}{2}}$ for $d>\mathrm{}4\mathrm{}$ and ${(T-T_f)}^{\frac{2}{d-1\mathrm{}}}$ for $2<d<\mathrm{}4\mathrm{}$.

The spin polarization of electrons field emitted from single-crystal iron surfaces was observed to be (+25±5)% for emission along [100], (+25±5)% along [111], and (-5±10)% along [110] (the polarization is positive for majority-spin electrons). These results correlate qualitatively with expectations from the bulk band model. However, quantitative calculations with bulk wave functions gave strongly parameter-dependent results. For meaningful calculations the effect of the surface on the wave functions must be included.