Raman spectra have been obtained on $2H-\mathrm{Ta}{\mathrm{Se}}_2$ at room temperature and in the commensurate charge-density-wave, superlattice state. At low temperatures six new low frequency modes are seen having symmetries consistent only with a symmetric, three-charge-density wave. A microscopic model of the lattice dynamics of this state is fitted to the four strongest new Raman lines. These are assigned to amplitude and phase modes of the charge-density wave.

Voltage measurements made inside a superconducting weak link suggest that under ac conditions a nonequilibrium quasiparticle current exists and stimulates a quasiparticle potential.

Piezoelectric powders excited by two or more rf pulses are found to exhibit echoes characteristic of anharmonic oscillators. At high rf powers, the echoes show complex dependence upon the pulse separation, power, and widths.

The second-order Raman scattering and the temperature dependence of the ferroelectric soft mode in $\mathrm{AB}{\mathrm{O}}_3$ perovskites are explained quantitatively in terms of a nonlinear shell model by assuming an anisotropy in the oxygen polarizability. This model yields a new explanation for the origin of ferroelectricity in perovskites.

Photoemission-yield-spectroscopy measurements are reported for a set of cleaved $n$-GaAs(110) surfaces, clean or gradually oxygen covered. Depending on the cleavage, the Fermi level at the clean surface is observed at the bulk position or toward midgap. For the first time, the existence of surface states in the gap, close to the conduction band is directly demonstrated when the density of the defect-induced surface states found near the valence-band maximum is small.

The transverse correlation range $\xi$ and the susceptibility in the critical region has been measured by neutron scattering. A special technique required to resolve the superdiverging longitudinal correlation range has been utilized. The results for $\xi$ together with existing specific-heat data are in remarkable agreement with the renormalization group theory of systems with marginal dimensionality. The ratio between the susceptibility amplitudes above and below $T_c$ was found to be 2 in accordance with renormalization-group and meanfield theory.

The spins-2-spin-$\frac{3}{2}$ gauge multiplet for supergravity is coupled to the spin-1-spin-$\frac{1}{2}$ vector multiplet in a locally supersymmetric way.

The reaction $\overline{p}p\to 2\pi$ with polarized targets exhibits asymmetry. From this we extract the intrinsic parity of the antiproton.

Measurements of $R^{\nu }$ and $R^{\overline{\nu }}$, the ratios of neutral current to charged current $\nu$ and $\overline{\nu }$ cross sections, yield neutral current rates for $\nu$ and $\overline{\nu }$ that are consistent with a pure $V-A$ interaction but 3 standard deviations from pure $V$ or pure $A$, indicating the presence of parity nonconservation in the weak neutral current.

It has recently been predicted by De Rújula et al. that $m\left(D^{+}\right)-m\left(D^0\right)\sim 15$ MeV. The purpose of this Letter is to criticize this prediction and to examine in detail the mechanism responsible for meson mass splittings. It is concluded that electromagnetic splittings of hadrons cannot reliably be estimated using the present atomic models of hadrons. New terms are probably needed in the electromagnetic part of the potential.

The direct (Coulomb) formation of electron vacancies in the $1s\sigma$ state of superheavy quasimolecules is investigated for the first time. Its dependence on the impact parameter, projectile energy, and its contribution from excitations into the continum and higher bound states are determined.

The angular distribution of Xe $\mathrm{Xe} 5s\to \epsilon p$ photoelectrons has been measured yielding an asymmetry parameter $\beta =1.4\mathrm{}\pm 0.1$ at 304 Ă. This deviation from the intuitively expected result $\beta =2\mathrm{}$ (for a pure ${cos}^2\theta$ distribution) for photoionization of an $s$ subshell of a ${}_{}{}^1{}_{}{}^{}s_0^{}{}_{}{}^{}$ target is a direct manifestation of anisotropic interactions experienced by the electron during the ejection process. The combined effect of interchannel and spin-orbit coupling is proposed as the likely interaction mechanism.

For light focused on spherical plasmas, we obtain new results giving the power absorbed by inverse bremsstrahlung and resonance absorption as a function of the focusing scheme. For a given beam profile and lens, there is an optimum focus to maximize total absorption. Linearly polarized beams lead to asymmetric absorption. Good agreement with experimental absorption and scattered light data is obtained.

Acoustoelectrically amplified phonon beams were used to attenuate anomalous transmission of x-rays in InSb at 77°K. It is found that anomalous transmission is a very sensitive probe for phonon detection. The mean square atomic displacement and the associated strain produced by the intense phonon beam can be accurately determined. Weakly enhanced phonon modes other than those predicted by the acoustoelectric theory have also been detected using this x-ray technique.

Properties of the nematic-isotropic phase transition in surface-aligned nematic films are calculated using the Landau-de Gennes theory. It is shown that there is a critical thickness $D^c$ of the film below which the transition from the nematic phase to the isotropic phase becomes continuous. At the critical thickness the transition is second order and exhibits critical behavior. For $N-[e-\mathrm{methoxybenzylidine}]p-\mathrm{butylaniline}$, $D^c$ is calculated to be about 1000 Å, and the critical temperature is 0.17 K above the bulk first-order transition temperature. Magnetic and dimensional effects are discussed.

The phonon density of states of amporphous ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloys has been calculated for the entire range of concentrations. My calculation is based on the cluster Bethe lattice approximation in which the Bethe lattice leaves out everything but the short-range order. Effects of like-atom and unlike-atom bonds are studied. Localized resonant and extended states are identified.

Using light-beating spectroscopy we detect the "soft" mode associated with the secondorder smectic-$A$ to smectic-$C$ phase transition in undecylazoxymethylcinnamate. This mode corresponds to angular fluctuations of molecules with respect to the normal of the layers. In the $A$ phase both the amplitude and the relaxation time of these fluctuations diverge with a mean-field critical exponent $\gamma =1.0+0.15\mathrm{}$.

Inelastic electron scattering spectra of nitrogen, sulfur, and carbon core excitations in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) are presented. The spectra exhibit sharp excitonic transitions to final states localized on separate ${\mathrm{TTF}}^{+}$ or ${\mathrm{TCNQ}}^{-}$ ions followed at higher energies by a continuum of transitions to free-electron states. In contrast to prior photoemission experiments, we find no evidence for neutral molecules in the bulk of TTF-TCNQ.

The 49-K phase transition in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) is interpreted in terms of a phason instability of the charge-density-wave ground state. The order parameter couples linearly to an external strain, and the phase transition can be suppressed by clamping the crystal. Recent neutron scattering experiments show indications of this effect. Renormalization-group calculations yield pure classical critical behavior at this phase transition, and $\mathrm{XY}$ exponents for the 54-K transition.

The interpretation of contact-charge-exchange experiments using polymers requires the existence of electronic states contiguous to the Fermi energies of contacting metals. The atomic and electronic relaxations, both of the molecular moiety on which the charge is localized and of the surrounding polymer matrix, combine to provide suitable localized intrinsic electronic states.