Positron lifetime measurements were made in silicon powder samples with different grain size and doping. Two lifetime components were detected in low-resistivity $n$ samples and in high-resistivity $p$ samples having grain sizes smaller than about 50 μ. The faster component [${\tau }_1=(2.59\mathrm{}\pm 0.10)\times {10}^{-10\mathrm{}}$ sec] was easily ascribed to annihilations in the bulk; the slower one [${\tau }_2=(5.20\mathrm{}\pm 0.20)\times {10}^{-10\mathrm{}}$ sec] was ascribed to annihilation processes taking place in the oxide surface layer.

The intensity distribution of intense light beams was investigated in liquids. In C${\mathrm{S}}_2$ the diameter of the beam decreases from 700 to 5 μ when the laser power increases from 10 to 100 kW, in good agreement with theory. It is found that both stimulated Brillouin and backward-traveling stimulated Raman scattering terminate the duration of the self-focusing process.

We have observed the Mössbauer effect in the smectic liquid crystalline state. To our knowledge this is the first report of a Mössbauer-effect observation in this particular state of matter. Orientation effects of solute molecules are exhibited and methods of obtaining structure information for both the solute molecules and the liquid-crystalline solvent are indicated.

Convergent light observations have been made on three liquid crystalline substances with nematic and smectic phases. The nematic phase was observed to be uniaxial as expected, but smectic $C$ was found to be biaxial. The optic axial angle $2V$ for smectic $C$ was found to be on the order of 20° for all three compounds. Also, the tilt angle for smectic $C$ is nearly 45° for all three liquid-crystal systems. Anisotropy of the degrees of order is suggested as a partial explanation for the biaxial character of smectic $C$.

A general result for the feedback stabilization of surface instabilities of a highly conducting plasma is presented. Explicit form of feedback for stabilizing a linear pinch with a uniform axial static magnetic field is given.

Measurements of the longitudinal sound velocity $v_1$ in hcp ${\mathrm{He}}^4$ as a function of direction relative to the hexagonal axis are reported for molar volumes between 20.3 and 17.3 ${\mathrm{cm}}^3$/mole. The variation of $v_1$ with molar volume is much stronger than predicted by theory. Agreement is found with theoretical predictions for the anisotropy of $v_1$.

Spontaneous production of quantized vortices in He II by thermal fluctuations is considered. Implications of this theory for superflow in finite channels are discussed.

We have observed anomalously broad resistive transitions in superconducting films and have shown that the widths gradually reduce to the usual Aslamazov-Larkin value when a magnetic field is applied parallel to the film. These results are explained by adding to the Aslamazov-Larkin theory another fluctuation contribution to the conductivity of the type first proposed by Maki, which for thin films is logarithmically divergent with a cutoff proportional to the pair-breaking interactions.

The zero-temperature cyclotron resonance of the electron splits into two groups as the temperature is increased. One group moves rapidly toward very small mass, the other toward larger mass. This splitting has not been predicted by previous theories. Moreover, the first member of the small-mass group can be accounted for only qualitatively by these theories.

Rotation of volume elements within an acoustic wave can make a large contribution to the scattering of light from an acoustic shear wave in optically anisotropic media, thus removing the traditionally assumed symmetry between the last two (acoustic) indices of the usual photoelastic tensor $p_{\mathrm{ij}1\mathrm{}}$.

Scattering of surface waves by mass defects is estimated as a function of frequency and defect depth. Attenuation of surface waves of microwave frequencies on polished surfaces is discussed. Most of the energy is scattered into other surface waves, rather than body waves, so that energy of surface waves is retained, and leaks into the interior relatively slowly. This may explain the duration of seismic signals on the moon after the crash of the lunar excursion module during the Apollo 12 mission.

A new electronic configuration involving localized $6s$ electrons is hypothesized for the Sm ions in Sm${\mathrm{B}}_6$. The model is successful in explaining earlier results of susceptibility measurements and new Mössbauer spectroscopy data reported here.

A many-body formalism utilizing a tunneling operator is proposed for the photoemission process, thereby avoiding the artificial separation of the process into distinct steps. The lowest-order term in the formalism, corresponding to surface scattering, is developed in detail for simple metals. Energy-distribution curves from this "surface term" are presented for both free and interacting electrons. Many-body effects, including plasmarons, are prominent in the latter and have tentative experimental support.

Calculations of the rf surface impedance of a thin metal plate as a function of a dc magnetic field applied parallel to the faces of the plate reveal that one can measure experimentally the critical angle of collision with the surface, if such an angle exists, below (above) which electron surface scattering is mostly specular (diffuse).

An exact expression for the thermal average of the $S=\frac{1}{2}$ isotropic Heisenberg Hamiltonian has been derived which, in contrast to an exact result derived by Mills, is identical in form to a well-known result for boson systems with two-body interactions. A generalization of the result is found to be correct for $S>\mathrm{}\frac{1}{2}$ at low temperatures. It is shown that correct low-temperature results can be obtained from a wide range of approximate magnon spectra. This constitutes a major advantage over the result derived by Mills.

A simple model is used to describe the magnetic properties of cobalt impurities in copper. Isolated impurities are nonmagnetic with a high spin-fluctuation temperature. Isolated pairs of neighbor cobalt atoms are very nearly magnetic: This gives rise to a large susceptibility and a large electronic specific heat, varying as the square of the concentration $C$. At low temperatures, only the group of three (or more) cobalt atoms carries a magnetic moment.

Strong off-diagonal two-nucleon reaction-matrix elements imply appreciable depletion of the normally occupied single-particle states. Renormalization of the propagators in linked-cluster expansions by factors of occupation probabilities is shown to have important consequences for computed shell-model single-particle energies, separation energies, and spectroscopic factors for nucleon-transfer reactions.

The influence of the Coulomb and nuclear forces on the Coulomb barrier in heavy-ion reactions is studied in a dynamical classical model. It is shown that the fusion barrier is smaller than the conventional Coulomb barrier of two underformed nuclei. The model yields a dynamical picture of the excitation mechanism of surface vibrations and giant resonances. It is suggested that-due to nuclear forces-the excitation of the octupole mode is strongly enhanced over the excitation of the quadrupole mode in experiments at the Coulomb barrier.

Phenomenological analysis of Gamow-Teller transitions and magnetic moments is done for the ground states of the mirror nuclei, putting in evidence the linear quenching of the matrix elements $〈\Sigma i^A{\sigma }_{3i}〉$ and $〈\Sigma i^A{\sigma }_{3i}{\tau }_{3i}〉$. Configuration mixing techniques in the $j-j$ coupling scheme are discussed and used to explain the quenching and to fit the experimental data.

The motion and radiation of a relativistic classical charged particle in an intense magnetic field is analyzed when the radiative reaction force is comparable with the Lorentz force. Their significance for cosmic problems, especially the acceleration and radiation of cosmic-ray electrons in pulsars, is discussed.