Photoluminescence measurements with high spatial and spectral resolution on $n$-type GaAs at helium temperatures show the reduction of radiative quantum efficiency immediately at individual deformation-induced dislocations to be orders of magnitude stronger than near grown-in dislocations.

Electron paramagnetic resonance in the layered compound Hf${\mathrm{S}}_2$ doped with Ti is observed and attributed to weakly localized electrons associated with the three conduction-band valleys of Hf${\mathrm{S}}_2$.

A simple formula involving the geometric mean of the electronegativities of the two components is proposed for the determination of the Fermi energies and hence the photoelectric thresholds of simple binary compounds, and is found to be surprisingly accurate for all presently available data. A theoretical basis for the use of the "postulate of the geometric mean" is also given.

I consider the effects of the lattice on the total energy of a low-density exciton system. While only a weak attraction is contributed by acoustical-phonon coupling, polar optical phonons are found to cause a rather strong repulsion between the excitons, i.e., they increase the total energy. These lattice-induced terms provide a cancelation of the otherwise attractive forces. For intermediate or strong electron-phonon coupling, they can turn the total average exciton-exciton force into a repulsive one.

We suggest that the Pauli-force model potential defines a new electronegativity scale consisting of explicit orbital components. These are determined algebraically from atomic spectral data. We find that these components distinguish quantitatively the most stable crystal structures of 34 nontransition elemental solids and $59A^N{B}^{8-N}$ binary compounds.

Detailed measurements of the magnetization and sublattice magnetization of Fe${\mathrm{Cl}}_2$ in a magnetic field have been performed by use of polarized- and unpolarized-neutron-diffraction techniques. The phase diagram so determined is found to bear a close resemblance to that of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures near the tricritical point although there are a number of important differences which seem to require, at the minimum, an extension of present theories of tricritical phenomena.

Strong experimental evidence for the importance of multistep processes has been obtained from the reaction ${}_{}{}^{186}{}_{}{}^{}\mathrm{W}$(${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$)${}_{}{}^{184}{}_{}{}^{}\mathrm{W}$ at 70 MeV. The angular distributions are in good agreement with theoretical predictions and show dramatic influence from inelastic nuclear-Coulomb interference as well as from direct, two-step interference.

Detailed information on a new reaction mechanism is presented for a 600-MeV ${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$ bombardment of ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$. This reaction process represents a major fraction of the total-reaction cross section and is characterized by strong damping of energy degrees of freedom, but on the average, a relatively small amount of mass exchange compared to fission.

A ground-state band with spin up to $J^{\pi }={12}^{+}$ was observed in ${}_{}{}^{50}{}_{}{}^{}\mathrm{Cr}$ from the reactions ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}({}_{}{}^{16}{}_{}{}^{}\mathrm{O}, 2p\alpha ){}_{}{}^{50}{}_{}{}^{}\mathrm{Cr}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}({}_{}{}^{32}{}_{}{}^{}\mathrm{S}, 2p\alpha ){}_{}{}^{50}{}_{}{}^{}\mathrm{Cr}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}({}_{}{}^{14}{}_{}{}^{}\mathrm{N}, 3pn){}_{}{}^{50}{}_{}{}^{}\mathrm{Cr}$, and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}({}_{}{}^{12}{}_{}{}^{}\mathrm{C}, 2p){}_{}{}^{50}{}_{}{}^{}\mathrm{Cr}$. Spin and parity assignments were made from $\gamma -\gamma$ coincidences, angular distributions, and linear-polarization measurements. The higher-spin states are reasonably well reproduced in the ${\left(1\mathrm{}{f}_{\frac{7}{2}}\right)}^n$ shell-model frame while discrepancies exist for the lower ones. These levels have collective features at lower excitation but very pure shell-model characteristics towards the maximum spin expected in the ${\left(1\mathrm{}{f}_{\frac{7}{2}}\right)}^n$ space.

I have found a type-$N$ gravitational field for which the principal null direction has nonzero twist and which admits exactly one Killing vector.

The study of the vacuum polarization for strong magnetic fields reveals the existence of a massive, longitudinal photonlike resonance for magnetic fields exceeding $B\simeq \frac{m^2{c}^4}{e^3}$.

When a purely electromagnetic incoming wave is scattered by a charged black hole, the electromagnetic-gravitational coupling can lead to conversion to gravitational radiation in the outgoing wave. Analytic estimates and numerical results are given for this effect, which proves to be significant only for maximally or near-maximally charged black holes.

We discuss the theoretical total and differential cross sections for $Z^0$ production in the reaction $l+N\to N^{\prime }+Z^0+l$, including remarks about polarization effects and subsequent leptonic decays. Brief mention is made about production of $Z^0\text{'}\mathrm{s}$ by neutrino beams and of scalar mesons.

Order parameters have been measured in a homologous series of nematic liquid crystals, $p$-alkoxyazoxybenzenes, by ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR. The ordering exhibits an even-odd alternation along the series, and a comparison of our data at the isotropic-nematic transition temperatures is made for the first time with a recent theory of Marčelja which explicitly accounts for end-chain interactions between the molecules. Some conclusions are drawn concerning molecular conformational motion and ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ spin relaxation in the nematic phase.

The weak nonexponential relaxation $\propto t^{-a^{\prime }}$ recently found in computer experiments on the phase separation of alloys is explained in terms of a cluster reaction and diffusion process. The nonlinear features of this process can be accounted for by a time-dependent diffusion constant. Estimates for the resulting exponents [$a^{\prime }=\frac{1}{\mathrm{}(3+d)\mathrm{}}$] are consistent with the computer simulations. The experimental observability of this slowing down is discussed.

I report measurements of the longitudinal resonance in the $B$ phase of superfluid ${\mathrm{He}}^3$ at melting pressures over a broad temperature interval. I also present the results of a transverse NMR study in the $B$ phase which is designed to test the theory of textures in the $B$ phase and to provide accurate measurements of the $B$-phase susceptibility. Finally, I report the first measurements of a field-dependent frequency shift in the transverse $B$-phase NMR associated with the anisotropy energy of the rotation axis of the Balian-Werthamer state.

The general first-order parametric-instability equations for an inhomogeneous finite-size plasma are solved both analytically and numerically. It is shown that the wave-number mismatch introduced by plasma gradients does not necessarily convert absolute instability into convective instability, even for large systems. Absolute instability can also occur when the length of the system is determined by a smooth spatial dependence of the growth rate.

The effect of the ponderomotive force in the interaction of a capacitor rf field with a nonuniform plasma is investigated.

The spatial development of laser-driven shock waves in a plane solid-hydrogen target was directly observed by high-speed photography. From the measured shock-front velocity a peak pressure of 2 Mbar is evaluated at the onset of a 10-J, 5-nsec Nd laser pulse. The underlying mechanism of laser ablation and the relevance of such measurements to laser fusion and the behavior of matter at extremely high pressures are discussed.

On (110) GaAs surfaces cleaved in ultrahigh vacuum the changes of the ellipsometric angles $\delta \psi$ and $\delta \Delta$ induced by oxygen adsorption have been measured for photon energies $1.5<\mathrm{}\hslash \omega <3.5\mathrm{}$ eV. The main spectral structure is explained as due to the Franz-Keldysh effect at critical points of the bulk band structure induced by a change of the band bending. From a comparison with electroreflectance data information can be obtained about band-bending changes and about surface states.