We present numerical simulations of the evolution of a large-amplitude electrostatic wave packet in a hot collisionless plasma. When resonant particles are trapped by the wave potential during a transit time, the number of lobes in the packet is observed to increase. This is explained in terms of an equation of propagation for the packet amplitude and a dispersion relation involving the trapped particles.

Electrostatically driven oscillations near the lower-hybrid-resonance frequency have have been studied in a linear plasma device. The index of refraction of these waves is measured directly and is seen to peak at a critical density as the wave propagates radially inward. A strong, nonlinear interaction is observed between the driven oscillations and the low-frequency drift-wave-like fluctuations which exist in this plasma.

A mechanism for strong ion heating in oblique, collisionless shock waves is found in numerical-simulation experiments and identified with an essentially electrostatic two-ion-beam instability excited nonlinearly by the potential oscillations accompanying the whistler precursor.

Phonon interference has been observed in adsorbed helium films at frequencies above 100 GHz. Superconducting tunnel junctions were used as generators and detectors of the monochromatic phonons.

The shift in the complex frequency of a large-amplitude electron plasma wave is formulated in terms of a new subtraction procedure which reproduces the damping coefficient of Mazitov and of O'Neil directly from Poisson's equation. A time-dependent frequency shift is obtained which produces a phase shift that should be amenable to experimental observation. The results are interpretated in terms of simultaneous conservation of momentum and energy in the wave frame.

The pressure due to the magnetic ordering in solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is calculated using a Hamiltonian which includes triple exchange. This added degree of freedom supplies a simple explanation for the observed behavior of the pressure as a function of temperature.

A large-amplitude electron plasma oscillation, which is excited in a plasma as a result of its coupling to an external circuit, decays into ion modes and excites low-frequency fluctuations. Such a process is shown to contribute to dc anomalous resistivity.

It is shown that the residual high-frequency surface resistance found in the purest currently prepared superconductors can be explained in terms of the generation of sound waves by the incident electromagnetic radiation. This effect is negligible in normal metals, but is shown here to account quantitatively for the measured temperature and frequency dependence of the residual radio-frequency losses of superconductors.

A new oscillatory behavior of semiconductor transport properties in quantizing magnetic fields is predicted on general arguments. The "pseudoresonances" are due to the pairing off of single-LO-phonon emission and absorption processes ending at the infinite density-of-states region. An exact calculation of Ohmic longitudinal magnetoresistance shows that at "high" temperatures, the pseudoresonance amplitudes are comparable to the usual magnetophonon resonances. The pseudoresonances have probably been seen, but attributed to 2LO-phonon scattering processes.

In a recently reported measurement of the wavelength-modulated absorption of SrTi${\mathrm{O}}_3$, the results were interpreted as evidence for the coexistence of two phases over a temperature range of 60 K about the 105-K phase transition. We refute that interpretation by proposing an alternative which is necessary and sufficient to be fully consistent with the known properties of SrTi${\mathrm{O}}_3$ and which is unconnected with the phase transition. We also propose an experimental test of the validity of this new interpretation.

Hot exciton distributions, as much as 30°K in excess of the He ambient, are observed in optically pumped CdS. The effective exciton temperature is determined by analyzing the luminescence line shape for $E_x-2\mathrm{LO}$ recombination of free excitons. The exciton state is found to remain a viable excitation up to the maximum pump intensity of ${10}^7$ W/${\mathrm{cm}}^2$, with the only observed response to the increasing pump power being a rise in effective exciton temperature.

Data are presented for cyclotron-resonance signals in magnesium with the magnetic field oriented perpendicular to the specimen surface. The characteristics of these signals strongly suggest that they must arise from charge carriers on trajectories that lie entirely within the skin depth.

The free-exciton series $n=1,2,3\mathrm{}$ in high-purity GaAs has been observed in photoluminescence and photoconductivity. The $n=1\mathrm{}$ peak shifts upon changing the excitation level because of free-carrier screening. Anisotropy and exchange effects as well as two valence bands must be included to interpret the series spectrum of this Wannier-like exciton. New values are derived for the band gap at 1.6 K ($E_g=1.5189\mathrm{}\pm 0.0001$ eV) and for the $n=1\mathrm{}$ excitonic binding energy [$E_X\mathrm{}\left(1s\right)=3.77\mathrm{}\pm 0.05$ meV].

The random-phase-approximation formalism describing local fields developed by Adler and by Wiser is evaluated for an insulator for the first time. It is shown that umklapp effects decrease the low-frequency electronic dielectric constant, contrary to what is required to reconcile theory with experiment for most semiconductors. A simple prescription for effects of dynamical correlation is shown to improve agreement with experiment for diamond.

The thermodynamic properties of the antiferromagnetic Ising simple cubic and square lattices with nearest-neighbor and next-nearest-neighbor interactions have been studied using a Monte Carlo technique. Although both systems were found to possess magnetic tricritical points, their behavior in the "tricritical region" differed from that found in ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures.

The following result is stated: A black hole in its final state can be endowed with no exterior scalar, vector, or spin-2 meson fields. We conclude that such a black hole is not subject to the strong interactions which are mediated by meson fields such as the $\pi$, $\rho$, and $f$, and that consequently a useful definition of baryon number cannot be given for such an object.

When the average energy of the photon emitted by synchrotron radiation becomes appreciable compared to the energy of the particle, the particle will undergo straggling in its energy loss, which in turn broadens the radiation spectrum. The energy distributions of particles and the emitted photons are calculated using the method of quantum electrodynamics. The results are presented together with effects due to classical radiative reaction for experimental test. The significance of energy straggling in astrophysics is discussed briefly.

Inclusive reactions for multiparticles produced from cosmic-ray emulsion data at ∼ ${10}^{12}$ eV are presented and are compared with the predictions of limiting-fragmentation distribution.

We present the reason for setting $r=1\mathrm{}$ in Robertson's recent discussion of Wyler's expression for the fine-structure constant. Only for the choice $r=1\mathrm{}$ are all elements in the bounded domain $D^n$ also coset representatives and therefore group operations.

A technique is described for the study of electrons which are elastically scattered from a gas target into a small solid angle centered at 180° with respect to the direction of the incident electron beam. The method is applied to electron scattering from C${\mathrm{O}}_2$. Information is obtained concerning the nature of the compound state in the 4-eV region.