The collisionless interaction between particles undergoing $\overrightarrow{\mathrm{E}}\times \overrightarrow{\mathrm{B}}$ drift and Kelvin-Helmholtz instabilities propagating across a strong magnetic field is investigated using a test-wave technique. Orbit perturbations in phase space are observed with increasing instability amplitude. We also present the first direct measurements of finite—Larmor-radius effects.

Nonlinear evolution of a beam-plasma instability has been investigated by direct measurements on the energy distribution of beam electrons and the field intensity of an externally driven electrostatic wave. The initial exponential growth of the excited single wave is halted when the wave traps the electrons and rotates them in phase space. At the phase of spatial bunching of the beam, its harmonics are observed to have maximum values. The results agree with those predicted by a single wave model.

We derive and solve numerically the self-consistent set of equations for the nonlocal shear viscosity and the order-parameter decay rate near the critical point of fluids. This removes the ambiguity associated with the so-called "high frequency" shear viscosity entering the theoretical expressions for the Rayleigh linewidth, and improves the agreement with the measured linewidth of Chang et al.

The fluctuation-dissipation formula for the viscosity in terms of the stress-tensor fluctuations reproduces the experimental logarithmic temperature dependence of the hydrodynamic viscosity. Using the theoretical wave-number- and frequency-dependent viscosity in the problem of critical diffusion, we find that the effects of nonlocality and retardation practically cancel, resulting in satisfactory agreement with a recent light-scattering determination of the effective viscosity.

Observations of the rf emission from the interaction of a relativistic electron beam with a modulated magnetic field has led to a qualitative theory. This theory predicts emission of high-power radiation at high-frequency harmonics. An improved experiment has confirmed these predictions.

A spin-½ Ising-model "metamagnet" is studied by the method of high-temperature expansions. The phase boundary in the $H-T$ plane is obtained for a simple cubic lattic with in-plane ferromagnetic interactions but between-plane antiferromagnetic coupling, and a tricritical point is located. Along the critical line, the staggered susceptibility appears to have an exponent $\frac{5}{4}$ (consistent with the universality hypothesis), while at the tricritical point, the direct susceptibility shows a tricritical exponent of ½.

Positron-annihilation lifetimes were measured in pure aluminum and in aluminum—1.7 at.% zinc at temperatures between 20 and 550°C. The data were analyzed using an extension of the trapping model to obtain the vacancy formation energy in aluminum ($E_F=0.62\mathrm{}\pm 0.02$ eV). Under certain restrictive assumptions the binding energy of vacancies to zinc atoms in aluminum was found to be $E_B=0.019\mathrm{}\pm 0.004$ eV. A relaxation of these assumptions yielded only an upper bound ($E_B<0.04\mathrm{}$ eV).

Optical reflectivity measurements on mixed crystals of the system ${\mathrm{ZnS}}_{1-x}{\mathrm{Se}}_x (0\mathrm{}<~x<~1)$ were made at room temperature in the energy region of 2.5 to 9.5 eV. Indirect as well as direct optical transitions were observed in the ${\epsilon }_2$ spectra computed by a Kramers-Kronig analysis. Considering the complete series of mixed crystals, the very complex structure of the observed spectra can only be explained assuming separated energy bands for the different components of these mixed crystals.

The acoustic properties and low-temperature specific heat of a dense metallic glass in bulk form have been studied for the first time. A large softening of long-wavelength transverse acoustic phonons in ${\mathrm{Pd}}_{0.775}$${\mathrm{Si}}_{0.165}$${\mathrm{Cu}}_{0.06}$ glass, relative to the crystal, is observed which is not simply attributable to the small density difference between the two states.

It is shown that the photoionization process of inner-shell electrons by unpolarized radiation leads to an alignment of the ionized atoms. From the nonisotropic angular distribution of the following Auger electrons the relative partial photoionization cross sections can be determined.

Large deviations between distorted-wave Born approximation predictions and measured ${}_{}{}^{186}{}_{}{}^{}\mathrm{W}\mathrm{}(p,t)\mathrm{}$ reaction cross sections are explained by inelastic excitations in the entrance and exit channels when the reaction is treated by means of a coupled-channel calculation. The results presented include transitions to the ground-state, $\beta$ vibrational, and $\gamma$ vibrational bands of ${}_{}{}^{184}{}_{}{}^{}\mathrm{W}$. The presence of inelastic processes is shown to result in significant differences in angular distributions representing the same transferred angular momentum.

The crystal-blocking method for protons along a $〈110〉$ axis of a 30-keV-thick Ge crystal has been used to show sizable effects on mean compound-nuclear lifetimes of (1) the enhancement of an isobaric-analog resonance, and (2) the spin of the final state excited by inelastic scattering.

The apparent pressure dependence of the reduced mobility of potassium ions in various gases, reported by Elford, is explained in terms of diffusion effects and reversible reactions involving clustered ions.

The transient distortions, induced in a nematic film by a stepwise increase (or decrease) of the magnetic field around the threshold value $H_c$, are accurately measured by conoscopic techniques. The results can be interpreted by the Leslie equations and provide a measurement of a viscosity coefficient.

We show that elastic strain energy may account for the tendency of some nematic liquid crystals to lie parallel to the direction of rubbing on a solid surface that has been slightly deformed by rubbing, or perpendicular to a surface that is slightly rough in two dimensions.

Particle scattering due to a Rosenbluth-Post convective loss-cone instability is calculated using a slab model. This collective contribution to the scattering rate is found to be the classical scattering rate increased by a factor ${\left(\frac{T_i}{T_e}\right)}^{\frac{3}{2}}{\left(\frac{m_e}{m_i}\right)}^{\frac{1}{2}}{\left(\ln \Lambda \right)}^{-1\mathrm{}}\times \left(\mathrm{energy}\mathrm{amplification}\mathrm{of}\mathrm{the}\mathrm{convective}\mathrm{instability}\mathrm{in}\mathrm{the}\mathrm{finite}\mathrm{plasma}\right)$.

It is shown that a stream of charged vortex rings supports collective modes which are unstable below a critical wave number, thus leading to a charge-density wave structure. The negative effective mass of a gas of vortex rings plays a crucial role in the instability. For a singly charged stream, the situation is analogous to that of gravitational collapse.

Growing collective modes are observed in beams of charged vortex rings in superfluid helium. The growth of the waves is found to be dependent upon the path length of the beam and the modulation frequency, in agreement with theory.

NMR measurements of the molecular orientational order in the homologous series 4-$n$-alkoxybenzylidene-4′-phenylazoaniline are compared with the McMillan smectic-$A$ theory. The agreement is remarkable. The measurements show a discontinuous drop in the order parameter at the first-order smectic-$A$ ↔ nematic phase transition. The magnitude of the drop decreases with decreasing length of the alkyl end chain, finally approaching that characteristic of a second-order phase transition predicted by McMillan.

Direct measurements of the relaxation time of the birefringence within the self-focusing region of mode-locked laser pulses in several anisotropic liquids establish the importance of the orientational Kerr effect in the picosecond time regime. This result leads to some speculation as to the origin of the high degree of symmetry in the phase-modulated filament spectra observed with picosecond pulses.