Spontaneous reversal of the toroidal field in a reversed-field pinch as a result of low-$\beta$ (small $\frac{J_{\perp }}{J_{\parallel }}$) resisitive kink mode activity is investigated with use of a three-dimensional magnetohydrodynamics code. Helical and three-dimensional steady reversed states are obtained. In three dimensions quasisteady fluctuating states are observed above a critical value of the pinch parameter $\theta$.

The authors present a new formulation of the beam-density effect on energy loss by charged particles passing through matter, which exhibits an increased loss with a beam-shape dependence. This arises from a long-range dipolelike term contained in the two-particle vicinage function for cooperative energy loss by a pair of nonrelativistic particles. A new analytic expression for the vicinage function, which exhibits the long-range term, is also presented.

This paper reports the observation of the blocking effect of the host lattice on decay positrons from positive muons implanted into silicon crystals. High-purity and oxygen-doped wafers with [110] and [111] orientations were investigated, and axial and planar effects were seen both as blocking dips and as flux peaks. The results, together with muon spin-rotation experiments on the same samples, indicate a low-symmetry muonium state in pure material and an oxygen-trapped muonium state at a tetrahedral site in doped material.

An inelastic neutron-scattering study of spin relaxation in the dilute, two-dimensional Ising antiferromagnet ${\mathrm{Rb}}_2{\mathrm{Co}}_c{\mathrm{Mg}}_{1-c}{\mathrm{F}}_4$ has been carried out for $c$ very close to the magnetic percolation threshold $c_p=0.593\mathrm{}$. The dynamical exponent $z$ relating the order-parameter relaxation rate to the magnetic correlation length is found to be ${2.4}_{-0.1\mathrm{}}^{+0.2\mathrm{}}$. It is argued that $z$ has this anomalously high value because of the fractal geometry of percolating networks.

The nonisoelectronic nature of the semiconductor alloy ${\mathrm{Pb}}_{1-x}{\mathrm{Sr}}_x\mathrm{S}$ results in a dramatic band splitting near the Fermi energy. This paper presents experimental verification of a recent theoretical model of the electronic structure of ${\mathrm{Pb}}_{1-x}{\mathrm{Sr}}_x\mathrm{S}$. Absorption spectroscopy shows that the $X_{5^{\prime }}-X_3$ energy difference (which defines the direct energy gap in SrS) increases as Pb is substituted for Sr in SrS. Simultaneously, giant absorption tails extending up to 3 eV into the SrS gap are observed.

The concept of tunneling systems has been very successful in explaining phenomena which occur in disordered solids at small energies and with long relaxation times. It is shown in this Letter that fluctuations in the tunneling systems will also produce fluctuations in the electric resistivity and that the power spectrum of these resistivity fluctuations is inversely proportional to the frequency.

Simulations and effective-medium calculations were performed on the absorption of impurity particles by static sinks in three dimensions. The theory shows no qualitative effects due to two-sink clusters and agrees well with simulations. The conclusion is drawn that effective-medium theory is adequate for time-dependent impurity concentrations $C\mathrm{}\left(t\right)\mathrm{}$ above the lower bound recently demonstrated by several authors. Crossing of the concentration curves occurs at a time such that $\frac{C\mathrm{}\left(t\right)\mathrm{}}{C\mathrm{}\left(0\right)\mathrm{}}={10}^{-67\mathrm{}}$ or less.

Inelastic and elastic $\frac{J}{\psi }$ photoproduction on hydrogen are investigated at a mean energy of 105 GeV. The inelastic cross section with $\frac{E_{\psi }}{E_{\gamma }}<0.9\mathrm{}$ is significantly lower than the corresponding result for muoproduction on iron targets, but is consistent with a second-order perturbative QCD calculation.

Evidence is reported from the CLEO detector for the ${\chi }_b\mathrm{}\left(1P\right)\mathrm{}$ states from radiative decays of the $\Upsilon \mathrm{}\left(2S\right)\mathrm{}$. Photons are detected by pair conversion in the beam pipe or a lead radiator, and the pair momenta are measured in the drift chamber. Two photon lines are well resolved, with energies 109.5±0.7±1 and 129.0±0.8±1 MeV.

Positive pions were stopped in a Ge detector and the energy of their decay muons was measured with a resolution of 6.7 keV. The energy spectrum was analyzed to obtain an upper limit for the branching ratio for the decay into a muon and a heavy neutrino. For neutrino masses between 2.5 and 10 MeV/${\mathit{c}}^2$ this upper limit is a few parts in ${10}^4$.

Results are presented of a measurement of the proton-proton elastic-scattering spin parameter $C_{\mathrm{LL}}=(L,L;0,0\mathrm{})\mathrm{}$ at 11.75 GeV/c and ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}=48\mathrm{}°-90\mathrm{}°$. The value of $C_{\mathrm{LL}}$ is nearly constant and is approximately -0.16 in this angular region. This behavior is consistent with only one of the many models proposed describing the interaction via the hard scattering of two quarks.

Molecular photoelectrons have been observed to be spin polarized even if ejected from a beam of randomly oriented molecules by unpolarized radiation. The polarization and the asymmetry parameter $\beta$ for ${\mathrm{Br}}_2$, ${\mathrm{I}}_2$, C${\mathrm{H}}_3$Br, and C${\mathrm{H}}_3$I are compared with general predictions of a nonrelativistic model. While all available data of the differential cross section agree with this prediction, spin-polarization results partly disagree, manifesting the influence of the spin-orbit interaction upon the phases of molecular continuum wave functions.

The authors propose the application of time-resolved nonlinear optical techniques to the selective study of the dynamics of vibrational overtones and two-phonon states in condensed media. They present the first experimental demonstration of picosecond coherent anti-Stokes higher-order Raman scattering on the first overtone and the bound two-phonon state of the ${\nu }_2$ mode in liquid and crystalline C${\mathrm{S}}_2$, respectively, and describe its dynamics and deexcitation channel in terms of a simple effective Hamiltonian.

The development of the Rayleigh-Taylor hydrodynamic instability was studied in laser-accelerated targets by introduction of mass thickness variations in foil targets. Observations made by side-on flash x radiography showed target structures and mass redistribution effects which resemble Rayleigh-Taylor bubbles and spikes, including not only advanced broadening of the spike tips on the laser-irradiated side of the foil but also projections of mass on the unirradiated side. The observations compare well with numerical simulations.

The effect of laser-beam spatial modulations on plane targets is analyzed as a function of modulation scale length and target material by means of x-ray shadowgraphy. At short laser wavelength it is shown that lateral energy smoothing is clearly improved because of the strong x-ray emission from the irradiated high-$Z$ side of layered targets. The results, interpreted in terms of radiation-driven thermal transport, are in agreement with numerical simulations.

Ion thermal conductivity for a tokamak plasma in the collisional regime with arbitrarily large toroidal rotation is calculated from Braginskii's equations. It is found to decrease to the classical value as ${\nu }_{\mathrm{ii}}^{-1\mathrm{}}$ for high enough collisionality.

It is argued that a proper description of the vibrational properties of systems comprised of solid spheres interacting via forces exerted at their points of contact must take account of the rotational motion of the individual grains. The spectrum obtained by coupling these additional degrees of freedom to the familiar translational motion is studied in both ordered and disordered packings and, in the latter case, used to interpret data on compressional- and shear-wave speeds in systems with pressure-generated contacts.

A neutron-scattering study of phase separation in the Fe-Cr alloy system clearly indicates dynamical scaling. The results are compared with the predictions of recent scaling theories. Both the time evolution of the characteristic wave vector and the scaling function of the structure factor show crossover phenomenon. The crossover is attributed to a transition from a coagulation to a condensation process at a late stage of phase separation in alloy systems.