By use of an rf electric resonance technique, the fine structure of the 1s4f multiplet of He i and its splitting in a static electric field have been investigated. Our results for the fine-structure intervals ${\nu }_{1\mathrm{J}}$ between the ${}_{}{}^1{}_{}{}^{}\mathrm{F}_3^{}{}_{}{}^{}$ level and the ${}_{}{}^3{}_{}{}^{}\mathrm{F}_{\mathrm{J}}^{}{}_{}{}^{}$ levels are ${\nu }_{12}$=232.8(2.0) MHz, ${\nu }_{14}$=490.8(2.0) MHz, and ${\nu }_{13}$=704.3(1.0) MHz. Furthermore, a singlet-triplet mixing ratio Ω=0.757(3) and the exchange interaction 2K=158(3) MHz have been derived from the measurements.

Modulation instability that leads to breakup of intense cw radiation into a train of ultrashort pulses during propagation in optical fibers occurs only in the presence of anomalous group-velocity dispersion. It is shown that a new kind of modulation instability can occur even in the normal-dispersion regime when two copropagating optical fields interact with each other through cross-phase modulation initiated by the nonlinearity. The quantitative aspects of this cross-phase–modulation–induced modulation instability are discussed and illustrated by use of a realistic experimental example.

We identify the defects of waves by means of topological arguments and study them in the framework of Landau-type analysis. It is shown that they correspond to sinks, sources, or dislocations of traveling waves, and to dislocations of standing waves.

We introduce a simple model for the consumption and fragmentation of a reactive porous medium, and investigate various distinct regimes of such processes. The model simulates the reaction-consumption process in a realistic way, and is capable of simulating the entire range of possible kinetic limitations. We show that a dynamic scaling governs the time evolution of the fragments distribution and calculate the critical exponents. They vary continuously with the reactivity of the porous medium. This dynamic scaling may also hold for the fragmentation of a system as a result of mechanical breakage, such as grinding of minerals, or stress-induced fragmentation of disordered solids, or of rocks and polymers.

We have formed dense Z pinches from frozen deuterium fibers ranging in diameter from 80 to 125 μm and at peak currents of up to 640 kA. The pinch remains stable for the entire 130 nsec of the current rise. This anomalously long stable period corresponds to about 100 MHD growth times. As soon as the current peaks, i.e., when dI/dt=0, the pinch goes rapidly m=0 unstable and produces nearly ${10}^{12}$ neutrons in a 30–60-nsec-wide pulse. The instability occurs only at dI/dt=0 and is independent of either the current magnitude or the time to peak.

Local measurement in magnetized plasma of the ion velocity distribution function synchronous with an externally launched electrostatic wave carries information concerning the wavelength perpendicular to the magnetic field as well as the wave amplitude and phase.

The distribution of drag forces over a rigid fractal structure such as a colloidal aggregate in a flowing liquid is considered. The force p(α) exceeded on a fraction α<(1/2 of the aggregate is not a constant multiple of the median force p((1/2), but diverges in an unusual way with the size of the aggregate. This behavior is interpreted in terms of the time t(α) required to rinse a coating from a fraction α of the aggregate. Diffusing fields and stress fields near an arbitrary fractal are expected to behave similiarly.

cw NMR measurements have been made on ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He solutions of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentration (${\mathrm{X}}_3$) between 1.3% and 8.6%, at pressures of 0, 7, or 8 bars and at temperatures down to 0.2 mK. In the collisionless region, the NMR absorption line exhibits several satellite peaks in addition to its narrowing. Numerical analysis of the phenomenon as an excitation of standing spin-wave modes leads to the determination of the Fermi-liquid interaction parameter λ. Its concentration dependence supports the quasiparticle interaction potential proposed by Murdock, Mountfield, and Corruccini.

A new approach to modeling glass structures is presented, with specific application to a-${\mathrm{SiSe}}_2$ and the medium-range order exhibited by this system. A systematic investigation of the parameter-space describing amorphous structures has been undertaken, and the effect of different structural features on the total pair correlation function studied. This work suggests that the structure of a-${\mathrm{SiSe}}_2$ consists of both randomly oriented chains of edge-sharing Si(${\mathrm{Se}}_{1/2}$${)}_4$ tetrahedra and cross-linked chain-cluster units.

The effect on the superconducting transition temperature of exchanging ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ has been studied. The concentration of the two isotopes has been determined by Rutherford backscattering spectroscopy and Raman spectroscopy confirmed the mass-related phonon frequency shift. The ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ substitution is found to be the same on the two O sites. Magnetic measurements of the transition temperature reveal a nonzero isotope effect (α=0.16±0.02). The exponent α has also been calculated within a conventional electron-phonon interaction model and was found to be significantly larger.

An oxygen isotope shift is observed in superconducting ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ when ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ is substituted partially for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$; the superconducting transition temperature $T_c$ is lowered by 0.3 to 1.0 K in different samples. We examine these results using conventioanl phonon-mediated BCS theory and conclude that, for ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$, phonons play an important role in the pairing mechanism.

Optical and neutron-diffraction studies of superconducting and nonsuperconducting samples of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$ are described. Superconducting samples, with y≊0.1, showed two strong electronic transitions, at 3000 ${\mathrm{cm}}^{\mathrm{-}1}$/0.37 eV and at 20 000 ${\mathrm{cm}}^{\mathrm{-}1}$/2.5 eV, which are interpreted as charge-transfer bands (i.e., excitons). Nonsuperconducting samples, with y≊0.8 (prepared by heating of superconducting samples in vacuum to deplete the oxygen content), did not show these features. Our measurements support an excitonic mechanism for the superconductivity in high-$T_c$ copper oxide compounds.

It is shown that the effective antiferromagnetic exchange interaction associated with the Hubbard model of a metal can give rise to antiferromagnetism and superconductivity simultaneously. This is a suggested model for the newly discovered high-temperature superconductors.

Two-dimensional open-orbit metals can undergo a spin-density-wave transition in a magnetic field which leaves the Fermi energy pinned in a gap. The result for a pure system would be a quantized Hall resistance and vanishing longitudinal resistance. However, in contrast to the closed-orbit case, where impurities are required to observe the quantum Hall effect, in the present case impurities lead to conducting extended states in the gap, Hall plateaus with nonintegral values, and nonvanishing longitudinal resistance.

Extreme critical slowing down of the order-parameter fluctuations in a d=3 random-field Ising model system is observed in quasielastic-neutron-scattering studies of ${\mathrm{Fe}}_{0.46}$${\mathrm{Zn}}_{0.54}$${\mathrm{F}}_2$. This is manifest in the logarithmic time dependence of I(q) in the 30–${10}^4$-sec time domain at, and near, ${\mathrm{T}}_{\mathrm{c}}$(H) in zero-field–cooled measurements. Contrary to a report by others, ${\mathrm{T}}_{\mathrm{eq}\left(\mathrm{H}\right)>\mathrm{}{\mathrm{T}}_{\mathrm{c}}(\mathrm{H}}$) and no evidence for a first-order phase transition is found in this virtually gradient-free crystal.

Symmetry arguments and analytical calculation show that—contrary to common belief—photoelectrons emitted normal to (111) surfaces of nonmagnetic centrosymmetric cubic crystals by normally incident linearly polarized light may be spin polarized. Their spin is parallel to the surface and rotates by an angle 2α upon rotation of the light polarization by α. Numerical calculations using a relativistic multiple scattering formalism explicitly predict the effect for Pt(111), with a polarization up to 70%, and identify spin-orbit coupling in half-space intial states as its main cause.

The neutrino signatures obtained from extended hydrodynamic calculations of stellar-core collapse are described and compared with the data of Hirata et al. and Bionta et al. The comparisons suggest that SN1987A resulted from the collapse of a small core, and marginally suggest the delayed shock mechanism.

The formation of fractal patterns during an amorphous structural transition in Ni-Mo thin films after 200-keV xenon-ion irradiation is presented. The dimension of the ion-induced fractals is 1.72±0.07. The fractal patterns, together with other microscopic features, are described and characterized. Our results are compared to the cluster-diffusion-limited-aggregation model. The growth habit is attributed to long-range correlation which occurs at a critical condition during the amorphous-crystal transition.

It is argued that the global, uniform semiclassical approximation to quantization afforded by a coherent-state approach is well suited for an analysis of classically chaotic systems. Special attention is given to the interpolating action which incorporates in a single expression the classical action for each and every contributing classical trajectory. Approximate expressions for the configuration-space propagator are presented for small, intermediate, and large propagation times.

We set an upper limit on the branching fraction B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$$K^{\mathrm{-}}$$K^0$)<0.26% at the 95% confidence level. The data sample was obtained with the TPC/Two-Gamma detector facility at the SLAC $e^{+}$$e^{\mathrm{-}}$ storage ring PEP. The process ${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$$K^{\mathrm{-}}$$K^0$ is related via SU(3) to the second-class-current decay ${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$${\pi }^{\mathrm{-}}$η. Our limit is nearly 20 times smaller than the recently reported branching fraction B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$${\pi }^{\mathrm{-}}$η) of (5.1±1.0±1.2)%, whereas SU(3) symmetry predicts the ratio of πη to KK¯ production to be at most 5:1. We also measure the branching fraction B(${\tau }^{-}$→${\nu }_{\tau }$$K^{*-}$)=(1.5±0.4*0.4%.