The $2^3$${\mathrm{S}}_1$→$2^3$${\mathrm{P}}_1$ and $2^3$${\mathrm{S}}_1$→$2^3$${\mathrm{P}}_0$ transition frequencies (${\nu }_1$ and ${\nu }_0$, respectively) in positronium have been measured for the first time and an improved measurement of the $2^3$${\mathrm{S}}_1$→$2^3$${\mathrm{P}}_2$ transition frequency (${\nu }_2$) has been made. The results ${\nu }_0$=18504.1±10.0±1.7 MHz, ${\nu }_1$=13001.3±3.9±0.9 MHz, and ${\nu }_2$=8619.6±2.7±0.9 MHz are in reasonable agreement with the predictions of quantum electrodynamics which are, to order ${\mathrm{\alpha }}^3$Ry, ${\nu }_0$(${\mathrm{\alpha }}^3$)=18496.1 MHz, ${\nu }_1$(${\mathrm{\alpha }}^3$)=13010.9 MHz, and ${\nu }_2$(${\mathrm{\alpha }}^3$)=8625.2 MHz. More precise measurements are planned to test the, as yet uncalculated, order ${\mathrm{\alpha }}^4$ Ry corrections.

The first measurements of electron-loss cross sections σ(q→q+1) for symmetric collisions between multicharged heavy ions have been performed by use of a folded-beam ion-ion collider. The absolute values of σ(3→4) for the ${\mathrm{Ar}}^{3+}$+${\mathrm{Ar}}^{3+}$ and the ${\mathrm{Kr}}^{3+}$+${\mathrm{Kr}}^{3+}$ systems at 60 keV c.m. energy were determined from the measurements.

We develop a quantum theory of propagation in dispersive nonlinear media. Quantum fluctuations are handled via the coherent-state positive-P representation. A stochastic nonlinear Schrödinger equation in the field variables is obtained which predicts wide band squeezing in the region of anomalous dispersion. For soliton inputs, fluctuations are reduced over the soliton bandwidth. This leads to quantum solitons which have quadrature fluctuations less than the level of vacuum fluctuations.

A new type of two-dimensional cellular automation method is introduced for computation of magnetohydrodynamic fluid systems. Particle population is described by a 36-component tensor referred to a hexagonal lattice. By appropriate choice of the coefficients that control the modified streaming algorithm and the definition of the macroscopic fields, it is possible to compute both Lorentz-force and magnetic-induction effects. The method is local in the microscopic space and therefore suited to massively parallel computations.

We present a theory which provides an explanation for recent laboratory observations of a ‘‘new electromagnetic mode in a non-Maxwellian high-beta plasma’’ and suggests a likely relevance of these modes to space plasmas. We find unstable parallel-propagating waves, which can be beam modes or whistlers, depending mainly on the degree of velocity anisotropy in a weak electron beam. In the beam-mode limit relevant to the experiment, this is a kinetic Weibel instability, enhanced by a parallel magnetic field. The frequency of the mode can be above or below the electron cyclotron frequency.

Measurements of the hot-electron (T=450 keV, n=2×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$) equilibrium in the Constance B minimum-B magnetic mirror show that the pressure profile is peaked off the axis and is shaped like the seam on a baseball. This curve is the drift surface of the deeply trapped electrons and the location of the strongest microwave heating. The configuration is stable and decays quiescently on the hot-electron collisional time scale (1–2 s) after the microwave power is turned off. According to 1D pressure-weighted F dl/B analysis this plasma configuration is expected to be unstable.

Self-consistent phonon and Monte Carlo calculations show that the three-body exchange interaction is important in dense helium. Together with a realistic pair potential and the Axilrod-Teller three-body dispersion interaction, it brings calculations into agreement with the experimental equation of state. This interaction stabilizes the hcp structure for pressures greater than about 60 GPa. A speculative phase diagram of high-pressure helium is proposed.

We present a new scheme to study the linear response of crystals which combines the advantages of the dielectric-matrix and supercell (‘‘direct’’) approaches yet avoids many of their drawbacks. The numerical complexity of the algorithm is of the same order as that of a self-consistent calculation for the unperturbed system. The method is not restricted to local perturbations as the dielectric-matrix one nor to short wavelengths as the direct one. As an application, we calculate the long-wavelength optical phonons in Si and GaAs, both transverse and longitudinal, using norm-conserving pseudopotentials, and without any use of supercells.

We have used quasielastic light scattering to study a binary-liquid critical mixture in a porous medium which undergoes a random-field transition upon warming to a history-dependent frozen domain state. Finite-size effects and a crossover to activated dynamics with nonexponential relaxations are observed. The relaxation time is found to increase very rapidly at the random-field transition, in agreement with recent theories of activated dynamics in disordered systems.

Picosecond Raman studies on the prototypic alloys ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As and ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As show that substantial nonequilibrium phonon effects occur in spite of the break-down in wave-vector conservation engendered by alloy disorder. Phonon lifetimes, measured to be the same as in pure GaAs, also are unaffected by the disorder. Explicit expressions for the Fröhlich couplings of the two LO phonon modes in ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As are obtained to compare with the generation rates of these phonons measured in our experiments.

Incoherent inelastic neutron-scattering experiments on liquid sodium at high temperature revealed that atomic motions in simple liquids are governed by hydrodynamic shear modes leading to measurable deviations from Fick’s law of diffusion. The experiments for the first time verify earlier predictions of a ‘‘long-time tail’’ behavior of the velocity-autocorrelation function of liquid particles as derived from computer simulation data and theory. A proper analysis of the experimental data demonstrates the existence of a corresponding low-frequency cusp in the velocity-autocorrelation spectrum.

Low-energy electron-diffraction and infrared-absorption measurements of H/Mo(100) and H/W(100) surfaces show that, at low hydrogen coverages, the effective H-H interactions are very different for the two substrates, leading to island formation on Mo(100) and to a uniform H layer on W(100). The results can be explained by a phonon-mediated interaction mechanism for the hydrogen adatoms, according to which the difference in behavior is due to the difference in the coupling between substrate distortions induced by the individual adatoms. It is concluded that a realistic treatment of the adatom interactions should go beyond the rigid-substrate model and include the crucial role of soft surface phonons.

A low-temperature expansion is derived for the ferromagnetic phase boundary of an hcp Ising model with competing interactions, near its multiphase point, to second order in x=exp(-2D/T), where D is the ferromagnetic interplane coupling. The coefficient of ${\mathrm{x}}^2$ is a slowly converging two dimensionally infinite sum. The resulting phase boundary coincides to order ${\mathrm{x}}^2$ with the line onto which Domany mapped a two-dimensional kinetic Ising model. This lends support to the conjecture of Domany and Gubernatis that this special line coincides with the exact ferromagnetic phase boundary.

Superconducting onsets near 90 K are observed in multiphase samples with nominal composition ${\mathrm{Lu}}_{1.8}$${\mathrm{Ba}}_{0.2}$${\mathrm{CuO}}_4$. Results of electrical resistance, mutual inductance, and dc magnetization measurements are reported. Magnitude of superconducting transitions indicates that about 2% of the best sample is superconducting.

Ten new superconducting cuprate perovskites with ${\mathrm{T}}_{\mathrm{c}}$’s above 77 K are reported. The highest ${\mathrm{T}}_{\mathrm{c}}$ of the new group, and for any superconductor, is observed in ${\mathrm{Ba}}_2$${\mathrm{Eu}}_{0.9}$${\mathrm{Y}}_{0.1}$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ which has a resistive midpoint of 94.5 K with a width of 0.7 K. Miessner effect and ac susceptibility measurements indicate bulk superconductivity in these materials. Each of these cuprates has a two-to-one ratio of divalent to trivalent ion on the A site of the perovskite-type structure.

We have found superconductivity in the 90-K range in A${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$ with A=La, Nd, Sm, Eu, Gd, Ho, Er, and Lu in addition to Y. The results suggest that the unique square planar Cu atoms, each surrounded by four or six oxygen atoms, are crucial to the superconductivity of oxides in general. In particular, the high ${\mathrm{T}}_{\mathrm{c}}$ in A${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$ is attributed mainly to the quasi two-dimensional assembly of the ${\mathrm{CuO}}_2$-Ba-${\mathrm{CuO}}_{2+\mathrm{x}}$-Ba-${\mathrm{CuO}}_2$ layers sandwiched between two A layers, with particular emphasis in the ${\mathrm{CuO}}_{2+\mathrm{x}}$ layers. Higher-${\mathrm{T}}_{\mathrm{c}}$ oxides are predicted for compounds with bigger assemblies of ${\mathrm{CuO}}_2$ layers coupled by Ba-layers.

Elastic light scattering has been used to measure the spin-correlation length ξ, in the reentrant spin-glass ${\mathrm{Eu}}_{0.54}$${\mathrm{Sr}}_{0.46}$S. ξ is found to change only modestly as one goes from the ferromagnetic to the spin-glass (SG) phase, i.e., from 2700 to 2000 Å. This result may be reconciled with neutron experiments which find ξ∼100 Å in the SG regime by ascribing the light scattering in the SG regime to the remanent magnetization of the original ferromagnetic regions.

Translational invariance requires that the electron density operator is not diagonal in a Wannier representation. We show that this leads to important terms in the electron-electron interaction Hamiltonian, terms which involve the bond-charge density and which are not included in the widely studied extended Hubbard model. When these bond-charge interactions are included, we find that electron-electron repulsion tends to stiffen the lattice (for physically reasonable parameters) and to oppose dimerization in polyacetylene and in other charge-density-wave systems. A related decrease in the Peierls gap for fixed dimerization is also obtained. These conclusions are in contrast with previous results obtained from the extended Hubbard model.

We report the discovery of an oscillatory instability in the dynamics of incommensurate structures. The oscillations survive the thermodynamic limit. The instability occurs for both long- and short-range interactions. The frequency and stability of the oscillations are studied.

A neutrino signal from the supernova SN1987A is used to place an upper limit on the neutrino mass. If most of the neutrinos must have been emitted within several seconds, as suggested by astrophysical models, the last three of the eleven events observed by the Kamioka detector must correspond to noise or to the tail of a distribution in emission times. If the remaining eight events (which arrived within two seconds of one another) are due to neutrinos emitted within four seconds (a conservative upper limit), bound &≤12 eV/$c^2$ is obtained. The Irvine-Michigan-Brookhaven data, with a higher energy threshold, primarily provide information regarding the total duration of the burst.