A new Hamiltonian density formulation of a perfect fluid with or without a magnetic field is presented. Contrary to previous work the dynamical variables are the physical variables, $\rho$, $\overrightarrow{\mathrm{v}}$, $\overrightarrow{\mathrm{B}}$, and $s$, which form a noncanonical set. A Poisson bracket which satisfies the Jacobi identity is defined. This formulation is transformed to a Hamiltonian system where the dynamical variables are the spatial Fourier coefficients of the fluid variables.

This Letter reports the first observations of stimulated Brillouin scattering in microwave interaction with a plasma, as verified by the satisfaction of the frequency- and wavelength-matching rules and growth rate. A small amount of chamber reflectivity causes ion fluctuations due to the standing-wave ponderomotive force, which then serve as an enhanced noise level for the initiation of the instability.

The current-driven electrostatic ion-cyclotron instability in a $Q$ machine operated in the low-density regime ${\left(\frac{{\omega }_{\mathrm{pe}}}{{\omega }_{\mathrm{ce}}}\right)}^2\ll 1$ is stabilized by externally generated rf electrostatic fields in the lower-hybrid range. Stabilization is due to a resonant ponderomotive force, which reduces the instability frequency, thus increasing the ion-cyclotron damping. The experimental results are in good agreement with an analysis which describes the effect of rf power on the instability frequency shift and growth rate.

A simulation study of the electrostatic interaction of a field-aligned electron current sheet with ambient hot ions ($\frac{T_e}{T_i}\lesssim 0.6$) and a conducting boundary shows that a $V$-shaped potential double layer spontaneously develops and accelerates electrons to the ion thermal energy. It is proposed that this double-layer process is responsible for the aurora energy supply. Only as $\frac{T_e}{T_i}$ approaches unity, the potential exceeds $T_i$ via resistivity enhanced by wave trapping of electrons.

It is shown that the repulsive part of the He-surface interaction is given by an almost linear, surface-independent function of the surface electron density at the He site. This gives a very simple connection between the measured low-energy He-beam scattering potentials and the substrate electron densities. The scheme is applied to an analysis of the He-scattering potential outside Al.

The phase diagram of an Ising fcc antiferromagnet in a field is investigated by a study of the ground state and Monte Carlo simulation of a lattice with 16 384 sites. At the critical field between the two "ordered" phases, the disordered phase is stable down to zero temperature due to "frustration" effects. The corresponding alloy phase diagram disagrees with all previous calculations. Order parameters and internal energy are briefly compared to experimental data on the Cu-Au system.

Principal planes of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ solid-superfluid interface are expected to undergo roughening transitions at temperatures of about 1 K. An experiment is described in which two such transitions were observed in the hcp-superfluid interface: first for the basal plane at 1.08 K and second for an orthogonal face at 0.85 K.

This Letter presents an approximate closed-form solution to the density-functional theory for the surface tension and surface width of simple liquids. The results for both are in excellent agreement with experiments on Ar over wide range of temperature and density. The theory is extended via an ion density-functional formalism to simple metals. Good agreement is obtained for the surface tensions of ten liquid metals and for the surface width of liquid Hg.

The main problems underlying the experimental study of particles with sizes between single atoms and the solid state have been solved: the formation of metal clusters, their individual detection, and the separation of beams with uniform cluster size. By inertgas condensation of metal vapors the clusters ${\mathrm{Sb}}_1$-${\mathrm{Sb}}_{500}$, ${\mathrm{Bi}}_1$-${\mathrm{Bi}}_{280}$, and ${\mathrm{Pb}}_1$-${\mathrm{Pb}}_{400}$ have been produced. The high intensity observed over the whole mass range henceforth allows a systematic size-dependent investigation of metal clusters.

The first determination of two-dimensional adsorbate-induced corrugation functions from He-diffraction data is reported. Local maxima in the corrugations of both H(2×6) and H(2×1) on Ni(110) directly reflect adatom positions due to the localized charge distribution of the chemisorbed H atoms. Hence, genuine information on the adsorption sites is obtained leading to novel insight into coverage and adsorbate-adsorbate interactions. The transition to the reconstructed (1×2) saturation phase is discussed.

The electrical conductivity of two linear-chain organic compounds in their semiconducting states is found to increase drastically witht the application of electric fields of ∼ 10 mV/cm and with the increase of frequency to ∼ 10 GHz. Also, the electron-spin-resonance line is found to switch abruptly from unresolvable to fully resolved (narrow and metal-like) at fields of 0.1 V/cm. One speculative explanation of these observations is the presence of spin-density waves.

Granular A1 films have been measured in a microwave cavity at 9.4 GHz together with their dc resistance, $R\left(T\right)\mathrm{}$. The change in resonant frequency is proportional to the number of superconducting electron pairs which is finite above the temperature ${T_c}^{\rho }$, where $R\left(T\right)\mathrm{}\simeq 0$. Microwave loss has been identified which appears to be specific to granular materials and vanishes only well below ${T_c}^{\rho }$. This is shown to be due to normal conduction between grains, which upon cooling becomes progressively more Josephson-like.

The structure and dynamics of the Anderson plasma mode in a high-$Q$, high-current-density, low-capacitance Josephson junction is examined. It is found that if the junction is subjected to a microwave field which is resonant with a plasmon transition, the relevant levels will be split in a manner analogous to the Stark effect.

Stable trivalent lead impurities have been observed at room temperature in "flux-grown" single crystals of YP${\mathrm{O}}_4$ and LuP${\mathrm{O}}_4$ by electron paramagnetic resonance. Because of a fortuitous set of circumstances, a unique measurement of the hyperfine constant of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}_{}^{3+}{}_{}{}^{}$ in these materials was possible. The resulting values are the largest ever observed for the trivalent lead ion in a solid. A super hyperfine interaction with ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ neighbors has also been measured and, surprisingly, this interaction is larger for the second ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ neighbors than for the first ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ neighbors.

A diagrammatic theory is presented for the density response function of a system of independent particles moving in a random potential in terms of a current relaxation kernel $M(\overrightarrow{\mathrm{q}},\omega )$ (essentially the inverse of the diffusion coefficient). In the presence of time-reversal invariance, $M(\overrightarrow{\mathrm{q}},\omega )$ is shown to have infrared divergencies in $d<~2$ dimensions. A self-consistent treatment of the divergent terms yields a finite static electric polarizability $\alpha$, a dynamical conductivity $\sigma \left(\omega \right)\propto {\omega }^2(\omega \to 0)$, and a finite localization length in $d<~2$ for arbitrarily weak disorder.

The exact frequency dependence of the ac hopping conductivity of a one-dimensional chain with random interruptions is obtained. The real and imaginary parts of the ac conductivity are shown to vanish quadratically and linearly, respectively, with frequency at the static limit. Critical behavior of the ac conductivity in the limit of no interruptions is discussed.

The two-point function of the two-dimensional Ising model at arbitrary temperature is expressed in terms of the solution of a nonlinear partial difference equation. From this difference equation the known results for the two-point function of the Ising field theory may be regained as a special case.

It is argued that the leading corrections to scaling for planar Ising models, which occur as analytic factors, arise from the quadratic terms of the nonlinear thermal and ordering fields (rather than from irrelevant variables). This yields ${{\alpha }_{\mathrm{eff}}}^{\prime }+2\mathrm{}{\beta }_{\mathrm{eff}}+{{\gamma }_{\mathrm{eff}}}^{\prime }=2\mathrm{}$ for the effective critical exponents, with $\mathrm{no}$ leading corrections, and is confirmed by exact square-lattice results for arbitrary anisotropy $\frac{J_2}{J_1}$. For isotropic lattices the ratios of correction amplitudes and quadratic nonlinear-field terms appear universal.

Interactions of 209-GeV muons in the multimuon spectrometer at Fermilab have yielded 20 072 dimuon final states, with (81±10)% attributed to production of charmed states decaying to muons. The cross section for diffractive charm muoproduction is ${6.9}_{-1.4\mathrm{}}^{+1.9\mathrm{}}$ nb. Extrapolated to $Q^2=0\mathrm{}$, the effective cross section for 178- (100-) GeV photons is ${750}_{-130\mathrm{}}^{+180\mathrm{}}$ (${560}_{-120\mathrm{}}^{+200\mathrm{}}$) nb.

We present the dimuon mass spectrum from 102 678 three-muon final states produced by muon interactions within a magnetized steel calorimeter. The data place a 90%-confidence-level upper limit on the production of $\Upsilon$ states by muons: $\sigma (\mu N\to \mu \Upsilon X)B(\Upsilon \to {\mu }^{+}{\mu }^{-})<\mathrm{}22\mathrm{}\times {10}^{-39\mathrm{}}$ ${\mathrm{cm}}^2$, consistent with a photon-gluon-fusion model calculation.