The two-plasmon decay of light propagating in filaments in a laser-produced plasma and the resulting $\frac{3{\omega }_0}{2}$ light emission is discussed with use of a simplified model. Experimental observations are consistent with the predictions of the model. Implications for use of the $\frac{3{\omega }_0}{2}$ doublet splitting as a temperature diagnostic are discussed.

A charged particle trapped in an electrostatic wave is accelerated in the plane perpendicular to the wave vector $\overrightarrow{\mathrm{k}}$. It is found that there is an optimum angle $\theta ={\theta }_m$ at which the particle gains a maximum energy which is about four times larger than that at $\theta =\frac{\pi }{2}$, $\theta$ being the angle between $\overrightarrow{\mathrm{k}}$ and the magnetic field.

An x-ray-diffraction study of in-plane structures in K-graphite has revealed a remarkable chemical potential (${\mu }_{\mathrm{K}}$) dependence of the in-plane density in pure stages 1 and 2, and some continuous nature of the stage-1 melting transition. A simultaneous growth of intralayer and interlayer correlations of K atoms in stage-1 liquid is observed as ${\mu }_{\mathrm{K}}$ approaches the solidification poínt. This transition is weakly first order. A new stacking transition from $\alpha \beta$ to $\alpha \beta \gamma \delta$ K layers in stage-1 solid is also reported.

We have observed narrow, distinct resonances in the NMR spectrum of dilute spinpolarized atomic hydrogen gas ($n\sim {10}^{16}$ atoms/${\mathrm{cm}}^3$). The dependence of the observed spectra on temperature, density, polarization, and magnetic field gradient is consistent with theoretical predictions for spin-wave excitations damped by diffusion. We have measured the parameter $\mu$, which is a measure of the importance of exchange effects in spin-transport processes, and the diffusion coefficient $D_0$, both of which are in reasonable agreement with theory.

The spin dynamics of weakly interacting, spin-polarized quantum gases are discussed in terms of a quasiparticle picture of these systems. Quantum mechanical exchange between identical particles introduces "molecular" magnetic fields which give rise to the precession of the spin current; this in turn leads to coherent oscillations of the magnetization. The theory is used to discuss magnetic resonance experiments in spin-polarized atomic hydrogen.

The method outlined by Kerstein has been used to simulate percolation of the void region between overlapping, randomly located spheres. The computed threshold is in agreement with the previous result of Kertész. In addition, three critical exponents are computed and are found to be in agreement with the universality hypothesis. This constitutes the first such evaluation for a three-dimensional nonlattice problem and the first test of universality for a percolation problem with no underlying network defined a priori.

The three-body recombination rate in spin-polarized hydrogen has been measured at temperatures from 0.25 to 0.60 K in magnetic fields from 5 to 9 T. In contrast to theoretical expectation, the three-body rate is a weakly decreasing function of field in this region. At 7.6 T the gas and surface three-body rate constants are $L_g=7.5\mathrm{}\left(3\right)\mathrm{}\times {10}^{-39\mathrm{}}$ ${\mathrm{cm}}^6$ ${\mathrm{s}}^{-1\mathrm{}}$ and $L_s=2.0\mathrm{}\left(6\right)\mathrm{}\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^4$ ${\mathrm{s}}^{-1\mathrm{}}$. The large value of $L_s$ can account for effects previously attributed to an anomalously large surface two-body nuclear relaxation rate.

The effects of solitons on the spectrum of fermion excitations in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$A$ are investigated. It is found that there is a two-dimensional manifold of bound states with energies within the gap of the bulk superfluid. The bound-state spectrum lacks inversion symmetry parallel to the wall.

We have shown that rotons as well as phonons can evaporate ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms in a single-quantum process. Measurements of the time of flight and the angular distribution of the evaporated atoms clearly distinguish between evaporation by phonons and rotons. The results indicate that energy and the parallel component of momentum are conserved at the free liquid surface.

To assess the importance of pair and three-body interactions between adatoms we have made field-ion-microscopic observations comparing the behavior of two with three and more rhenium atoms on the W(110) plane. For two adatoms, close dimers are not stable, and measurements of the radial distribution function confirm that pair interactions are repulsive out to ≈ 10 Å. Despite that, Re trimers form from three adatoms. Quantitative studies of trimer dissociation lead to a binding energy due to trio interactions of ≈ 0.25 eV.

An $x-y$ model with nearest-neighbor interaction $V\left(\theta \right)=2J\{1-{\left[{cos}^2\right(\frac{\theta }{2}\left)\right]}^{p^2}\}$ is studied by Monte Carlo simulation. The model has a transition from a paramagnetic to an algebraic (massless) phase. An increase of the parameter $p$, which changes the shape of the potential, brings about a change in the nature of the transition from continuous to first order.

An absolute determination of the superconducting penetration length in niobium has been carried out by a novel technique, in which the reflectivity of a polarized-neutron beam from a polished surface is measured. The result thus obtained, $\Lambda \mathrm{}\left(0\right)=410\mathrm{}\pm 40$ Å, is in substantial agreement with the earlier work.

The lower critical dimension for a random-field Ising model cooled from the paramagnetic region is found to be 4, although it is 2 at equilibrium. The coherence length $R$ is proportional to $H^{{-\nu }_H}f\left(t\right)\mathrm{}$, where $H$ is the random-field amplitude and $t$ is the time. At low temperature, ${\nu }_H=2\mathrm{}$ and $f\left(t\right)=\ln \left(\frac{t}{\tau }\right)$. At the transition temperature $T_{c0}$ of the pure system, ${\nu }_H\simeq 1$ and $f\left(t\right)=1\mathrm{}$. The agreement with experiment is acceptable.

The dynamics of a continuum interface model in a random medium are studied and the results applied to the random-field Ising model. We find that if the dimensionality $d<\mathrm{}5\mathrm{}$, the interface will move only if a force beyond a finite depinning threshold $F_c\sim h^{\frac{4}{\mathrm{}(5-d)\mathrm{}}}$ is applied, where $h$ is the random field strength. Thus, when the random-field Ising model is quenched to low temperatures, there is a critical value $R_c\sim \frac{1}{h^{\frac{4}{\mathrm{}(5-d)\mathrm{}}}}$ for the average radius of curvature $R$ of the domain walls. If $R>\mathrm{}{R}_c$, the domain structure is frozen. If $R<\mathrm{}{R}_c$, the domain structure evolves until $R\sim R_c$.

Measurements of the magnetic susceptibility, specific heat, and electrical resistivity of ${\mathrm{U}}_2$${\mathrm{Zn}}_{17}$ reveal a magnetic phase transition of this compound at 9.70 K. Details of the specific heat indicate that the ordering occurs among electrons whose energy spectrum shows an anomalously large density of states at the Fermi energy. This result is of interest with regard to the occurrence of superconductivity in similar systems.

In contrast to our earlier measurements as well as the band calculated from the $\pi$-bonded chain model, a very narrow dangling-bond band has been reported by Solal et al. from a recent photoemission experiment on the Ge(111)-2 × 1 surface ($35<~\hslash \omega <~50$ eV). We present new photoemission measurements for a broad range of photon energies ($8.6<~\hslash \omega <~35$ eV) in which a highly dispersive dangling-bond band is found, in good agreement with the $\pi$-bonded chain model.

An angle-, energy-, and spin-resolved photoemission experiment has been performed for Pt(111) with the circularly polarized synchrotron radiation of the new German dedicated storage ring BESSY. The photoelectron polarization measured as function of the kinetic energy at different photon energies between 6.5 and 24 eV for normal incidence and normal emission shows a pronounced spectral variation up to ±55%. The electron intensities and polarizations observed allow a symmetry-resolved band characterization of Pt in the $\Gamma -L$ direction.

The upper critical magnetic field of Eu${\mathrm{Mo}}_6$${\mathrm{S}}_8$ in its high-pressure superconducting phase was measured resistively at 16 kbar in dc magnetic fields to 28 T. The data can be described by multiple pair-breaking theory including a negative exchange field due to the ${\mathrm{Eu}}^{2+}$ ions that acts on the superconducting electrons. Measurements of the ac heat capacity of Eu${\mathrm{Mo}}_6$${\mathrm{S}}_8$ above ∼ 13 kbar revealed that the compound exhibits bulk superconductivity, but no Meissner effect was observed in magnetization measurements.

The need for inducing a probability distribution from partial data and the complementary problem of the analysis of an observed distribution in terms of fewer relevant variables occur in many branches of physics. For reproducible experiments, consistency conditions which must be satisfied by any algorithm for inferring a discrete probability distribution with given averages are formulated. The only consistent algorithm is the one leading to the distribution of maximal entropy subject to the given constraints.

A new analytic method for the determination of quasinormal oscillations of a black hole is presented. It is based on a connection between the quasinormal modes and the bound states of the inverted black-hole potentials. With use of this method, the quasinormal frequencies of a Schwarzschild, a Reissner-Nordström, and a slowly rotating Kerr black hole are determined in the eikonal approximation, Moreover, we provide evidence for the stability of an extreme Kerr black hole.