A comprehensive study of odd-A rotational bands in normally deformed rare-earth nuclei indicates that a large number of seniority-one configurations (30% for odd-Z nuclei) at low spin have moments of inertia nearly identical to that of the seniority-zero configuration of the neighboring even-even nucleus with one less nucleon. It is difficult to reconcile these results with conventional models of nuclear pair correlation, which predict variations of about 15% in the moments of inertia of configurations differing by one unit in seniority.

Based on the systematics for hard photon production a scheme is suggested which relates the hard-photon multiplicity observed in intermediate energy heavy ion reactions to the impact parameter. The method is applied to data obtained in an exclusive experiment of the reaction ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{158}$Gd at 44 MeV/nucleon, demonstrating the usefulness of this new approach.

Angular correlation functions for the ${\mathrm{He}}^4$(${\mathrm{\pi }}^{\pm }$,${\mathrm{\pi }}^{\pm \prime }$t)p reaction at the incident energy ${\mathrm{T}}_{\mathrm{\pi }}$=180 MeV and at four pion scattering angles have been measured. The ${\mathrm{He}}^4$(${\mathrm{\pi }}^{\pm }$,${\mathrm{\pi }}^{\pm \prime }$t)p data complement previous ${\mathrm{He}}^4$(${\mathrm{\pi }}^{\pm }$,${\mathrm{\pi }}^{\pm \prime }$p)t results by greatly extending the available range of proton emission angles. We calculate angular correlation functions of the cross sections with three different quasifree knockout models, and we show that direct triton knockout is crucial to an understanding of the data. © The American Physical Society

We report on the first femtosecond time-resolved experiments in cluster physics. The photofragmentation dynamics of small sodium cluster ions ${\mathrm{Na}}_{\mathit{n}}^{+}$ have been studied with pump-probe techniques. Ultrashort laser pulses of 60-fs duration are employed to photoionize the sodium clusters and to probe the photofragments. We find that the ejection of neutral dimer ${\mathrm{Na}}_2$ and, observed for the first time, neutral trimer ${\mathrm{Na}}_3$ photofragments occur on ultrashort time scales of 2.5 and 0.4 ps, respectively. This and the absence of cluster heating reveals that direct photoinduced fragmentation processes are important at short times rather than the statistical unimolecular decay.

The correlation between fluorescence photons emitted by an optically pumped single molecule of pentacene in a p-terphenyl host has been investigated at short times. The correlation function shows photon antibunching, a unique feature of a nonclassical radiation field, which decreases if two molecules rather than one are pumped at the same time. The peculiarities of the correlation function for a three-level molecule are discussed and the theoretical description outlined.

We show analytically and numerically that the linearly stable flat interface in the Saffman-Taylor problem becomes unstable against finite amplitude perturbations when the surface tension is modified in proportion to the local curvature. This nonlinear instability is identified with unstable saddle points in a flow diagram in a weakly nonlinear approximation. Numerical simulations confirm this prediction and reveal that this instability leads to spiky celluar patterns.

We report a real-time, two-dimensional light scattering study of the evolution of structure in a concentrated electrorheological fluid during the ‘‘liquid-solid’’ phase transition. We find that after particle chaining along the electric field lines, strong light scattering lobes appear at a finite scattering wave vector q orthogonal to the field lines, and then brighten as they move to q=0. This indicates the existence of an unstable concentration fluctuation that signifies the segregation of chains into columns. We suggest a theoretical explanation for our results.

Nonresonant fast ions (${\mathit{E}}_{\mathit{i}}$/${\mathit{T}}_{\mathit{i}}$∼100) with large ratios of perpendicular-to-parallel energy (${\mathit{E}}_{\mathrm{\perp }}$/${\mathit{E}}_{\mathrm{\parallel }}$∼100) have been observed in rf-produced, electron-cyclotron-resonance-heating-produced and low-density Ohmic plasmas in the Continuous Current Tokamak device by means of a scannable fast neutral spectrometer. Superthermal ions (${\mathit{E}}_{\mathit{i}}$/${\mathit{T}}_{\mathit{i}}$∼20) are also observed in high-harmonic ion cyclotron-heated tokamak discharges (ω/${\mathrm{\Omega }}_{\mathit{i}}$∼20), but appear to be isotropic, possibly due to pitch-angle scattering. These measurements strongly suggest the existence of a universal mechanism of perpendicular ion acceleration.

The adsorption of potassium on Al(111) at 90 K and at 300 K has been investigated by low-energy electron diffraction (LEED). Although a (√3 × √3 )R30° structure is formed at each temperature, a detailed LEED analysis has revealed that the adsorbate positions are quite different and unusual in each case. At 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites. An irreversible phase transformation from the former to the latter structure occurs on warming to 300 K. These results are discussed in the light of recent density-functional-theory calculations.

A polymer in a random velocity field is investigated in application to turbulent drag reduction. The model employed constrains the links to a constant length. The results differ strongly from the usual linear model and show novel behavior. With no thermal fluctuations included, and with finite stiffness, the steady state of the chain is a straight line. When the stiffness is zero the straight configuration is marginally stable. When thermal fluctuations are included, the amplitude of bending modes has a power-law probability distribution, whose exponent depends continuously upon parameters.

The correlation lengths of vitreous structures estimated from the low-frequency (boson) peak in Raman spectra and from the width of the first sharp diffraction peak are compared. This first comparison shows that the two estimates, which are based on dynamic and static properties, respectively, are in good quantitative agreement. The present result supports the concepts that (a) a decisive scale for structural order in glasses is ∼10 Å, and (b) the low-frequency excitations are defined by the characteristic length in vitreous structures.

Interface dynamics are analyzed in directional growth of liquid crystals in a two-mode interaction picture. We find that this simple model retains many interesting features observed experimentally including order and chaotic motion. We discover that the parity-broken mode undergoes an instability characterized by a permanent hopping between q and 2q states (where q is the wave number). On increase of the two-mode coupling the interface motion enters a chaotic regime via a quasiperiodicity route. Chaos manifests itself by an erratic change of the drift direction of the pattern between left and right. We tentatively call it phase chaos.

We report results from computer simulations of the kinetics of domain growth and wetting in a binary liquid mixture contained inside a model porous medium which has an interconnected and tortuous structure resembling Vycor glass. We find that the growth process slows down dramatically when the average size of the domains is comparable to the average pore radius and dynamical scaling breaks down at late stages. We argue that the random field model cannot be invoked to explain these results. On the other hand, the single-pore model seems to be reasonably successful in explaining the source of this slow growth.

We study the nonequilibrium dynamics of line liquids as in the nonlinear motion of flux lines of a superconductor driven by an applied electric current. Our analysis suggests a transition in the dynamics of the lines from a smooth, laminar phase at small driving forces, to a rough, turbulent phase when the drive is increased. We explore the nature of these phases and describe interesting analogies to driven diffusion and growing interfaces.

We perform a one-loop, self-consistent calculation of the density-wave and director fluctuations above the nematic–smectic-A phase transition, find the renormalization of the perependicular and parallel correlation lengths, and for the first time calculate values of the anisotropic critical-point exponents ${\nu }_{\mathrm{\parallel }}$ and ${\nu }_{\mathrm{\perp }}$. As the transition is approached several distinct regions of scaling behavior are found with increasingly anisotropic critical behavior. A plot of effective exponents from experiment versus the anisotropy ratio ${\nu }_{\mathrm{\parallel }}$/${\nu }_{\mathrm{\perp }}$ reveals a systematic correlation that agrees with our calculation.

We present the muon-spin-relaxation studies in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the temperature range 0.5–1.5 K. At temperatures below 1 K we observed muonium spin precession. From the magnetic-field dependence of the amplitude of the muonium component we estimate a muonium formation rate of about 5×${10}^7$ ${\mathrm{s}}^{\mathrm{-}1}$ at 0.5 K. We find that external electric fields, preventing the convergence of muon and electron, reduce the amplitude of the muonium precession signal.

We suggest that the herringbone reconstruction of the Au(111) surface results from the spontaneous formation of ‘‘stress domains.’’ The surface is described theoretically by a 2D Frenkel-Kontorova model. Upon including long-range elastic interactions, the stress-domain pattern is energetically favored. The surface topography and structure factor obtained by relaxing atomic coordinates using molecular dynamics bear a strong resemblance to scanning tunneling microscopy pictures and x-ray scattering data, respectively. Our numerical estimate for the separation between domain walls is consistent with experimental results.

Local-density-functional (LDF) calculations show that on Al(331), a surface whose narrow (111)-like terraces are separated by (110)-like steps, a trivalent Al adatom is strongly attracted to step bottoms, while a monovalent H adatom prefers step edges. H-adatom potential energies in the three-fold terrace sites on Al(331) and Al(111) differ by only about 0.1 eV. This is because H atoms are small and must sit close to substrate atoms, where step effects are rapidly screened. Al adatoms, being larger, prefer the terrace sites of Al(331) by more than 0.25 eV.

We have developed a theory of surface acoustic phonon normal modes and of Brillouin-scattering ripple cross section for a periodically corrugated surface of an opaque material, in the small corrugation limit. Both the discrete and continuous spectra of acoustic modes on a grating have been studied within the elasticity theory. In the continuum, the Rayleigh wave becomes a resonance and hybridizes with the longitudinal pseudomode of the flat surface, giving rise to a gap. The theory explains quantitatively recent experimental results obtained for a shallow grating on a Si(001) surface.

The structural order of the two-dimensional (2D) charge-density-wave (CDW) phase in ${\mathrm{Nb}}_{\mathit{x}}$${\mathrm{Ta}}_{1\mathrm{-}\mathit{x}}$${\mathrm{S}}_2$ materials has been determined as a function of impurity concentration from the quantitative analysis of scanning tunneling microscopy images. We show that the CDW phase evolves through crystalline, hexatic glass, and liquidlike states as the impurity concentration in the lattice increases to x=0.10. These results address systematically the structural manifestations of quenched disorder in 2D systems.