Neutron scattering experiments on an insulating single crystal of ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{NiO}}_4$ show the presence of strong two-dimensional incommensurate magnetic correlations in the ${\mathrm{NiO}}_2$ planes. At low temperatures, T=2 K, these exist on time scales τ>${10}^{\mathrm{-}11}$ s. The typical side length of the correlated regions in the direction parallel to the incommensurate modulation is equal to the average spacing between strontium atoms.

We have measured 1/f noise of single-grain-boundary junctions in superconducting ${\mathrm{Y}}_1$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial films. The origin of 1/f noise is identified and the data are used to rule out recent suggestions for new mechanisms which generate 1/f noise. Using ozone annealing, both 1/f noise and critical current density of the grain boundary were found to improve. However, the persistence of both noise and Josephson coupling suggest that oxygen deficiency at the boundary is not the sole cause of the anomalous coupling in these materials. We speculate that the depression of the order parameter is connected with the cores of the dislocations forming an array.

Single crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ with different oxygen concentrations were investigated by Raman scattering. Dependent on the polarizations of the incident and the scattered light, the electronic spectra measured well below ${\mathit{T}}_{\mathit{c}}$ show either zero or finite intensity at small energy transfers. In oxygen-annealed crystals, the superconducting order parameter Δ is temperature dependent. In contrast, Δ is almost constant in the essentially two-dimensional argon-annealed samples.

It is numerically shown that there may be a new type of ordered state (in some sense glassy) in far-from-equilibrium systems which can be identified with a large population of coupled limit-cycle oscillators, provided couplings are not only random but also frustrated. It is characterized by quasientrainment and algebraic relaxation.

We construct a 19-bit lattice-gas model which is shown to exhibit spontaneous fluctuation correlations as produced in a thermal fluid system. Thus the lattice gas can be considered—in the same sense as a real fluid—as a reservoir of excitations with wavelengths and frequencies ranging from the microscopic level to the hydrodynamic scale. In particular the dynamic structure factor obtained from lattice-gas simulations is in agreement with the classical Rayleigh-Brillouin spectrum of real fluids.

A classical version of the Car-Parinello method is proposed, combining density-functional theory for the counterions and molecular dynamics for the macroions, to simulate charged colloidal suspensions. The classic Derjaguin-Landau-Verway-Overbeek (linear screening) theory is shown to fail for large macroion packing fractions.

A dynamic thermally induced cooperative first-order phase transition between ripple phases is described for fully hydrated mixtures of dipalmitoylphosphatidylcholine and 5-mole % cholesterol bilayers. The ripple-ripple phase transition has not been observed for pure phosphatidylcholine systems. This transition has not been observed for pure phosphatidylcholine systems. This transition is more likely driven by changes in headgroup-water-headgroup interactions than by a change in acyl chain tilt. Implications for mechanical processes within a lipid bilayer are discussed.

A new stochastic method for the direct computation of real-time Green’s functions is proposed. The inherent sign problem is circumvented by partitioning the path integration into two parts, one of which involves conventional stochastic sampling, and the other explicit or analytical summation. Using this method, the dynamics of the spin-boson model may be computed up to several tunneling periods. The results reveal surprisingly complex relaxation behaviors near the coherent-incoherent boundary at low temperatures.

We show that the metric and Berry's curvature for the ground states of $N=2\mathrm{}$ supersymmetric $\sigma$ models can be computed exactly as one varies the Kahler structure. For the case of ${\mathrm{CP}}^n$ these are related to special solutions of affine Toda equations. This allows us to extract exact results. We find that the ground-state metric is nonsingular as the size of the manifold shrinks to zero, suggesting that 2D quantum field theory makes sense even beyond zero radius. Thus it seems that manifolds with zero size are nonsingular as target spaces for string theory (even when they are not conformal).

We consider the inclusive reaction pp→ΛX in the fragmentation region and present a dynamical mechanism which leads to a fair description of all the basic features of the observed Λ spectrum. We also show in a simple manner why it generates a nonzero Λ polarization whose magnitude and $sign— are consistent with existing data.

We performed a numerical investigation of the dynamics of the Bloch equations, extended for dense media (media having many atoms within a cubic resonance wavelength), for optical pulses whose duration is much less than an induced-dipole dephasing time. We find a signature of near dipole-dipole interaction which may provide a useful method for validating the first-principles model, for measuring the strength of the interaction, and for coherent pumping. Further, we demonstrate a new and unique optical switching mechanism which may lead to the development of important devices.

Electrons scattered by $3^2$${\mathit{P}}_{3/2}$, ${\mathrm{M}}_{\mathrm{L}}$=±1 sodium has been measured in the collision energy range up to 5 eV and in the angular range 0° to 30°. Absolute differential cross sections are reported for deexcitation to $3^2$${\mathit{S}}_{1/2}$ and excitation to $4^2$${\mathit{S}}_{1/2}$, obtained without normalization. The 3P→4S cross sections are over a factor of 2 times larger than those for the corresponding 3S→3P reaction, for the same final electron energy. Comparison with available theory is presented.

The two-photon excitation spectrum in polycrystalline α-sexithienyl (${\mathit{T}}_6$) thin films has been investigated at 4.2 K in the spectral range between 910 and 1180 nm of the fundamental laser radiation. The intense two-photon absorption band at 18 350 ${\mathrm{cm}}^{\mathrm{-}1}$ is assigned to the $2^1$${\mathit{A}}_{\mathit{g}}$ exciton band origin. The comparison of the one-photon and two-photon excitation spectra shows that the lowest ‘‘gerade’’ exciton level lies at 898 ${\mathrm{cm}}^{\mathrm{-}1}$ above the lowest one-photon-allowed 1 ${}_{}{}^2{}_{}{}^{}$${\mathit{B}}_{\mathit{u}}$ exciton level.

We show that photorefractive media can support a new type of spatial soliton, in which the diffraction is balanced by the self-scattering (two-wave mixing) of the beam spatial frequency components. This photorefractive soliton possessses some unique properties, such as independence of the absolute light intensity, and can experience absorption (or gain) with no change in its transverse structure.

The quadrupole moments of ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ are determined by combining the calculated electric-field gradients of Na(3p${;}^2$P°) and Al(3p${;}^2$P°) with experimental quadrupole coupling constants. Q${(}^{25}$Mg) was determined previously using the same method [Nucl. Phys. A534, 360 (1991)]. Contrary to Q${(}^{25}$Mg) the ‘‘atomic’’ values for Q${(}^{23}$Na) and Q${(}^{27}$Al) lie outside the error limits of the ‘‘muonic’’ values. The obtained values for Q${(}^{23}$Na), Q${(}^{25}$Mg), and Q${(}^{27}$Al) are 0.1089(32), 0.1994(20), and 0.1402(10) b, respectively, as compared to the muonic values of 0.1006(20), 0.201(3), and 0.150(6) b.

The Goos-Hänchen longitudinal shift at total reflection of an optical Gaussian beam is experimentally investigated for only one reflection. The differential experimental method uses the high sensitivity of the eigenstates of a quasi-isotropic laser to small perturbations to measure an intracavity Goos-Hänchen effect for angles of incidence both below and above the critical angle. The measurements are in good agreement with our calculations of the longitudinal shift for Gaussian laser beams.

The overall propagation rate of growing interfaces subject to weak random stirring is investigated, based on a variant of the Kardar-Parisi-Zhang equation. Scaling analysis, supported by computations, indicates that random stirring increases the overall propagation rate in proportion to 4/3 power of the ratio of stirring rate to growth rate. The generally assumed quadratic dependence on this ratio is valid only for special cases such as periodic flow. On this basis, a modified turbulent-flame speed formula is proposed. Implications concerning interpretation of measurements are noted.

Homodyne photon correlation spectroscopy (HCS) and laser Doppler velocimetry (LDV) were used to study the probability density function of velocity differences δv(l) between points in the fluid separated by a distance l. The two measuring schemes yield different results for the probability density, P(δv(l))dδv(l). HCS probes the spatial fluctuations directly, whereas LDV records temporal fluctuations in velocity and relates them to the spatial fluctuations through the Taylor hypothesis. The measurements therefore imply the failure of this hypothesis in the Reynolds number range where we have applied it.

The propagation of an intense electromagnetic pulse in a large underdense plasma has been investigated using spatially and temporally incoherent laser light. Spatial incoherence was obtained using the induced spatial incoherence (ISI) technique. The first observation of the occurrence of thermal whole-beam self-focusing with ISI laser light is presented. Thermal imaging with a multiframe x-ray camera (140 ps gate time) was used to record the propagation characteristics of the interaction beam through the plasma. A direct measurement of the self-focusing growth length has been made.

We have demonstrated that an underdense ionization front can significantly upshift the frequency of an impinging electromagnetic wave. Source radiation at 35 GHz was shifted to more than 116 GHz when a relativistically propagating, laser-produced ionization front interacted with microwave radiation that was confined in a resonant cavity. The amount of upshift was proportional to the plasma density in the front, in agreement with theoretical predictions.