Direct numerical simulations with up to ${1024}^2$ resolution are performed to study statistical properties of the inverse energy cascade in stationary homogeneous two-dimensional turbulence driven by small-scale Gaussian white-in-time noise. The energy spectra for the inverse energy cascade deviate strongly from the expected ${\mathit{k}}^{\mathrm{-}5/3}$ law and are close (somewhat flatter) to ${\mathit{k}}^{\mathrm{-}3}$. The reason for the deviation is traced to the emergence of strong vortices distributed over all scales. Statistical properties of the vortices are explored.

A recent experiment in the IUCF cooler ring studied the adiabatic turn-on of a partial Siberian snake at 370 MeV, where the spin tune, ${\nu }_{\mathit{s}}$ is 21/2 for all snake strengths. The snake consisted of two rampable warm solenoid magnets in series with a superconducting solenoid; this combination allowed varying the snake strength between about 0 and 25% at 370 MeV. We measured the beam polaraization after varying the snake either 1, 2, or 10 times; we found with good precision that no polarization was lost. This supports the conjecture that a Siberian snake can be ramped adiabatically at an energy where the spin tune is a half integer.

Raman forward scattering of short-pulse relativistic-intensity laser pulses is investigated. Differential equations which model the instability for arbitrarily large pump strengths are derived. Exact solutions are obtained for a set of physically relevant initial conditions. The growth rate is found to asymptotically approach zero for ultrarelativistic laser intensities. The relevance of the results to present experiments and the limitations of the quasistatic equations are discussed

For the first time, a model-free procedure is developed to analyze polarized neutron diffraction data pertaining to acentric crystals. It consists of a two-step process, featuring first an effective flipping ratio and second a linear inverse problem. The latter is solved either by a new generalized inverse Fourier transform or by using maximum entropy. Using metamagnetic UCoAl as a test case, we find the following results: (i) the U and Co(2) moments increase with an applied magnetic field whereas the Co(1) moment remains almost constant, (ii) the U and Co(2) magnetic densities are weakly anisotropic.

We report scanning tunneling microscopy (STM) studies of Si substitutional donors (${\mathrm{Si}}_{\mathrm{Ga}}$) in GaAs that reveal delocalized and localized electronic features corresponding to ${\mathrm{Si}}_{\mathrm{Ga}}$ in the top few layers of the (110) cleavage surface. The delocalized features appear as protrusions a few nm in size, superimposed on the background lattice. These features are attributed to enhanced tunneling due to the local perturbation of the band bending by the Coulomb potential of subsurface ${\mathrm{Si}}_{\mathrm{Ga}}$. In contrast, STM images of surface ${\mathrm{Si}}_{\mathrm{Ga}}$ show very localized electronic structures, in good agreement with a recent theoretical prediction [J. Wang et al., Phys. Rev. B 47, 10 329 (1993)].

In the course of atomistic simulations of the dislocation array at the Ge/Si(001) interface, we have generated a new closed symmetric defect structure comprising eighteen atoms that may be found in a variety of circumstances including dislocation intersections and grain boundaries. The structure maintains tetrahedral bonding with reasonable changes in bond lengths and angles, and may have interesting electronic properties. At the Ge/Si interface, these extended point defects may reach a very high planar concentration of ∼${10}^{12}$/${\mathrm{cm}}^2$.

A thin bilayer of two coexisting polymer phases, in contact with a surface which favors and is partially wetted by one of them, was studied by nuclear reaction analysis and phase contrast microscopy. We find that when the less-favored phase is initially in contact with this surface, it is entirely replaced by the partially wetting phase. This inversion evolves with time very differently than in the case of complete wetting. The results suggest that a microscopic surface layer enriched in the surface-preferred component may not be a stable signature of partial wetting in small binary mixtures.

The buckling of a two-dimensional polymer network at the air-water interface has been evidenced by grazing incidence x-ray scattering. A comprehensive description of the inhomogeneous octadecyltrichlorosilane polymerized film was obtained by atomic force microscopy and x-ray scattering measurements. The buckling occurs with a characteristic wavelength ≃10 μm.

Inelastic light scattering spectra reveal a new behavior of electron-electron interactions in two-dimensional double quantum well semiconductor structures. It manifests in intersubband excitations when both subbands are densely populated. In such high density limit, spin excitations, well defined when only one subband is populated, cannot be distinguished from single-particle transitions. A quantitative analysis shows a collapse of vertex corrections (exciton shift), which we attribute to the cancellation between contributions from electrons in the two subbands.

A single model is presented which represents both of the two apparently unrelated localization problems of the title. The phase diagram of this model is examined using scaling ideas and numerical simulations. It is argued that the localization length in a spin-degenerate Landau level diverges at two distinct energies, with the same critical behavior as in a spin-split Landau level, and that all states of a charged particle moving in two dimensions, in a random magnetic field with zero average, are localized.

A unified model of phonon-assisted and zero-phonon radiative transitions in nanoscale silicon structures is presented. For characteristic sizes above 15–20 Å, phonon-assisted transitions dominate, while zero-phonon transitions, allowed due to the finite-size effect, are more important for smaller length scales. Light emission from porous silicon is analyzed on the basis of these results showing that phonon-assisted transitions should dominate the emission in the observed red band.

New tunneling structures are proposed consisting of magnetic barriers that can be created by lithographic patterning of ferromagnetic or superconducting films. The form of the equivalent potential for such a barrier depends on the wave vector of the incident electron. This renders the transmission through such structures an inherently two-dimensional process since the tunneling probability depends not only on the electron’s momentum perpendicular to the tunneling barrier but also on its momentum parallel to the barrier. Pronounced resonances are obtained for the tunneling probability and the conductance of a resonant tunneling device consisting of such magnetic barriers.

Magnetotunneling of electrons in an asymmetric double quantum well is investigated by time-resolved luminescence spectroscopy. The tunneling time decreases strongly when the bottom of the lowest electronic subband of the narrow well is brought to intersect the lowest wide well subband at nonzero wave vector by a magnetic field perpendicular to the growth direction. This decrease is particularly pronounced when longitudinal optical phonon emission is possible. Transfer becomes then as fast as in electrically tuned resonances in zero magnetic field.

It is shown that a sizable areal density of midgap states exists on a {110} surface of a ${\mathit{d}}_{\mathit{x}\mathit{a}}^2$-${\mathit{x}}_{\mathit{b}}^2$-wave superconductor, which can either have vacuum or an insulator at the surface, or be separated from vacuum or an insulator by a clean, size-quantized, normal metal overlayer. These ‘‘midgap’’ states have many observable consequences—some of which are briefly discussed here—which can be used as a clear signature to distinguish between d-wave and anisotropic s-wave superconductors.

The effect of weak impurity disorder on flux lattices at equilibrium is studied in the absence of free dislocations using both the Gaussian variational method and, to O(ε=4-d), the functional renormalization group. We find universal logarithmic growth of displacements for 2<d<4:〈u(x)-u(0)${\mathrm{〉}}^2$¯∼${\mathit{A}}_{\mathit{d}}$ln‖x‖ and persistence of algebraic quasi-long-range translational order. When the two methods can be compared they agree within 10% on the value of ${\mathit{A}}_{\mathit{d}}$. We compute the function describing the crossover between the ‘‘random manifold’’ regime and the logarithmic regime. A similar crossover could be observable in present decoration experiments.

The quantum tunneling of vortices in superconductors and superfluids is discussed. Inertial effects are shown to be generally negligible. The equation of motion of a vortex line in two dimensions can then be written in Hamiltonian form and this leads to a natural method for quantization of the motion. Using a simple model we discuss quantum tunneling in clean and dirty superconductors. Expressions which interpolate between the clean and dirty cases and the quantum and classical regimes at low and high temperatures are obtained for the tunneling action.

The effects of photoinduced hole doping, caused by persistent photoconductivity, on the normal and superconducting properties of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ (6.35≤y≤6.70) thin films are studied using a high-power-density He-Ne laser light. Substantial photoinduced enhancements of both the normal conductivity and ${\mathit{T}}_{\mathit{c}}$ are found for all the films. Hall measurements reveal that the number of photoinduced holes depends only on the photon dose and is independent of the oxygen content. The enhanced ${\mathit{T}}_{\mathit{c}}$ has a nearly quadratic correlation with the hole density similar to that observed in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ when the hole density is varied by adding oxygen to the ${\mathrm{CuO}}_{\mathit{x}}$ chain.

A general method to find, in a systematic way, efficient Monte Carlo cluster dynamics among the vast class of dynamics introduced by Kandel et al. [Phys. Rev. Lett. 65, 941 (1990)] is proposed. The method is successfully applied to a class of frustrated two-dimensional Ising systems. In the case of the fully frustrated model, we also find the intriguing result that critical clusters consist of self-avoiding walk at the θ point.

We present a powerful method for calculating the thermodynamic properties of infinite-dimensional Hubbard-type models using an exact diagonalization of an Anderson model with a finite number of sites. The resolution obtained for Green’s functions is far superior to that of quantum Monte Carlo calculations. We apply the method to the half-filled Hubbard model for a discussion of the metal-insulator transition, and to the two-band Hubbard model where we find direct evidence for the existence of a superconducting instability at low temperatures.

Using high temperature expansions for the equal time correlator S(q) and static susceptibility χ(q) for the t-J model, we present evidence for quantum critical (QC), z=1, behavior at intermediate temperature in a broad range of t/J ratio, doping, and temperatures. We find that the dynamical susceptibility is very close to the universal scaling function computable for the asymptotic QC regime, and that the dominant energy scale is temperature. Our results are in excellent agreement with measurements of the spin-echo decay rate, 1/${\mathit{T}}_{2\mathrm{G}}$, in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, and provide qualitative understanding of both 1/${\mathit{T}}_1$ and 1/${\mathit{T}}_{2\mathrm{G}}$ nuclear relaxation rates in doped cuprates.