Preferential annihilation of mobile surface vacancies at the ends, rather than the sides, of dimer rows leads to a new, nonequilibrium, single-domain phase of Si(100) that is not accessible by epitaxial growth, but is stable at moderate temperatures (T∼450 °C). These results emerge from a tunneling-microscope study of layer-by-layer removal of Si from Si(100) under 225-eV Xe-ion bombardment.

We have measured and calculated the full intensity profiles for Si 2p photoelectron emission from Si(111), and have analyzed these via a two-dimensional Fourier-transform holographic inversion procedure. The resulting images associated with two symmetry-inequivalent emitter types exhibit elongated features corresponding to near-neighbor atoms in the first (111) double layer above a given emitter, and are found to improve in quality if strong forward-scattering peaks are removed in a smooth way.

The radiative-excited-state dynamics in crystals slabs is studied as a function of the slab thickness L. A general dispersion relation is derived that interpolates between the superradiant-exciton regime at thicknesses small compared to an optical wavelength λ and bulk polaritons and thick crystals. It is shown that previous results predicting superradiance in thick slabs (L≫λ) arise from invalid pole approximations.

The structure of the edge excitations of a quantum Hall droplet, confined in a double quantum well, is studied using a generalized version of the Wen-Stone chiral boson action. As the interwell tunneling, t, is increased, one finds a ‘‘metal-insulator’’ transition from a phase where the optic edge mode is ungapped to a phase where the optic mode is gapped. The large-t phase exhibits edge quasiexcitons whose charge is half that of a quasiparticle-quasihole bound state. The critical behavior of the conductances, the exciton gap, and the excitonic order parameter is discussed.

Hot electrons in ${\mathrm{Al}}_{0.6}$${\mathrm{Ga}}_{0.4}$As were produced in the ${\mathit{X}}_6$ valley from a photoexcited indirect transition by a 585-nm femtosecond pump pulse. The time evolution of the population of electrons in the bottom of the ${\mathit{X}}_6$ alley was monitored by a femtosecond infrared probe pulse. The ${\mathit{L}}_6$→${\mathit{X}}_6$ intervalley scattering time of ∼200 fs and the ${\mathit{L}}_6$${\mathit{X}}_6$ intervalley deformation potential of ∼2.7×${10}^8$ eV/cm were determined from the measured kinetic data.

Photoexcited states in transpolyacetylene [t-(CH${)}_{\mathit{x}}$], and in alkoxy derivatives of poly(paraphenylenevinylene) (PPV) nondegenerate ground-state polymers, have been studied by the technique of absorption detected magnetic resonance. In addition to the stable charged photoexcitations, which are charged solitons in t-(CH${)}_{\mathit{x}}$ and bipolarons in PPV, we clearly identify neutral photoexcitations with nonzero spins. In t-(CH${)}_{\mathit{x}}$ these are spin-1/2 neutral solitons ${\mathit{S}}^0$(S¯ ${}_{}{}^0{}_{}{}^{}$) with an optical transition at 1.35 eV. The neutral photoexcitations in PPV form triplet excitons which cannot dissociate into ${\mathit{S}}^0$S¯ ${}_{}{}^0{}_{}{}^{}$ pairs due to the confinement caused by the nondegenerate backbone structure.

Analytic results are presented for the dynamic behavior of sliding charge-density waves (CDW’s) and related extended nonlinear systems with randomness. It is shown that, in the limit of long times, a sliding configuration approaches a unique solution. In CDW models, the velocity of this asymptotic solution is periodic in time. These results explain previous numerical observations, simplify further analysis and simulation, and have direct experimental implications.

Direct measurements of the compressibility K of the 2D electron gas at both zero magnetic field B and in the high-field extreme quantum limit, ν<1, are reported. A new method is used to determine both the magnitude and IsignR of K. At B=0 we find ${\mathit{K}}^{\mathrm{-}1}$ changes sign and becomes negative as the 2D density is reduced. At high B, ${\mathit{K}}^{\mathrm{-}1}$ exhibits negative anomalies near both ν=0 and 1. Compressibility features near the fractional fillings ν=1/3 and 2/3 give thermodynamic evidence for dilute interacting quasiparticle gases.

The effect of finite sample size on the Kondo resistance anomaly has been explored in the CuCr system. The resistivity of thin CuCr films was studied as a function of width from 0.5 to 20 K. The magnitude of the Kondo anomaly is significantly depressed as the film width is reduced below 10 μm. This length scale is consistent with the radius of the electron-spin correlation cloud, suggesting a crossover from two- to one-dimensional behavior. A concurrent decrease in the temperature of the resistivity maximum suggests a diminished interimpurity interaction strength.

On the basis of first-principles (local-spin-density-approximation) calculations of the magnetocrystalline anisotropy energy, we have predicted a perpendicular orientation of the magnetization in a [111${]}_{\mathrm{fcc}}$ ${\mathrm{Co}}_1$/${\mathrm{Ni}}_2$ multilayer. This multilayer was then grown by vapor deposition and found to have the predicted properties. The saturation magnetization of 1 T is much larger than that of known magnetic multilayers with perpendicular anisotropy.

The response of magnetization to the application of a magnetic field has been measured above the Curie temperature ${\mathit{T}}_{\mathit{C}}$ in epitaxial Co films on Cu(100). Magnetic domain images have been acquired up to ${\mathit{T}}_{\mathit{C}}$. The results suggest that for temperatures T>${\mathit{T}}_{\mathit{C}}$ the system behaves like a two-dimensional Heisenberg magnet.

We measure the longitudinal (${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$) and Hall (${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$) resistivities as a function of magnetic field and temperature in the mixed state of epitaxial ${\mathrm{YBa}}_2$${\mathrm{Ca}}_3$${\mathrm{O}}_7$ films. We observe a striking power-law behavior ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$(T)∝[${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$(T)${]}^{\mathrm{\alpha }}$ at fixed field, with α=1.7±0.2. Our results link the Hall behavior to collective behavior in the presence of pinning. In addition, this power-law dependence is consistent with new calculations for the vortex-glass transition, and therein may provide the first measurement of a new critical exponent.

We discuss the Hall effect in the mixed state of type-II superconductors with an emphasis on the regime near the vortex-glass transition. This transition is argued to have a ‘‘hidden’’ particle-hole symmetry, and a new exponent is introduced to characterize the irrelevance of the particle-hole asymmetry. This leads to a natural explanation of the scaling of the Hall and Longitudinal resistivities observed in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. At the transition, the nonlinear Hall voltage is predicted to vary with a universal power of the current, and the linear ac Hall conductivity with a universal power of the frequency.

Nuclear electric quadrupole measurements of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation rates (${\mathit{R}}_1$) in zero applied magnetic field and nuclear magnetic resonance ${\mathit{R}}_1$ in 8.2 T are reported for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$. A direct effect of the magnetic field is seen for the first time both above and below ${\mathit{T}}_{\mathit{c}}$. Above ${\mathit{T}}_{\mathit{c}}$ the effect is ascribed to the partial suppression of antiferromagnetic (AF) spin fluctuations. For T<${\mathit{T}}_{\mathit{c}}$ the effect goes in the opposite sense and may be related to the spin-temperature diffusion for the core of the fluxons or to the enhancement of AF spin fluctuations persisting in the superconducting phase.

The authors report mmeasurements of the planar ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation rates ${}_{}{}^{63}{}_{}{}^{}$${\mathit{W}}_{1\mathit{a}}$ ${(}^{63}$${\mathit{W}}_{1\mathit{c}}$) for the static field oriented along the a (c) axis in the superconducting state of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. These measurements were made on a single sample as a function of temperature and magnetic field. The weak-field results are well explained by several recent calculations in terms of a BCS spin-singlet, orbital-d-wave pairing state. They also find that at low temperatures ${}_{}{}^{63}{}_{}{}^{}$${\mathit{W}}_{1\mathit{c}}$ varies linearly with the applied magnetic field, suggesting that at high field it is dominated by fluxoid cores.

We have extended a weak-coupling theory of antiferromagnetic fluctuations to the superconducting state and analyzed NMR data for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. Within this framework we find that while an s-wave gap structure with 2Δ(0)/${\mathit{kT}}_{\mathit{c}}$=4 can fit the Knight-shift data for Cu(2) and O(2,3), it does not provide a satisfactory fit to the ${\mathit{T}}_1^{\mathrm{-}1}$ data. Using a ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$-wave gap with 2Δ(0)/${\mathit{kT}}_{\mathit{c}}$ of order 6 to 8 provides a reasonable fit to both the Knight-shift and ${\mathit{T}}_1^{\mathrm{-}1}$ data.

We studied flux lattice melting in a model high-${\mathit{T}}_{\mathit{c}}$ superconductor by Monte Carlo simulation in terms of vortex variables. We identify two melting curves in the B-T phase diagram and evaluate a density-dependent Lindemann criterion number for melting. We also observe that the transition temperature shifts downward toward the two-dimensional melting limit as the density of flux lines increases. Although the transition temperature does not change, a significant difference in shear modulus is observed when flux cutting or reconnection is allowed.

We report on numerical evidence for intermittent bursting phenomena in a one-dimensional reaction-diffusion system that mimics spatiotemporal pattern formation in the Couette flow reactor. This bursting regime is attained via an interior crisis when decreasing the diffusion coefficient. The intermittent occurrence of spatially localized structures can be understood in terms of Shil’nikov’s homoclinic chaos. This diffusion-induced chaotic bursting is likely to be observed in bench experiments.

Chaos generated by the internal dynamics of a large neural network can be correlated over large spatial scales. Modulating the spatial coherence of the chaotic fluctuations by the spatial pattern of the external input provides a robust mechanism for feature segmentation and binding, which cannot be accomplished by networks of oscillators with local noise. This is demonstrated by an investigation of synchronized chaos in a network model of bursting neurons responding to an inhomogeneous stimulus.

The parametric motion of eigenvalues of two chaotic quantum systems, a stadium billiard and a kicked rotator, is studied to investigate the curvature distribution introduced by Gaspard et al. [Phys. Rev. A 42, 4015 (1990)]. By using an average of a statistical quantity over the parameter values along the motion, we obtain the curvature distribution and confirm the predicted universality of its tail behavior. We also show a nonuniversal characteristic at small curvatures, which is attributed to the existence of solitonlike structures.