We report measurements of exciton dynamics in GaAs multiple-quantum-well structures at low temperature using three-pulse picosecond four-wave mixing. The time-resolved measurements of the emission show both a delayed signal (stimulated photon echo) and a prompt signal (free polarization decay) with different decay times. The results suggest that, in addition to the inhomogeneously broadened resonance, a homogeneously broadened resonance contributes to the optical response of the exciton.

Magnetization of small ferromagnetic clusters at finite temperatures has been studied using the Ising model and Monte Carlo techniques. The magnetization of finite clusters is reduced from the bulk value, and increases with the external magnetic field and with the cluster size. The results explain qualitatively the recent observations by de Heer, Milani, and Chatelain of the reduction with decreasing cluster size of the average magnetic moment in small iron clusters.

A spin-split interface state in the Pd 4dband of epitaxial Pd(111) films on Fe(110) has been identified in spin-resolved and spin-integrated photoemission. It is most pronounced for Pd thicknesses between 1 and 2 atomic layers (AL), with an exchange splitting of about 130±50 meV that is inverted compared to that of Fe. Bulklike states of Pd(111) at Pd thicknesses between 1.5 and 2.7 AL also appear spin polarized, but without a resolvable exchange splitting. Above 2.7-AL Pd thickness no spin polarization attributable to Pd is evident.

Quantum Monte Carlo simulations are used to explore the phase diagram of the negative-U Hubbard model. As the system is doped away from half filling, 〈n〉=1, the s-wave pair-field correlations are enhanced and the charge-density-wave correlations suppressed. There is no indication of phase separation. Away from half filling, the pair-field correlations are consistent with Kosterlitz-Thouless scaling and a finite ${\mathit{T}}_{\mathit{c}}$ which peaks near 〈n〉=1 but vanishes at exactly half filling.

We consider the effect of interplane coupling on the energetics of the relative sign of statistics of anyons in adjacent planes. We offer qualitative arguments as well as numerical results which support the idea that opposing chirality (i.e., ‘‘antiferromagnetic’’ order of the statistics) is favored over a wide range of interplane coupling V–including both attractive and repulsive interactions between planes. We argue that our results apply to the normal (nonsuperfluid) phase in the thermodynamic limit.

Using a SQUID picovoltmeter, we have measured current-voltage curves of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ microtwinned crystals as a function of temperature in fields from 1 to 6 T. The data constitute evidence for a finite-temperature phase transition in the vortex state. Exponents derived within the framework of the vortex-glass model are found to be similar to thin-film values. Strongly temperature-dependent correlations involving up to ${10}^5$ vortices near the transition, combined with the qualitative failure of thermal-activation models to fit our data, support the existence of a phase transition.

It is shown that translational disorder in the interior bulk of real sandpiles is a relevant perturbation on self-organized surface fluctuations. In two simplified models, this disorder destroys the flat phase of the surface; surface fluctuations are instead described at long wavelengths for all parameter values by a simple diffusion equation with noise, which implies a logarithmically rough surface in d=2. This result suggests that real sandpiles may also be described by a simple diffusion equation, and proves that bulk translational disorder is important for their surface fluctuations.

We calculate the partition function for a relativistic Bose-Einstein gas. The excitation energies are identified and compared with known results in various limits. In particular, this treatment shows how the Goldstone bosons of a spontaneously broken symmetry and the quasiparticles of the nonrelativistic gas emerge as special cases of one general formula. New results are obtained for the spectrum of Goldstone modes in a medium of nonzero density, and the fully relativistic case is discussed along with a possible cosmological application.

The analysis of very recent and accurate deep-inelastic scattering data leads us to conclude that isospin is strongly violated in the proton sea-quark distributions. Such findings have severe implications for reliable global parton-distribution fitting programs.

For the first time, definite evidence for the existence of two-phonon collective states in deformed nuclei has been established through the measurement of the absolute transition rate for the decay of the double-γ ${\mathit{K}}^{\mathrm{\pi }}$=$4^{+}$ vibration in ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$.

Nonexponential photon-echo decays are observed in ruby, ${\mathrm{Er}}^{3+}$:${\mathrm{YLiF}}_4$, and ${\mathrm{Er}}^{3+}$:${\mathrm{LaF}}_3$ with samples of low ion concentrations in high (10–50 kG) magnetic fields. At the highest fields, photon-echo decays are field independent and their temporal forms are identical for all three systems. This behavior is attributed to optical dephasing arising from electron-spin nuclear-spin superhyperfine interactions in the presence of a frozen core.

We report the observation of coherent emission from a Cherenkov free-electron laser driven by a 5-MeV radio-frequency microtron. Power up to 50 W in pulses of 4-μs duration has been generated at the wavelengths of 1.6 and 0.9 mm using two different dielectric-loaded waveguides.

The kinetic fluctuations of a stable interface driven by the gradient of a Laplacian field are investigated. In three and more dimensions the interface width is finite. In two dimensions the width diverges logarithmically with time and system size. Its scaling form is derived in agreement with simulations of diffusion-limited erosion (anti diffusion-limited aggregation). A crossover to algebraic roughness with an extended intermediate scaling regime is predicted for diffusion with a drift towards the interface. Capillary effects are discussed in relation to recent experiments on fluid displacement in porous media.

The toroidal current observed during electron cyclotron heating in the Advanced Toroidal Facility torsatron is identified as bootstrap current. The observed currents, ranging between +4 and -2 kA, agree well with predictions of neoclassical theory in magnitude and parametric dependence, as determined by systematic scans of quadrupole (shaping) and dipole (magnetic axis shift) moments of the poloidal magnetic field. It has been shown that the bootstrap current in a stellarator can be externally controlled, and zero-current operation can be achieved.

We report the first consistent observations of internal tilt instabilities in field-reversed configurations. Detailed comparisons with numerical calculations establish that data from an array of external magnetic probes are signatures of these destructive plasma instabilities. As suggested by finite-Larmor-radius theory, field-reversed configurations appear grossly stable when s/e [(average number of ion gyroradii)/(separatrix elongation)] is less than 0.2–0.3 and show MHD-like tilt instabilities when s/e∼1.

Films of silicone oils (or alkanes) deposited on low-energy surfaces (grafted silicone wafers) are metastable. We nucleate a dry spot at the center of the wafer and observe the rate of growth V=dR/dt of the dry patch of radius R. We find that V is (a) independent of R, (b) independent of the film thickness, (c) inversely proportional to the oil viscosity, and (d) very sensitive to the value of ${\mathrm{\theta }}_{\mathit{e}}$, the equilibrium contact angle between oil and wafer. At small ${\mathrm{\theta }}_{\mathit{e}}$, V≊${\mathrm{\theta }}_{\mathit{e}}^3$. All these features are in agreement with a recent hydrodynamic theory.

Using small-angle neutron-scattering techniques, the scaling laws between the thickness of a grafted polymer layer h, its graft density σ, and the molecular weight of the grafted chains M have been determined for polydimethylsiloxane chains end grafted on porous silica. The observed laws h≊Mσ in bad solvent and h≊M${\mathrm{\sigma }}^{1/3}$ in good solvent provide the first experimental evidence of the ‘‘brush’’ regime, where the grafted chains, confined by their neighbors, are stretched normal to the surface.

${}_{}{}^2{}_{}{}^{}\mathrm{H}$ NMR of a liquid crystal confined to submicron cylindrical cavities, in a 17 K interval above the nematic-isotropic transition, reveals a weakly orientationally ordered molecular layer at the cavity wall governed by local interactions. The order (${\mathit{S}}_0$=0.02) is temperature independent so that a partial wetting is realized with no prewetting transition expected. A molecular exchange rate between the surface layer and the bulk is measured to be ∼${10}^3$ ${\mathrm{s}}^{\mathrm{-}1}$. The coherence length measuring the order penetrating further into the cavity is described by α=0.4 and ${\xi }_0$=0.6 nm.

We examine the effect of long-range surface interactions on surface wetting phase transitions in polymer mixtures. Within a perturbation scheme, we have found an analytic solution for the volume-fraction profile near the surface which can be compared with that obtained from a model with short-range surface interactions only. We find that the long-range interactions should not be ignored in the interpretation of recent experimental results.

We analyze density fluctuations of a self-avoiding chain in a random environment by a computer simulation. The density at a site is obtained by computing the number of times a monomer visits a particular site on a finite lattice. Although the chain obeys self-avoiding-walk statistics, the density measure appears to be multifractal. We compute τ(q) and f(α).