A new spectroscopy based on the resonant coherently induced electron loss to the continuum in ion-surface scattering under grazing incidence is proposed. A series of peaks, corresponding to the energy differences determined by the resonant interaction with the rows of atoms in the surface, is predicted to appear in the energy distribution of electrons emitted from electronic states bound to the probe. Calculations for MeV ${\mathrm{He}}^{+}$ ions scattered at a W(001) surface along the 〈100〉 direction with a glancing angle of 0–2 mrad show a total yield close to 1.

Equilibrium and dynamical properties of a storage-ring free-electron laser are shown as resulting from the combined effects of two different saturation mechanisms. The first one is the laser-induced energy spread of the electron beam. The second mechanism is due to an imperfect longitudinal synchronism between the electron and laser pulses. Consequences are briefly described.

Experiments have demonstrated the reduction of the beam-breakup (BBU) instability growth rate by coupling the main cavities, through which an electron beam passes, to identical dummy cavities. When seven of the ten main cavities are coupled, an average reduction of about 6 dB in BBU microwave growth, from 36 to 30 dB, is observed. This reduction is consistent with a simple mode coupling theory. The electron beam parameters are 750 kV, 210 A, 0.5 μs, with solenoidal focusing of 3.4 kG.

The beta-degradation phase of a high-${\mathrm{\beta }}_{\mathit{p}}$ plasma in the TFTR tokamak is analyzed with x-ray and electron cyclotron emission imaging techniques. Medium-n (toroidal mode number) instabilities with ballooning characteristics are observed near and within the q=1.5 surface during a slow degradation in the plasma β and precede a sudden partial collapse in the central plasma pressure. This is the first reported observation of a ballooning instability in the interior of a large, collisionless tokamak plasma.

We present analytical and numerical results that explain the transient turbulent dynamics observed in the complex Ginzburg-Landau equation. Just below the transition to turbulence, we observe that metastable turbulent states break down by the nucleation and growth of single-vortex droplets, leading to a ‘‘frozen’’ state with a low (but finite) density of stationary vortices. We derive the relation between nucleation time and radius, and determine their dependence on the distance to the turbulence transition line.

The surface-induced melting of the closed-packed (100) face of the anisotropic molecular crystal caprolactam has been studied using x-ray reflectivity. A thin-to-thick film prewetting transition is observed at about 13 K below the bulk melting point. Only above this transition does the thickness of the quasiliquid layer increase continuously with temperature. We speculate that initially the surface melting proceeds via layering transitions.

We present Monte Carlo simultions of binary mixtures of hard parallel spherocylinders with length-to-width ratios ${\mathit{L}}_1$/D=1.0 and ${\mathit{L}}_2$/D ranging from 1.3 to 2.1. The composition is such that the partial volume fractions are equal. For ${\mathit{L}}_2$/D>1.6, we observe a thermodynamically stable columnar phase which is not formed by the monodisperse components separately. The nematic-smectic transition, which is postponed by increasing the ${\mathit{L}}_2$/D ratio, is eventually preempted by a nematic-columnar transition for ${\mathit{L}}_2$/D>1.9, showing that in these mixtures bidispersity favors columnar order over smectic order.

The shape of the free surface of rotating superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ has been investigated optically. The drastic change in viscosity at the superfluid transition was seen directly. Focusing of light reflected from the liquid surface and expansion of the light beam when it travels through the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ sample show that the meniscus of the rotating superfluid has the same parabolic shape as that of the normal liquid, which implies an equilibrium density of vortices. The fountain effect in ${}_{}{}^3{}_{}{}^{}\mathit{B}$ could be seen directly as well: Liquid accumulates into the illuminated region.

We consider a one-dimensional two-band model of electrons on a lattice with nearest-neighbor hopping t constrained by the excluded multiple occupancy of the sites, an interband splitting, Δ, and intraband and interband exchange J between neighboring sites. This model is integrable at the supersymmetric point and reduces to the SU(4)-invariant supersymmetric t-J model with crystal field splitting. We present the exact solution via Bethe’s ansatz and discuss ground-state properties and the excitation spectrum as a function of Δ.

We theoretically investigate excitonic effects on the optical properties of silicon quantum wires, based on ab initio electronic structure calculations. The Si wires have a direct, allowed band gap in the visible energy range and exhibit a strong optical anisotropy. Taking into account the electron-hole Coulomb interaction, the geometrical restriction of excitons dramatically enhances the oscillator strength of the optical transitions. Comparisons with recent experimental results are also made.

Direct evidence of space-charge dipole formation resulting from minority-carrier injection in a high-purity relaxation semiconductor has been observed for the first time. Small signal ac measurements are utilized to monitor the capacitance of a high-purity silicon structure as the temperature is decreased from 70 to 11 K. An increase in capacitance at 22 K is attributed to the formation of a charge dipole as a consequence of majority-carrier depletion in the relaxation regime.

We have measured the specific heat of multilayers of superconducting amorphous ${\mathrm{Mo}}_{77}$${\mathrm{Ge}}_{23}$ layers separated by insulating amorphous germanium. We observe a fluctuation regime in quantitative agreement with predictions for two-dimensional superconductivity. The fluctuation peak is rapidly suppressed by the application of small magnetic fields perpendicular to the layers, and the transition becomes extremely broad as the field is increased. The transition widths scale as expected for a field-induced 2D to 0D crossover, and are in excellent agreement with the exact result for 0D fluctuations.

The universal dynamic and static properties of two-dimensional antiferromagnets in the vicinity of a zero-temperature phase transition from long-range magnetic order to a quantum-disordered phase are studied. Random antiferromagnets with both Néel and spin-glass long-range magnetic order are considered. Explicit quantum-critical dynamic scaling functions are computed in a 1/N expansion to two-loop level for certain nonrandom, frustrated square-lattice antiferromagnets. Implications for neutron scattering experiments on the doped cuprates are noted.

Applying a renormalization-group (RG) procedure to the 2D antiferromagnetic Heisenberg (AFH) model, we derive an effective AFH model in which spins on a 1D ring experience antiferromagnetic nearest-neighbor interactions and iteract individually with an effective central spin. Using analytic and numerical arguments, we establish the existence of staggered long-range order in the effective model and hence, given our RG prescription, in the 2D AFH. Our RG calculations give a value of the magnetic moment consistent with Monte Carlo results for the 2D AFH.

We use mappings onto soluble models and Abelian bosonization to determine the phase diagram and correlation functions of a model of two coupled spin chains. We find quantum disordered singlet phases and magnetic phases and determine the phase boundaries, universality classes of transitions, and correlation functions. We believe our model captures the essential physics of the interplay between the Kondo effect and magnetic ordering, which may be important for the heavy fermion materials.

Crystalline ${\mathrm{C}}_{60}$, after exposure to air, is shown to exhibit the characteristic luminescence spectrum of singlet molecular oxygen. The role of oxygen is verified by measuring the large isotope shift of the ${\mathrm{O}}_2$ vibrational mode luminescence replica in a sample prepared with ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}_2^{}{}_{}{}^{}$. Detailed spectroscopy reveals novel bands in the spectrum which arise from vibronic coupling of the ${\mathrm{O}}_2$ transition to the ${\mathrm{C}}_{60}$ molecules. This allows measurement of the ${\mathrm{C}}_{60}$ vibrational spectrum with unparalleled sensitivity and precision from mg-sized samples.

Field evaporation of silicon as positive and negative ions in the field ion microscope and scanning tunneling microscope configurations is investigated with the charge-exchange model using atomic potentials from an empirical potential due to Tersoff [Phys. Rev. B 37, 6991 (1988)] and an environment dependent potential developed by Bolding and Andersen [Phys. Rev. B 41, 10 568 (1990)]. For the geometry of the field ion microscope, ${\mathrm{Si}}^{+}$ should be the observable ion species. In the close-spaced electrode geometry of the scanning tunneling microscope, ${\mathrm{Si}}^{2\mathrm{-}}$ should be the favored ion species since it requires the lowest evaporation field.

Dry, noncohesive granular materials were very slowly driven to the threshold of instability where they produced avalanches. We have used digital image analysis to record avalanches that occurred on the surface of the incline. Results show large slides every 10–13 min which reset the surface from the maximum angle of stability, ${\mathrm{\theta }}_{\mathit{c}}$, back to the angle of repose, ${\mathrm{\theta }}_{\mathit{r}}$. We also find a power-law distribution of sizes for smaller avalanches occurring between the large sliding events. The number of these small avalanches follows a power-law dependence on the reduced angle, (${\mathrm{\theta }}_{\mathit{c}}$-θ)/(${\mathrm{\theta }}_{\mathit{c}}$-${\mathrm{\theta }}_{\mathit{r}}$), or time, which demonstrates that ${\mathrm{\theta }}_{\mathit{c}}$ behaves like a critical point.

We discuss the general conditions under which a Jahn-Teller effect can be meaningfully associated with the solvation shell of an ion in solution. Unlike the Jahn-Teller effect in solids, no distortion of the solvation shell exists on the longest time scales. On intermediate time scales, the solvation shell exhibits cubic (${\mathit{O}}_{\mathit{h}}$) symmetry and on the shortest scales it shows the Jahn-Teller distortion (${\mathit{D}}_{4\mathit{h}}$). A molecular-dynamics simulation of a model in which the frame of reference for the Jahn-Teller potential rotates in time exhibits these features and shows how the time scales involved vary with parameters in the potential.

The coalescence of monodisperse silicone oil-in-water emulsions stabilized with sodium dodecyl sulfate has been studied. We report the existence of a sharp destabilization threshold, controlled by surfactant chemical potential, osmotic pressure, and droplet diameter, at which the rate of coalescence increases dramatically. We present evidence that the stability of the emulsions can be characterized by two microscopic parameters: a minimum stable value of the surfactant chemical potential and a maximum value of the pressure exerted upon a droplet-droplet interface.