The chaotic properties of the interacting-boson model of nuclei are studied both classically and quantum mechanically. Classical phase diagrams are constructed in the general parameter space of the Hamiltonian known as Casten’s triangle. They lead to a discovery of a nearly regular region which is probably related to an unknown approximate symmetry. Analysis of the quantum fluctuations of the spectrum and of the electromagnetic B(E2) intensities confirms the classical results. The phase diagrams may be useful in identifying ‘‘chaotic’’ nuclei within the nuclear periodic table.

Double ionization of krypton leaving the ion in its ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{e}}$ ground state has been studied at 2.26 eV above threshold, the two photoelectrons being measured in coincidence with angle and energy selection. The results are compatible with the angular dependence of the favored ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ state of the pair of electrons, which can be selected by maintaining the angle between the two detectors, ${\mathrm{\theta }}_{12}$, equal to 180°. The observed angular correlation is maximum at ${\mathrm{\theta }}_{12}$=180° and appears to originate from the three-body ion-electron-electron interaction, in qualitative agreement with the Wannier theory.

The first experimental demonstration of a harmonic free-electron-laser amplifier utilizing a periodic position instability is described for a planar wiggler configuration. The interaction occurs at the even harmonics of the fundamental. A maximum gain of 7 dB was observed over a frequency band ranging from 14 to 15 GHz. The experimental results are compared with predictions from the three-dimensional simulation code wigglin with excellent agreement. Improvements due to a tapered wiggler for this interaction are discussed.

Current continuum models based on the theory of plasticity have failed to describe the observed gravitational flow patterns of granular particles in a confined geometry. We propose a random-walk model that correctly predicts the shape of the free surface of the granular media and the solid stream lines with and without obstacles. We present simulation results that qualitatively exhibit many of the unique features of granular flows. The simulation results are also compared with experimental data.

The coupling between different oscillation branches of a waveguide-mode free-electron laser has been studied. It is observed that the wave form of the upper branch of a waveguide-mode free-electron laser is modulated at the rate of the period of the mode-locked oscillation of the lower branch for a wide range of experiment parameters. This phenomenon is explained as the temporally periodic modulation of the gain of the upper-branch oscillation by the electromagnetic radiation of the lower-branch oscillation through the periodic bunching of the electron beam.

We report on a mean-field thermodynamic analysis of AB random copolymer melts with (backbone) composition correlations that reflect realistic polymerization conditions. Depending on the strength and sign of these correlations, multiple homogeneous phases, periodic mesophases, or disordered mesophases are possible. We identify an isotropic Lifshitz point and the critical lines that emerge from it. Our results should facilitate the design of new copolymeric materials.

A closed-cycle distillation column was operated on a helium-3–helium-4 mixture in an attempt to ‘‘separate’’ the two nuclear spin states of helium-3. For the atoms in the dilute phase, a steady-state polarization 3.5 times larger than that of the equilibrium was obtained; possible ways to improve this result are discussed.

We use quantitative chemical mapping and statistical analysis to explore the microscopic consequences of the passage of energetic ions through AlAs/GaAs multilayers. We thus quantitatively characterize and microscopically identify the damage cascades produced by individual 320-keV ${\mathrm{Ga}}^{+}$ ions. Our results reveal a strong drift of the damage in the applied electric field, indicating the effects to be charged. We exploit this effect to show that the defects produced by the passage of energetic ions may be microscopically steered in a solid.

The locally restricted motion of hydrogen and deuterium which are trapped at Cd acceptors in silicon has been studied by perturbed-angular-correlation spectroscopy. Close Cd-H and Cd-D pairs were identified by their characteristic electric-field gradients. The kinetics of these complexes was extracted directly from the time spectra. In the temperature region between 270 and 430 K clear evidence has been found for thermally activated motion of hydrogen or deuterium about the Cd with an activation energy of 0.21 eV for both hydrogen as well as deuterium. The isotropic-mass-dependent attempt frequencies are in the order of ${10}^9$ ${\mathrm{s}}^{\mathrm{-}1}$. The ratio of the frequency for hydrogen relative to that for deuterium is ${\nu }_{\mathrm{H}}$/${\nu }_{\mathrm{D}}$=3.3(1.3).

The orientation of hydrocarbon chains deposited on surfaces has been studied by near edge x-ray-absorption fine structure (NEXAFS). The so-called ‘‘building-block’’ scheme, which is generally used to predict angular dependences of NEXAFS resonances, is found to be inadequate for C-C ${\mathrm{\sigma }}^{\mathrm{*}}$ resonances in larger molecules, in contrast to previous work [D. A. Outka, J. Stöhr, J. P. Rabe, J. D. Swalen, and H. H. Rotermund, Phys. Rev. Lett. 59, 1321 (1987)]. Using the results of ab initio electronic structure calculations consistent results for several systems containing long hydrocarbon chains could be obtained.

He diffraction and scanning-tunneling-microscopy investigations on the growth of the Cu{110}-(2×1)O added-row structure reveal a novel phenomenon: the long-range spatial self-organization of two-dimensional islands. In a wide coverage range the anisotropic Cu-O islands arrange themselves in a striped periodic supergrating, with the stripes running along the 〈001〉 direction. The spacing of the supergrating depends on oxygen coverage and temperature and varies between 140 and 60 Å.

Surface extended x-ray-absorption fine structure is used to determine the crystallographic structure of iron films grown on Cu(100). We show that these metastable films have structural and vibrational properties strongly dependent on their thickness and their growth temperature: Either a well-defined fcc structure or an anisotropically distorted fcc structure is observed. A strong reordering of the thin iron films prepared at room temperature is observed when they are coated with copper; this shows that it may be incorrect to assume that a thin film is a good simulation of an interface.

A new method of imaging surface topologies of nonconductive bulk materials, especially biological specimens, by scanning tunneling microscopy, is presented. The realizability of this technique relies mainly upon surface condensation of water molecules on the specimen and substrate, with the measured current carried by ions rather than tunneling electrons. The theoretical model shows that temperature and humidity control play a crucial role in the image formation. The experimental conditions are also discussed.

We have applied first-principles molecular-dynamic relaxation to the Si(111) surface in the 5×5 dimer–adatom–stacking-fault structure. We have determined the relaxed atomic surface geometry and the corresponding electronic structure. We find that the filled adatom dangling-bond states are segregated, with the charge density residing on the faulted side of the unit cell, and that the adatoms on the faulted side are higher than the adatoms on the unfaulted side. Surprisingly, we find that, at low temperature, the adatoms on each side of the unit cell are corrugated in a variety of patterns.

A perturbed-shell-model calculation for the impurity system KI:${\mathrm{Ag}}^{+}$ predicts three nearly-degenerate localized gap modes of different symmetry and unusual amplitude patterns: The displacements of the defect’s fourth-nearest neighbors are more than an order of magnitude larger than those of the impurity or its nearest neighbors. Far-infrared isotope-effect measurements directly confirm the existence of these localized pocket modes.

Dynamical studies of Bi ultrathin films prepared on carbon at 100 K exhibit a phase transition from nanocrystalline to amorphouslike clusters. Evidence for the transition at a thickness of ∼8 Å is provided by the form, intensity, and polarization dependence of in situ Raman-scattering measurements in ultrahigh vacuum. A substantial increase in frequency of the opticlike band with decreasing thickness indicates increased covalent interactions. This contrasts with superconducting Bi films, suggesting a semiconducting amorphous state in clusters.

Inelastic scattering of x-ray photons from standing-wave fields is used to measure nondiagonal elements of the dielectric-response matrix of Si. A peak-valley structure for q<${\mathit{q}}_{\mathit{c}}$ is found, due to the fact that bulk plasmon bands, split near the zone boundary, contribute with different signs; hence, direct evidence for the existence of plasmon bands is obtained. Such detailed studies of the response matrix, together with the proposed plasmon-band model, offer a new access to dynamical screening in an inhomogeneous electron system.

The critical exponent ν of the localization length in the integral quantum Hall regime can be measured directly using the Hall-bar geometries with different sizes. We obtain a universal behavior for the three lowest Landau levels. This is in agreement with the universality prediction of the field-theoretic approach to the metal-insulator transition in the quantum Hall effect. The value of ν=2.3±0.1 agrees with recent numerical studies for the lowest Landau level. The measured value of the temperature exponent of the inelastic-scattering rate is found to range from p=2.7±0.3 to p=3.4±0.4.

We report the first experimental evidence for the indirect fundamental band gap developed when an in-plane magnetic field is applied to a modulation-doped quantum well. The observed photoluminescence peak for the lowest subband undergoes a large, approximately quadratic shift with field, a consequence of the behavior of the allowed transitions in the increasingly indirect-gap band-structure. A good theoretical fit is obtained by calculating the transition energies using a realistic self-consistent Hartree potential for the quantum-well profile.

We report a study of the temporal evolution of the luminescence from an electron-hole plasma confned in GaAs quantum wires under high-power excitation. In the spectra 1D intersubband transitions with quantum numbers as high as n=5 are resolved. The spectral positions are in excellent agrement with Kohn-Luttinger calculations based on an active wire width of w=60±5 nm, accurately determined by an x-ray diffraction method employed for 1D systems. The decay of the luminescence and the hot-carrier cooling rate, which is obtained from line-shape analysis, are found to be slower than the corresponding processes in quantum wells.