We investigate the energy exchange between a Rydberg electron and the molecular core in the regime where the Born-Oppenheimer approximation is violated. The theory developed allows the possibility for a strong energy exchange even for high orbital momentum of the electron when quantum defects are small. We establish the existence of a classical chaos border and of a quantum delocalization border above which diffusive autoionization of the electron takes place. The relevance of theory for autoionization of doubly excited Rydberg atoms is also discussed.

A technique for empirically determining optimal coordinates for modeling a dynamical system is presented. The methodology may be viewed as a nonlinear extension of the Karhunen-Loève procedure and is implemented via an autoassociative neural network. Given a high-dimensional system of differential equations which model a dynamical system asymptotically residing on a stable attractor, the task of the network is to compute a reembedding of the attractor and the dynamics into an ambient space which reflects the intrinsic dimensionality of the problem. The method is demonstrated on the unforced Van der Pol oscillator, the forced Van der Pol, and the Kuramoto-Sivashinsky equation.

Resonance lines of hydrogenlike aluminum ions emitted from a high-power laser-produced plasma have been recorded using a novel ultrahigh-resolution double-crystal x-ray spectrometer. The resolution provided by this instrument allows, for the first time, well resolved emitted line profiles and positions to be measured as a function of observation angle. Interpretation of the measured line shapes, positions, and intensities require simulations which include the transfer of radiation in a plasma with a large velocity gradient.

Small-angle neutron scattering has been used to study phase separation in a polymer blend made miscible by a dilute, random distribution of hydrogen bonds between the two components. In the one-phase region, scattering below a well-defined wave vector ${\mathit{q}}_{\mathit{c}}$ remains weak as the critical point is approached. During the early stages of spinodal decomposition, the formation of domains of a characteristic size R(t)≥1/${\mathit{q}}_{\mathit{c}}$ is apparent as a peak in the time-dependent structure factor, which scales as ${\mathit{q}}^{\mathrm{-}2.40}$ independent of quench depth, raising interesting questions about the morphology of early-stage spinodal decomposition in these mixtures.

It has recently been observed that a diblock copolymer melt can undergo a second microphase separation with increasing temperature. We use an equation-of-state description along with the random phase approximation to study the stability of the disordered system, and calculate the lower critical ordering temperature as a function of the polymer interaction parameters, chain lengths, and volume fractions.

Nematic circular drops of the liquid crystals 5CB and MBBA suspended in their isotropic phases exhibit an electric-field-induced deformation into an elliptical shape extended normal to the applied field which is many times larger than that found in nonliquid crystal systems, a result of the low nematic-isotropic interfacial tension. Isotropic drops suspended in a nematic phase also exhibit large deformation, but of opposite sign. In both cases the deformation is proportional to the applied field squared and decreases in amplitude with increasing frequency, a result of the conductivity relaxation of the liquid crystal.

We construct trace maps for products of 2×2 matrices generated by arbitrary substitution sequences. The dimension of the underlying space of our trace map is the minimal possible, namely 3r-3 for an alphabet of size r≥2.

We have employed Mössbauer spectroscopy and low energy H implantation to study the Sb-H complexes in n-type Si. The different Mössbauer components were studied as a function of H dose, H-implantation temperature, and annealing temperature. To understand the observed data, it is necessary to introduce, in addition to the well-known SbH complex, an ${\mathrm{SbH}}_{\mathit{n}}$ complex (n≥2), which provides experimental evidence for the existence of donor-multihydrogen complexes. We show that these complexes are in thermal equilibrium with a larger hydrogen reservoir (${\mathrm{H}}_2^{\mathrm{*}}$), which governs their thermal stability.

The general evolution equation describing spinodal decomposition in systems with conserved or nonconserved order parameter is developed. It is shown that if in the system thermodynamic potential expression the coefficients at order parameter square and at order parameter gradient square can change sign near the spinodal point, some different decomposition scenarios are possible, which result in system transformation from disordered to ordered, modulated, patterned, or ordered-patterned states.

We compute by a path integral Monte Carlo calculation the isotropic shift of helium melting pressure in the temperature range (T>100 K) where a discrepancy between theory and experiment has been recently reported. We use a realistic Aziz pair potential together with Bruch-McGee three-body forces for the interaction. The isotopic shift predicted in this work is in agreement with experiment; its measurement provides a good test of the interatomic potential of helium, as the isotopic shift is sensitive to the kinetic energy, which is determined by the short-range pair interaction.

We simulate the vapor deposition of the first three monolayers of Au on Ag(110) with the molecular dynamics code damsel using surface-embedded-atom potentials and utilizing simulated annealing. The simulations show interdiffusive Stranski-Krastanov growth, which is consistent with recent ion scattering and scanning tunneling microscopy studies. Most of the first deposited monolayer of Au burrows to the substrate layer, with further deposition leading to 3D growth resembling a 3×1 missing-row reconstruction. We discuss the dynamic mechanisms accounting for this growth.

We report unambiguous atomic scale evidence demonstrating that the atom density in the ‘‘1×1’’ high temperature phase of Si(111) is ∼6% higher than the 7×7. Such evidence is provided by scanning tunneling microscopy observation of excess adatom density and related island formation on surfaces prepared by a novel method which provides a low step density. The presence of the excess adatom density is also correlated to the observation of areas of metastable reconstructions, i.e., 9×9, 2×2, c2×4, and √3 × √3 , much larger than previously reported and for the first time, the existence of metastable 11×11 on Si(111).

Positron annihilation induced Auger electron spectroscopy (PAES) performed on submonolayer films of Au deposited on Cu(100) provides direct evidence for preferential annihilation with Au atoms. The PAES intensity of Au is anomalously large at low surface concentrations (e.g., the PAES signal from Au deposited at 173 K reaches ∼40% of the signal obtained for a complete monolayer at ∼0.07 monolayer). We infer from this behavior and comparisons with model calculations that positrons are localized at sites containing many contiguous Au atoms.

A potential ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ pairing state in the 2D Hubbard model is investigated within a conserving extension of Hartree-Fock theory known as the fluctuation exchange approximation. The self-consistent field equations are solved with the aid of a recently developed renormalization group approach for general models which lack a Migdal theorem. The calculations yield an anisotropic gap which saturates rapidly below ${\mathit{T}}_{\mathit{c}}$ to a peak value of 2${\mathrm{\Delta }}_{\mathbf{k}}$/${\mathit{kT}}_{\mathit{c}}$∼10–12 for k along the ${\mathit{k}}_{\mathit{x}}$ and ${\mathit{k}}_{\mathit{y}}$ axes.

Results of a conserving fluctuation exchange calculation for the superconducting state of a two-dimensional Hubbard model are presented. Self-consistent solutions for the full momentum- and frequency-dependent gap, renormalization, and frequency shift parameters have been obtained. The resulting frequency- and momentum-dependent gap is found to have ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ symmetry. Below ${\mathit{T}}_{\mathit{c}}$, the maximum gap opens rapidly to a low temperature 2Δ(0)/${\mathit{kT}}_{\mathit{c}}$ value of order 10. We find that the effective interaction arising from the exchange of spin and charge fluctuations is stabilized by self-energy feedback effects as the superconducting gap opens.

Hybridization of the ${\mathrm{\sigma }}^{\mathrm{*}}$ and ${\mathrm{\pi }}^{\mathrm{*}}$ states of the graphene network is shown to be as important as band-folding effects in determining the metallicity of small radius carbon nanotubes. Using detailed plane-wave ab initio pseudopotential local density functional (LDA) calculations, we find that the electronic properties of small tubes are significantly altered from those obtained in previous tight-binding calculations. Strongly modified low-lying conduction band states are introduced into the band gap of insulating tubes because of strong ${\mathrm{\sigma }}^{\mathrm{*}\mathrm{-}}$${\mathrm{\pi }}^{\mathrm{*}}$ hybridization. As a result, the LDA gaps of some tubes are lowered by more than 50%, and a tube previously predicted to be semiconducting is shown to be metallic.

The structure of the dominant N pair defect in Si is determined from channeling, infrared local vibrational mode spectroscopy, and ab initio local density functional theory. Channeling experiments show that the N atoms are displaced by 1.1±0.1 Å from lattice sites along 〈100〉. Annealing experiments reveal that this N site is associated with two N-related local vibrational modes originating from the N pair. The ab initio calculations demonstrate that the pair consists of two neighboring 〈100〉 oriented N-Si split interstitials, arranged in an antiparallel configuration, and with four N-Si bonds forming a square lying on {011}.

We construct a nonlinear σ model to describe a system of noninteracting electrons propagating in the presence of random magnetic flux. We find a term describing the long range logarithmic interaction between the topological density of the nonlinear sigma model, and argue that this could give rise to a Kosterlitz-Thouless transition from the localized phase to a phase with power law correlations and continuously varying conductances. We provide a physical interpretation of our results in terms of the scattering of edge states of the magnetic domains in different regions.

We present a new approach to the problem of Bloch electrons in magnetic field, by making explicit a natural relation between magnetic translations and the quantum group ${\mathit{U}}_{\mathit{q}}$(${\mathit{sl}}_2$). The approach allows us to express the spectrum and the Bloch function as solutions of the Bethe-ansatz equations typical for completely integrable quantum systems.

A novel type of a random matrix ensemble originating from a partial breaking of the spin rotation invariance is considered. Universal conductance fluctuations, density of states correlation function, and the persistent current in mesoscopic rings are studied for this type of symmetry of a disordered system.