Photon contribution to the “super-hard” pomeron

It is shown that direct photons provide a leading twist mechanism for diffractive jet production in which the jets carry away all of the momentum lost by the proton. Two-photon processes are thus expected to asymptotically dominate “super-hard” pomeron events in ep collisions. We report the expected rates from these events for recent ZEUS and H1 data cuts. We also estimate the direct photon contribution to the “super-hard” pomeron events observed by the CERN UA8 group for collisions. It is again argued that direct photons are the leading mechanism for these events. We find that direct photons are an appreciable fraction of the events seen by UA8.     

Molecular-dynamics investigation of structural transformations of a Cu201 cluster in its melting process

We perform molecular-dynamics calculations to investigate the structural transformation of a copper cluster containing 201 atoms in its melting process within the framework of the embedded-atom method (EAM). Concerning melting, the obtained results reveal that its structural changes are different from those of larger-size clusters containing several hundreds or more atoms and smaller-size clusters containing tens of atoms. The melting process of this Cu₂₀₁ cluster involves three stages, firstly some atoms in inner regions of this cluster move into outer regions accompanying the structural transformation of the local atom packing, followed by the continuous interchange of atomic positions, and finally this cluster is wholly disordered. During the temperature increase, the structural changes of different regions determined by atom density profiles result in apparent increases in internal energy. By decomposing peaks of pair distribution functions (PDFs) according to the pair analysis (PA) technique, the local structural patterns are identified for the melting of this cluster.     

Imaging benzene on nickel and copper {110} surfaces with low temperature STM:: the adsorption site

Single benzene molecules have been imaged on the {110} surfaces of both copper and nickel with an Eigler-type low temperature scanning tunnelling microscope. Conditions were chosen such that the molecule and the lattice atoms could be resolved at the same time within one image frame. On Ni{110} benzene was found to adsorb in the hollow site between the close-packed rows. For the more weakly bound molecule on Cu{110} the imaging conditions had to be changed since, for the tunnelling conditions under which the lattice atoms are resolved, a strong interaction between the tip and the benzene takes place. This results in the molecule being “dragged” along the surface. By changing the tunnelling conditions within one image frame, however, it is possible via extrapolation of the substrate lattice to show that the molecule adsorbs in the long-bridge site. The shapes of the images of individual molecules are discussed.     

The lattice dynamics of systems with several identical atoms per unit surface cell: a unit cell spectral density

In studies of the lattice dynamics of surfaces in which there are many identical atoms in the unit cell, for example a reconstructed surface, there is a problem in presenting the results. The problem arises because each atom in the unit cell has a different spectral density and it is not, in general, clear which of the many densities should be presented. We propose here that in such studies a “unit cell spectral density”, one which replaces the collection of densities for individual atoms with a single density, should be used. The relationship of this unit cell spectral density to experimental phonon measurements is also discussed.     

Coherent noise, scale invariance and intermittency in large systems

We introduce a new class of models in which a large number of “agents” organize under the influence of an externally imposed coherent noise. The model shows reorganization events whose size distribution closely follows a power law over many decades, even in the case where the agents do not interact with each other. In addition, the system displays “aftershock” events in which large disturbances are followed by a string of others at times which are distributed according to a t⁻¹ law. We also find that the lifetimes of the agents in the system possess a power-law distribution. We explain all these results using an approximate analytic treatment of the dynamics and discuss a number of variations on the basic model relevant to the study of particular physical systems.     

Temperature-dependent surface X-ray diffraction on K/Ag(001)-(2 × 1)

Temperature-dependent surface X-ray diffraction experiments have been performed on the K/Ag(001)-(2 × 1) adsorption system. The structure is characterized by a missing-row geometry in which alternate Ag rows along [1 0] are missing. The K atoms reside within the large grooves coordinated by six Ag atoms at a distance of 3.44(5) Å, corresponding to an effective K-radius of 2.00(5) Å. Large anisotropic disorder is observed for both the K-atoms and the top-layer (“ridge”) Ag atoms. The K-atom displacements are largest in the direction along the grooves, whereas for the Ag atoms the vibrations along [110] are significantly larger. The temperature dependence of the Ag vibrations is in accordance with Debye theory for the [110] direction, but deviates from it for the [1 0] vibrations at high temperature. In contrast to the K-atoms, the out-of-plane vibrations of the top-layer Ag atoms are larger than the in-plane vibrations. The inclusion of anharmonic contributions to describe the Ag disorder significantly improves the fits. It is shown that if anharmonicity is neglected the interlayer contraction is overestimated (Δd₁₂/d₁₂ only −3.2%, instead of −12.7% if anharmonicity is neglected). Due to the anharmonicity, different definitions of the atomic position arise (mean, mode and equilibrium position), which are discussed on the basis of the results.     

The role of point defects in melting of solid He

We have recently observed that the resistance to shear of solid ⁴He decreases dramatically near the first-order BCC–HCP transition. In our view, the solid shears via a diffusive counter-flow of atoms and point defects. The mechanism of self diffusion couples point defects with one specific phonon which softens as the transition is approached. Since such a scenario can possibly lead to melting, it is important to understand (a) which type of point defect is associated with the reduction of shear resistance, and (b) can the presence of point defects lead to the softening of phonons. We report here the results of numerical simulations and analytic modeling. Our results indicate that split interstitials are much more effective than vacancies in lowering the resistance to shear. We suggest that these excitations can be generated as a result of a “local mode” excited in the crystal.     

Dynamic observations of the formation of thin Cu layers on clean and hydrogen-terminated Si(1 1 1) surfaces

The growth of Cu on the clean and hydrogen-terminated Si(1 1 1) surfaces is studied in situ by low-energy electron microscopy (LEEM). The dependence of the growth of the “5×5” layer on the clean Si(1 1 1) 7×7 surface upon the deposition temperature is investigated by combining LEEM with LEED. After completion of the “5×5” layer not only the regular-shaped three-dimensional islands reported before are observed but also irregular shaped more two-dimensional islands. On the hydrogen-terminated Si(1 1 1) surface the formation of the “5×5” structure is suppressed and nano-scale islands form preferentially at the step edges and domain boundaries. This is attributed to the enhancement of the surface migration of Cu atoms by the elimination of the surface dangling bonds.     

Local models of spatio-temporally complex fields

We investigate the ability of local models of the one space dimensional Kuramoto-Sivashinsky partial differential equation with periodic boundary conditions, obtained by projection on a small set of Fourier modes on a short subinterval, to reproduce coherent events typical of solutions of the same equation on a much longer interval. We find that systems containing as few as two linearly unstable modes can produce realistic local events in the short term, but that for more reliable short time tracking and long term statistics, three or four interacting modes are required, and that the length of the short interval plays a subtle rôle, certain “resonant” lengths giving superior results.     

Structure of monolayer silica on Mo(1 1 2) investigated by rainbow scattering under axial surface channeling

The structure of a monolayer crystalline silica film grown on a Mo(1 1 2) substrate is investigated via grazing scattering of fast atoms. For scattering along low indexed directions in the surface plane (“axial surface channeling”) the corrugation of the interaction potential leads to an azimuthal out-of plane scattering with an intensity enhancement for the maximum deflection angle, the so called “rainbow”. From the comparison of the experimental angular distributions for scattered projectiles with classical trajectory simulations we obtain information on the arrangement of atoms in the topmost surface layer. Our work provides evidence for the structural model of a two-dimensional network for the silica film.     

Photoionization of the first- and second-row atom hydrides using synchrotron radiation:β measurements for the outer- and inner-valence shells

Experimental results on the valence shell photoionization of argon- and neon-like molecules are reviewed and unpublished results are included. The outer lone-pair orbitals of the hydride secondrow atoms are characterized by a chlorine, sulfur or phosphorus 3p atomic character revealed by the occurrence of a Cooper minimum in the β trend associated with the first valence band in the photoelectron spectra; this atomic behavior is discussed as a function of the hydrogen environment. By comparing the β trends for the main band in the complex inner-valence photoelectron spectra of the first- and second-row atom hydrides we point out a similar behavior which reveals the main “s” atomic character of the inner valence orbital.     

Heating of condensation surface during magnetron sputtering

Chromium films deposited by magnetron sputtering on non-heated substrates from non-thermalized atoms crystallize in regular bcc Cr phase, with non-uniform microstructure and lattice constant along the thickness. These non-uniformities decrease with elevation of the substrate temperature and vanish at a certain value. However films deposited on non-heated substrates from thermalized atoms crystallize in a low-temperature Cr phase and have almost uniform microstructure. We have developed a model explaining this effect, which is based on the supposition of the formation of a “hot” layer on the growth surface during deposition, whose temperature depends on the flux of energy delivered to the condensation surface and can be noticeably higher than the substrate temperature. Detailed investigation of the structure of Cr films deposited at various temperatures and energy fluxes delivered to the growth surface, correlate well with the above model.     

Magnetic properties of Lu2Co17−xSix single crystals

The Co-sublattice anisotropy in Lu₂Co₁₇ consists of four competitive contributions from Co atoms at crystallographically different sites in the Th₂Ni₁₇-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu₂Co_17−xSi_x (x=0−3.4) grown by the Czochralski method. The SRT in Lu₂Co₁₇ was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T_SR1≈680 K and “easy-cone”–“easy-axis” at T_SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu₂Co₁₆Si (T_SR1≈75 K and T_SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu₂Co_17−xSi_x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.     

Reactions of methanol and ethylene glycol on Ni/Pt: Bridging the materials gap between single crystal and polycrystalline bimetallic surfaces

The catalytic decompositions of methanol and ethylene glycol on polycrystalline Ni/Pt surfaces were used as model probe reactions to explore oxygenate reforming for H₂ production. In the current study we evaluated whether favorable chemistry observed on single crystal Ni/Pt(111) can be extended to more commercially relevant polycrystalline surfaces, thus bridging the “materials gap”. Auger electron spectroscopy (AES) confirmed that two distinct bimetallic configurations can be formed for the Ni/Pt system, each possessing unique chemical properties: one with the surface enriched in Ni atoms, designated NiPtPt, and the other with the subsurface region enriched in Ni atoms, designated PtNiPt. Consistent with single crystal studies, temperature programmed desorption (TPD) revealed that the NiPtPt configuration was more active for reforming to CO and H₂ than either polycrystalline Pt or PtNiPt. High-resolution electron energy loss spectroscopy (HREELS) confirmed the presence of strongly bound reaction intermediates on NiPtPt, including aldehyde-like species, which was also observed on Ni–Pt–Pt(111). The strongly bound reaction intermediates most likely contribute to the high reforming activity observed on NiPtPt. Overall, TPD and HREELS results on polycrystalline surfaces were in general consistent with their single crystal counterparts for the reforming of oxygenates.     

Ultra-strong coupling effects with quantum metamaterials

We study a semiconductor-based quantum metamaterial which has the optical characteristics of a metal in two directions, but behaves like a collection of artificial atoms, whose properties can be designed-in using quantum theory, in the third. We find that it supports a type of guided collective plasma resonance (CPR) mode which exhibits efficient optical coupling and long propagation distances. Furthermore, the coupling of the CPR mode with the ‘artificial atom’ transition leads to a case of “Ultra-Strong-Coupling”, demonstrated by a record vacuum Rabi splitting of 65 meV, a sizable fraction (42%), of the bare intersubband energy.     

Protection of Quantum Correlation Through the Quantum Erasing Effect

By taking into account the quantum erasing effect(QEE), the quantum discord (QD) behavior of a two-qubit system with different initial states are investigated in detail. We find that the quantum correlation can be saved under a scheme of two spatially separated atoms, each located in a leaky cavity through the quantum erasing method. It is shown that QEE can weaken the effects of decoherence, and preserve the maximum information of the coherent item. No matter whether the two atoms are in the mixted or pure state, one can robusty save their initial quantum correlation even the number of erasing events is finite. If one limit the erasing events N → ∞, the QEE can be used to protect the initial quantum correlation independently of the state in which it is stored, the values of QD is always nearly equal to the initial QD values, and it is nearly independent of the decoherence, which imply us more encourage strategy for protecting the quantum correlation properties in some quantum systems.     

The Casimir-like size effects in ideal gases

The wave character of atoms can produce Casimir-like size effects in gases confined in a narrow box. Thus the pressure tensor is not isotropic anymore and size difference becomes a driving force for isothermal diffusion by a permeable wall. Such size effects give rise to “thermosize effects” similar to thermoelectric effects.     

Mean occurrence frequency and temporal risk analysis of solar particle events

The protection of astronauts from space radiation is required on future exploratory class and long-duration missions. For the accurate projections of radiation doses, a solar cycle statistical model, which quantifies the progression level within the cycle, has been developed. The resultant future cycle projection is then applied to estimate the mean frequency of solar particle events (SPEs) in the near future using a power law function of sunspot number. Detailed temporal behaviors of the recent large event and two historically large events of the August 1972 SPE and the November 1960 SPE are analyzed for dose-rate and cumulative dose equivalent at sensitive organs. Polyethylene shielded “storm shelters” inside spacecraft are studied to limit astronauts’ total exposure at a sensitive site within 10 cSv from a large event as a potential goal that fulfills the ALARA (as low as reasonably achievable) requirement.     

Functional integral for ultracold fermionic atoms

We develop a functional integral formalism for ultracold gases of fermionic atoms. It describes the BEC–BCS crossover and involves both atom and molecule fields. Beyond mean field theory we include the fluctuations of the molecule field by the solution of gap equations. In the BEC limit, we find that the low temperature behavior is described by a Bogoliubov theory for bosons. For a narrow Feshbach resonance these bosons can be associated with microscopic molecules. In contrast, for a broad resonance the interaction between the atoms is approximately pointlike and microscopic molecules are irrelevant. The bosons represent now correlated atom pairs or composite “dressed molecules”. The low temperature results agree with quantum Monte Carlo simulations. Our formalism can treat with general inhomogeneous situations in a trap. For not too strong inhomogeneities the detailed properties of the trap are not needed for the computation of the fluctuation effects—they enter only in the solutions of the field equations.