Search Results (66 total)

An investigation of the lifetime of the 3d⁹4s ³D₃ level along a part of the nickel-like isoelectronic sequence has been performed. The focus of this work has been to evaluate the importance of the hyperfine induced electric quadrupole channel of the 3d^10 1S₀−3d⁹4s ³D₃ transition for isotopes with nuclear spin. Comparisons between the magnetic octupole transition rate and the hyperfine-induced electric quadrupole transition rate shows that the hyperfine quenching is of importance along the whole isoelectronic sequence. At the low-Z end the hyperfine induced electric quadrupole transition channel dominates, being several orders of magnitude larger than the rate for the magnetic octupole transition channel, while for higher Z the two rates are of comparable size.

We investigate the influence of inhomogeneity in the pairing coupling constant U(r⃗) on dirty BCS superconductors, focusing on T_c, the order parameter Δ(r⃗), and the energy gap E_g(r⃗). Within mean-field theory, we find that when the length scale of the inhomogeneity is comparable to or larger than the coherence length, the ratio 2E_g∕T_c is significantly reduced from that of a homogeneous superconductor, while in the opposite limit, this ratio stays unmodified. In two dimensions, when strong phase fluctuations are included, the Kosterlitz-Thouless temperature T_KT is also studied. We find that when the inhomogeneity length scale is much larger than the coherence length, 2E_g∕T_KT can be larger than the usual BCS value. We use our results to qualitatively explain recent experimental observation of a surprisingly low value of 2E_g∕T_c in thin films.

We propose a dynamical gradient network approach to consider the synchronization in the Kuramoto model. Our scheme to adaptively adjust couplings is based on the dynamical gradient networks, where the number of links in each time interval is the same as the number of oscillators, but the links in different time intervals are also different. The gradient network in the (n+1)th time interval is decided by the oscillator dynamics in the nth time interval. According to the gradient network in the (n+1)th time interval, only one inlink’s coupling for each oscillator is adjusted by a small incremental coupling. During the transition to synchronization, the intensities for all oscillators are identical. Direct numerical simulations fully verify that the synchronization in the Kuramoto model is realized effectively, even if there exist delayed couplings and external noise.

We present a detailed investigation of the bulk electronic properties of Y_1−xPr_xBa₂Cu₃O₇ (YPrBCO) using x-ray absorption spectroscopy in the partial fluorescence yield mode (PFY-XAS) at Pr  L₃, Ba  L₃, and Cu  K edges together with Pr  2p_3∕23d_5∕2 and Cu  1s2p resonant inelastic x-ray scatterings. Pr  L₃ PFY-XAS spectra show that Pr is in the mixed-valence state (with the valence number slightly greater than 3) for the whole x=0.2–1 range, while the Pr valence decreases with increasing x. The decrease of the Pr⁴⁺ component upon Pr doping, hinting at a weakening Pr-O hybridization, is inconsistent with the scenario according to which the increase in Pr-O hybridization quenches the superconductivity in YPrBCO for x≥0.6. It rather suggests that the superconductivity may be suppressed by the total increase of the Pr-related states in the electronic structure due to the Pr ion concentration upon doping. No doping dependence is found in the electronic structure of the Ba and Cu sites, consistent with their invariant environment. No temperature dependence in our data is observed within the experimental accuracy.

The multiconfiguration Dirac-Fock (MCDF) method was used to calculate the excitation energies of the levels of ³P and ¹P in the lowest excited configurations for the two homolog elements of No I and Yb I. Also the transition probability of the 7s7p ¹P₁→7s^2 1S₀ transition and the ground-state ionization energy of No I were calculated. The results for Yb I agree very well with the available experiments, with deviations of below 0.6% for the triplets and below 4% for the singlet. The result for the No I excitation energies clears the situation of conflicting results between Borschevsky , 30 056 cm⁻¹ (3.726 eV) [Phys. Rev. A 75, 042514 (2007)], and Fritzsche, 3.36 eV [Eur. Phys. J. D 33, 15 (2005)] for the 7s7p ¹P₁ level, which is planned to be measured in the near future with a newly developed experimental technique by Backe [Eur. Phys. J. D 45, 99 (2007)]. The ionization energy result obtained in this work, 53 701 cm⁻¹, is in excellent agreement with the scaled result of 53 600(600) cm⁻¹.

We study charge delocalization and polarization effects in organic thin films by comparing surface- and bulk-sensitive, high-resolution C  1s photoemission data using tunable synchrotron radiation. In three cases, perylene-tetracarboxylicacid-dianhydride, coronene, and metal-free phthalocyanine, no surface core level shifts are observed (upper limit 100  meV). This unexpected finding shows that polarization effects, and hence polarization differences between surface and bulk, are rather small, and implies that the effective charge is not localized on the core-ionized molecule but delocalized over several neighboring molecules.

A resonant Raman spectroscopy study has been carried out under high pressure, using a diamond anvil cell, on carbon nanotube peapods (C₆₀@SWNT) synthesized in high yield in our laboratory. The Raman signal was excited by a near IR laser (830  nm) to avoid photopolymerization of C₆₀ and thus obtain the intrinsic vibrational information on the C₆₀ molecules in the nanotubes. It is found that the surrounding tubes create an effective pressure on the encapsulated C₆₀ due to tube-fullerene interactions, resulting in a shift of the intrinsic A_g(2) vibrational mode to 1474  cm⁻¹ at ambient pressure. High pressure Raman spectroscopy indicates that (C₆₀)₂ dimers form near 1  GPa, and that a further polymerization of C₆₀ occurs near 23  GPa, creating linear chains of covalently linked C₆₀ molecules in the tubes.

A detailed large-scale calculation on the resonant excitation rate coefficients from the ground state to the 106 fine-structure levels belonging to 3l¹⁷4l^′ (l=0,1,2; l^′=0,1,2,3) configurations of Ni-like tantalum have been performed using the relativistic distorted-wave approximation. The contributions through all possible Cu-like doubly excited states 3l¹⁷4l^′n^″l^″ and 3l¹⁷5l^′n^″l^″ (n^″≤15, l^″≤8) are calculated. The validity of n^″−3 scaling law is investigated. The radiative damping effects on resonant excitation rates are studied. The significant effects arising from decays to autoionizing levels are also investigated. The contributions from resonant excitation are found to be as important as direct excitation processes for most transitions. In some cases, the resonant excitation can enhance the excitation rate coefficients by an order of magnitude. Large discrepancies between the present resonant excitation rate coefficients with previously published values are found, and the present results should be more reliable and accurate.

We propose a procedure to distinguish quasiperiodic from chaotic orbits in short-time series, which is based on the recurrence properties in phase space. The histogram of the return times in a recurrence plot is introduced to disclose the recurrence property consisting of only three peaks imposed by Slater’s theorem. Noise effects on the statistics are studied. Our approach is demonstrated to be efficient in recognizing regular and chaotic trajectories of a Hamiltonian system with mixed phase space.

A Reply to the Comment by Thad G. Walker and Charles J. Goebel.

We report on a large scale calculation concerning the 5s^{2  1}S and 5s5p  ^1,3P levels in ⁸⁸Sr  I, using multiconfiguration Dirac-Fock methods. Our focus is on the ³P₂ meta-stable level, which has been shown to be extremely useful in atom cooling and trapping experiments. The lifetime for this level from our work (1071  s) agrees very well with a previous theoretical value, but not with the available experimental value (520₋₁₄₀⁺³¹⁰  s). The results for all the transitions between the 5s^{2  1}S and 5s5p  ^1,3P levels and within the 5s5p  ^1,3P terms are also given.

We report on the first theoretical investigation of M_F-dependent lifetimes due to interference between a magnetic octupole transition and a hyperfine induced electric quadrupole transition. Extensive multiconfiguration Dirac-Fock calculations are used to model the hyperfine quenching of the magnetic octupole decay of 3d⁹4s ³D₃ and the state mixing between the ³D₃ and ³D₂ due to hyperfine interaction in nickel-like Xe²⁶⁺.

We study the synchronizability of weighted aging scale-free networks with non-normalized and asymmetrical coupling matrices. We found that the synchronizability of such networks is improved when the couplings from older to younger nodes become dominant, where the out degrees of the nodes are heterogeneous and their in degrees are homogeneous, and that the synchronizabilty of the networks is seriously weakened or even lost when the couplings from younger to older nodes become dominant, where the out degrees of the nodes are homogeneous and their in degrees are heterogeneous. We also found that both the heterogeneity of nodes and a smaller average degree can improve the synchronizability of the networks with some appropriately chosen weighting parameter values. We finally show an example of the coupled Lorenz systems for illustration and verification of the theoretical analysis.

Studies of the dynamics of longitudinal space-charge waves in space-charge dominated beams propagating through a transport channel with a long solenoid are performed at the University of Maryland. In this paper, we report some experimental results on the energy modulations converted from density modulations. By changing the working conditions of the electron gun, pure initial density modulations are generated. Energy perturbation waveforms are measured with a high-resolution energy analyzer. The experimental results are compared with both the linear theory and the simulation results. Good agreements are achieved for the relationship between the energy and current perturbation strengths.

Strong dependence of the crystal orientation, morphology, and melting temperature (T_m) on the substrate is observed in the semicrystalline polyethylene thin films. The T_m decreases with the film thickness decrease when the film is thinner than a certain critical thickness, and the magnitude of the depression increases with increasing surface interaction. We attribute the large T_m depression to the decrease in the overall free energy on melting, which is caused by the substrate attraction force to the chains that competes against the interchain force which drives the chains to crystallization.

We present an equation-free dynamic renormalization approach to the computational study of coarse-grained, self-similar dynamic behavior in multidimensional particle systems. The approach is aimed at problems for which evolution equations for coarse-scale observables (e.g., particle density) are not explicitly available. Our illustrative example involves Brownian particles in a 2D Couette flow; marginal and conditional inverse cumulative distribution functions (ICDFs) constitute the macroscopic observables of the evolving particle distributions.

We report a new experimental study of the growth of longitudinal energy spread in a space-charge-dominated electron beam, with a beam energy of several keV and beam current of approximately 100 mA. At relatively low beam densities, we measure growing energy spreads with distance along the transport channel, which are in remarkably good agreement with the theory of energy relaxation via Coulomb collisions. At higher beam densities, however, anomalous energy spreads exceeding the predictions of the relaxation theory are observed, which, we believe, could be caused by collective longitudinal-transverse instabilities observed in computer simulation studies. The onset of these instabilities occurs after several plasma periods according to calculations.

We report a photoemission study of the systematics of the 4f electronic structure of a family of rare-earth organic compounds. Resonant photoemission has been used to determine the binding energies of the 4f^N−1 ground states, relative to those of the ligand orbitals, of a number of rare-earth tris-8-hydroxyquinolines (REQ₃’s). Using an empirical model these results have been extrapolated to the full series (Ce-Lu) of REQ₃’s. It is found that in all cases, with the possible exception of Ce, the energy of the 4f^N−1 ground state is less than that of the highest occupied molecular orbital, and therefore individual holes on lanthanide sites will not be stable.

We report additional rich fine structures in high-resolution near-edge x-ray-absorption fine structure (NEXAFS) spectra of large organic molecules using NTCDA on Ag(111) as an example. These fine structures are completely interpreted as vibronic coupling to electronic core excitations. The coupling is mode selective; predominantly one vibronic mode couples to each excitation. The fit results suggest the occurrence of a Davydov splitting, first observed for core excitons. Morphological differences substantially influence the electron-vibron coupling, indicating a strong intermolecular interaction. Thus NEXAFS becomes a more subtle probe for organic solids.

Characterization of beam energy spread in a space-charge dominated beam is very important to understanding the physics of intense beams. It is believed that coupling between the transverse and longitudinal directions via Coulomb collisions will cause an increase of the beam longitudinal energy spread. At the University of Maryland, experiments have been carried out to study the energy evolution in such intense beams with a high-resolution retarding field energy analyzer. The temporal beam energy profile along the beam pulse has been characterized at the distance of 25 cm from the anode of a gridded thermionic electron gun. The mean energy of the pulsed beams including the head and tail is reported here. The measured rms energy spread is in good agreement with the predictions of the intrabeam scattering theory. As an application of the beam energy measurement, the input impedance between the cathode and the grid due to beam loading can be calculated and the impedance number is found to be a constant in the operation region of the gun.

Radiative recombination transitions into the ground state of cooled bare and hydrogenlike uranium ions were measured at the storage ring ESR. By comparing the corresponding x-ray centroid energies, this technique allows for a direct measurement of the electron-electron contribution to the ionization potential in the heaviest He-like ions. For the two-electron contribution to the ionization potential of He-like uranium we obtain a value of 2248±9  eV. This represents the most accurate determination of two-electron effects in the domain of high-Z He-like ions, and the accuracy reaches already the size of the specific two-electron radiative QED corrections.

An experiment has been performed at the Gesellschaft für Schwerionenforschung accelerator facility using a beam-foil time-of-flight technique with a newly developed chemical vapor deposition diamond particle detector to measure the lifetime of the 2 ³P₀ state, τ(2 ³P₀), of the two-electron ion ¹⁹⁷Au⁷⁷⁺ (He-like gold) with the result τ(2 ³P₀)=22.16(0.81) ps. The mechanism for the decay of this state is by a hyperfine-induced radiative transition to the 1 ¹S₀ ground state (hyperfine quenching). The lifetime is therefore determined by a number of fundamental atomic and nuclear parameters not normally involved in radiative decay of allowed transitions. The experimental result is compared to several theoretical calculations.

Here we show that a wide range of states of phases and amplitudes exist for a circularly polarized (CP) plane wave to act on a classical hydrogen model to achieve infinite times of stability (i.e., no orbital decay due to radiation reaction effects). An analytic solution is first deduced to show this effect for circular orbits in the nonrelativistic approximation. We then use this analytic result to help provide insight into detailed simulation investigations of deviations from these idealistic conditions. By changing the phase of the CP wave, the time t_d when orbital decay sets in can be made to vary enormously. The patterns of this behavior are examined here and analyzed in physical terms for the underlying but rather unintuitive reasons for these nonlinear effects. We speculate that most of these effects can be generalized to analogous elliptical orbital conditions with a specific infinite set of CP waves present. The paper ends by briefly considering multiple CP plane waves acting on the classical hydrogen atom in an initial circular orbital state, resulting in “jump-like” and “diffusion-like” orbital motions for this highly nonlinear system. These simple examples reveal the possibility of very rich and complex patterns that occur when a wide spectrum of radiation acts on this classical hydrogen system.