Search Results (28 total)

Using different models for the deposition of energy on the lattice and a classical molecular dynamics approach to the subsequent transport, we evaluate how the details of the energy deposition model influence sputtering yield from a Lennard-Jones target irradiated with a MeV/u ion beam. Two energy deposition models are considered: a uniform, instantaneous deposition into a cylinder of fixed radius around the projectile ion track, used in earlier molecular dynamics and fluid dynamics simulations of sputtering yields; and an energy deposition distributed in time and space based on the formalism developed in the thermal spike model. The dependence of the sputtering yield on the total energy deposited on the target atoms is very sensitive to the energy deposition model. To clarify the origin of this strong dependence, we explore the role of the radial expansion of the electronic system prior to the transfer of its energy to the lattice. The results imply that observables such as the sputtering yield may be used as signatures of the fast electron-lattice energy transfer in the electronic energy-loss regime, and indicate the need for more experimental and theoretical investigations of these processes.

During the preparatory work for the optical-replica synthesizer experiment in the free-electron laser FLASH at DESY, we were able to superimpose a short, approximately 200 fs long pulse from a frequency-doubled mode-locked erbium laser with titanium-sapphire amplifier and an approximately 20 ps long electron bunch in an undulator. This induces an energy modulation in a longitudinal slice of the electron bunch. A magnetic chicane downstream of the undulator converts the energy modulation into a density modulation within the slice that causes the emission of coherent optical transition radiation from a silver-coated silicon screen. Varying the relative timing between electron and laser, we use a camera to record two-dimensional images of the slices as a function of the longitudinal position within the electron bunch.

We consider the problem of a degenerate electron gas in the background of a uniformly distributed positive charge, ensuring overall neutrality of the system, in the presence of noncommutativity. In contrast to previous calculations [F. S. Bemfica and H. O. Girotti, J. Phys. A 38, L539 (2005)] that did not include twisted formalism, we study the effects of noncommutativity from the points of view of both usual and braided twisted symmetry. We find corrections to the ground-state energy already at first order in perturbation theory when the usual twisted statistics is taken into account. The effects of noncommutativity, however, disappears if braided twisted symmetry is considered. In the former case these corrections arise since the interaction energy is sensitive to two-particle correlations, which are modified for the usual twisted anticommutation relations.

We demonstrated a novel optical switch to control the high-order harmonic generation process so that single attosecond pulses can be generated with multiple-cycle pulses. The technique combines two powerful optical gating methods: polarization gating and two-color gating. An extreme ultraviolet supercontinuum supporting 130 as was generated with neon gas using 9 fs laser pulses. We discovered a unique dependence of the harmonic spectra on the carrier-envelope phase of the laser fields, which repeats every 2π radians.

Commonly used dependence measures, such as linear correlation, cross-correlogram, or Kendall’s τ, cannot capture the complete dependence structure in data unless the structure is restricted to linear, periodic, or monotonic. Mutual information (MI) has been frequently utilized for capturing the complete dependence structure including nonlinear dependence. Recently, several methods have been proposed for the MI estimation, such as kernel density estimators (KDEs), k-nearest neighbors (KNNs), Edgeworth approximation of differential entropy, and adaptive partitioning of the XY plane. However, outstanding gaps in the current literature have precluded the ability to effectively automate these methods, which, in turn, have caused limited adoptions by the application communities. This study attempts to address a key gap in the literature—specifically, the evaluation of the above methods to choose the best method, particularly in terms of their robustness for short and noisy data, based on comparisons with the theoretical MI estimates, which can be computed analytically, as well with linear correlation and Kendall’s τ. Here we consider smaller data sizes, such as 50, 100, and 1000, and within this study we characterize 50 and 100 data points as very short and 1000 as short. We consider a broader class of functions, specifically linear, quadratic, periodic, and chaotic, contaminated with artificial noise with varying noise-to-signal ratios. Our results indicate KDEs as the best choice for very short data at relatively high noise-to-signal levels whereas the performance of KNNs is the best for very short data at relatively low noise levels as well as for short data consistently across noise levels. In addition, the optimal smoothing parameter of a Gaussian kernel appears to be the best choice for KDEs while three nearest neighbors appear optimal for KNNs. Thus, in situations where the approximate data sizes are known in advance and exploratory data analysis and/or domain knowledge can be used to provide a priori insights into the noise-to-signal ratios, the results in the paper point to a way forward for automating the process of MI estimation.

At the 1.7-GeV electron storage ring BESSY II, a first source of synchrotron radiation with 100 fs pulse duration, variable (linear and circular) polarization, tunable photon energy (300 to 1400 eV), and excellent signal-to-background ratio was constructed and is now in routine operation.

Femtosecond far-infrared radiation pulses in the THz spectral range were observed as a consequence of the energy modulation of 1.7 GeV electrons by femtosecond laser pulses in the BESSY storage ring in order to generate femtosecond x-ray pulses (”femtoslicing”). In addition to being crucial for diagnostics of the laser-electron interaction, the THz radiation itself is useful for experiments where intense ultrashort THz pulses of well-defined temporal and spectral characteristics are required.

A simple model in which immobilizing events are imposed onto otherwise free Brownian diffusion [R. Metzler and J. Klafter, Phys. Rep. 339, 1 (2000) and a recent adaptation due to S. Khan and A. M. Reynolds, Physica A 350, 183 (2005)] is shown to encapsulate the peculiar transport characteristics of individual cell receptors within plasma membranes observed in single-particle tracking (SPT) experiments. These characteristics include the occurrence of normal diffusion; non-Gaussian subdiffusion; confined diffusion; a superdiffusive mode of transport that is not due to flow of the membrane or molecular motor attachment; and the occurrence of transitions between these transport modes. Model predictions are shown to be in close agreement with a reanalysis of existing SPT data.

The energy modulation of 1.7-GeV electrons by femtosecond laser pulses was studied at the BESSY II “femtoslicing” source, a facility commissioned in 2004 for the purpose of producing sub-100 fs x-ray pulses. As a test case for future seeded free-electron lasers, the laser-electron interaction was investigated as function of various laser and electron beam parameters using different experimental methods.

We report observations of the zero-field superconductor-insulator transition in granular quench-condensed Pb for samples within 10–15 nm of relatively thick superconducting ground planes. Resistance vs temperature measurements of sufficiently thick Pb samples exhibit broadened superconductor transitions consistent with previous results on clean dielectric substrates. The lack of any measurable influence by the superconducting planes on the Pb film resistance is discussed within the context of zero-field vortex-antivortex unbinding explanations for the transition broadening.

We perform total-energy calculations based on the tight-binding Hamiltonian scheme (i) to study the structural properties and energetics of the extended {311} defects depending upon their dimensions and interstitial concentrations and (ii) to find possible mechanisms of interstitial capture by and release from the {311} defects. The generalized orbital-based linear-scaling method implemented on the Cray T3D is used for supercell calculations of large-scale systems containing more than 1000 Si atoms. We investigate the {311} defects systematically from few-interstitial clusters to planar defects. For a given defect configuration, constant-temperature molecular-dynamics simulations are performed at 300–600 K for about 1 psec to avoid trapping in the local minima of the atomic structures with small energy barriers. We find that interstitial chain structures along the 〈011〉 direction are stable interstitial defects with respect to isolated interstitials. The interstitial chains provide basic building blocks of the extended {311} defects, i.e., the extended {311} defects are formed by condensation of the interstitial chains side by side in the 〈233〉 direction. We find that successive rotations of pairs of atoms in the {011} plane are mechanisms with a relatively small energy barrier for propagation of interstitial chains. These mechanisms, together with the interstitial chain structure, can explain the growth of the {311} defects and related structures such as V-shape bend structures and atomic steps observed in transmission electron microscopy images.

The quantum theory of charged-particle beam transport through a magnetic lens system with a straight optic axis, at the level of single-particle dynamics and disregarding spin (or, when nonzero, assuming it to be an independent spectator degree of freedom), is presented, based on the Schrödinger and Klein-Gordon equations in a form suitable for analyzing the paraxial and aberration aspects in a systematic way using a Lie algebraic approach. In the classical limit, the well known Lie algebraic treatment of the corresponding classical theory is obtained. As examples, quadrupole and axially symmetric magnetic lenses are considered. An extension of the theory to the cases of electrostatic and other electromagnetic lens systems is outlined. This work is complementary to an already known similar approach to the spinor electron optics based on the Dirac equation and provides the corresponding framework when the optics of charged particles, with or without spin, is described with scalar wave functions in the nonrelativistic and relativistic situations.

Sub-Coulomb cross sections for ¹²C+¹²C→α+²⁰Ne, ¹²C+¹²C→p+²³Na, and ¹²C+¹²C→n +²³Mg are calculated in a semistatistical framework that allows a comparison to experiment in terms of the general energy dependence and magnitude. Intermediate structure previously produced in the generalized doorway model via calculations of the structure factor is not evident in the present approach because of the large energy dependence of the reaction cross sections. The compound nucleus is described in a generalized doorway model where the doorways are obtained by coupling the shape resonances in ¹²C+¹²C to the single and mutual excitation channels. The transmission coefficients for decay are determined in a modified form of the Hauser-Feshbach method. Agreement is good in terms of energy dependence and magnitude even though the experimental cross sections vary by about 4 orders of magnitude between 3 and 5 MeV c.m.

Macroscopic equations of motion describing the coupled dynamics and fluctuations of a dilute colloidal crystal and the underlying fluid in the presence of shear are analyzed for plane Couette flow. The system becomes unstable through a transverse acoustic resonance mechanism at a critical shear rate ω_cr. For large system size L, ω_cr∼[L ln(L)]^-1. The lattice displacement fluctuations excited by thermal motion only diverge logarithmically as the instability is approached, but cause the Debye-Waller factors to become periodic functions of time. The theory is discussed in terms of recent scattering experiments and other theoretical work.

A nuclear projection model that produces generalized doorways in the sub-Coulomb region is used to calculate the modified structure factor S̃(E) in ¹²C+¹²C down to about 3 MeV c.m. The formation of a compound nucleus is enhanced at energies near those of the generalized doorways which are obtained by coupling the shape resonances to the single and mutual excitation channels of the deformed ¹²C nuclei. The agreement between theory and experiment for S̃(E) in terms of magnitude and intermediate structure is dependent on the spreading width to complicated states as manifested in the energy averaging interval. The agreement in magnitude is striking in that the cross-section values are as small as 10^-3 mb at 3 MeV c.m. At low energies the nonresonant part of σ_comp is dominant, and the monotonic increase in S̃(E) with decreasing energy can be attributed to absorption under the barrier. Extrapolation of S̃(E) to the astrophysical energy of 1.7 MeV c.m. without a soft core suggests that the resonances will not introduce any dramatic structure. The presence of any prominent structure in this region may point to the need for a repulsive core in the nuclear potential.

It is found that exact and perturbative treatments of the imaginary potential at sub-Coulomb energies in ¹²C+¹²C provide close agreement in calculations of both the real and imaginary phase shifts at energies away from resonance.

A nuclear projection model is developed for heavy-ion reactions in the sub-Coulomb region. Generalized doorways are introduced as the set of resulting states obtained by coupling the shape resonances to the single and mutual excitation channels. The model is applied to the ¹²C+¹²C reaction by treating the ¹²C nucleus as deformed, and the ion-ion optical potential is taken as that of Kondo et al. with a soft repulsive core. The energies and continuum widths of the resonances below and near the Coulomb barrier are calculated and found to be consistent with observation. Intermediate structure is mainly caused by the shape resonances which are fragmented into the various generalized doorways. However, the major strength of the elastic channel remains concentrated around the energy where a potential resonance exists in the elastic channel. Levels are predicted down to 2.91 MeV c.m. and provide a first step in an impending extrapolation of ¹²C+¹²C reaction rates to astrophysical energies.

The static correlations in highly charged colloidal and micellar suspensions, with and without added electrolyte, are examined using the hypernetted-chain approximation (HNC) for the macro-ion–macro-ion correlations and the mean-spherical approximation for the other correlations. By taking the point-ion limit for the counter-ions, an analytic solution for the counter-ion part of the problem can be obtained; this maps the macro-ion part of the problem onto a one-component problem where the macro-ions interact via a screened Coulomb potential with the Gouy-Chapman form for the screening length and an effective charge that depends on the macro-ion–macro-ion pair correlations. Numerical solutions of the effective one-component equation in the HNC approximation are presented, and in particular, the effects of macro-ion charge, nonadditive core diameters, and added electrolyte are examined. As we show, there can be a strong renormalization of the effective macro-ion charge and reentrant melting in colloidal crystals.

A projection operator method is presented for the extraction of resonance energies, shifts, and widths of bound states embedded in the continuum for potentials that have a repulsive core. The procedure is applicable for heavy ion as well as nucleon-nucleus collisions. The original potential is modified beyond the radius where the repulsive core merges into the attractive part of the potential by a harmonic oscillator potential whose shape is determined primarily by the harmonic oscillator frequency. The method requires that the size of the modified well be fixed for only one particular l value. It is then automatically determined for a range of other l values. The extracted bound states can be used reliably in ensuing diagonalization procedures and in treating continuum-continuum coupling. The method is readily applicable to the determination of widths in those situations where the normal phase shift procedure cannot be employed. As a test case the procedure is applied to ¹²C+ ¹²C for l=0 to 8 and yields resonance energies and widths in very good agreement with a phase shift analysis.

Projection operator methods are used to obtain the elastic differential scattering cross section at 90° c.m. for ²⁸Si+ ²⁸Si in the region E_lab=100–120 MeV. Good agreement with the energies and structures of resonances seen experimentally is obtained by introducing doorway states for l=34–38 constructed from a particle-vibration model that includes both the 2⁺ and 3^- excitation energies of ²⁸Si. The sudden approximation is used in deriving the ion-ion potential and the density distribution is investigated. The calculated cross sections and widths support the association of the resonances with nuclear molecular formation.

The nucleon-energy correlation σ (K₁,K₂), where K₁ and K₂ are the kinetic energies of the outgoing nucleons, is studied in the weak neutral disintegration of the deuteron, ν+d→ν+n+p. The studies are made in all five (S, P, T, A, and V) variants of the neutral-current weak-interaction Lagrangian. The study in the region of low kinetic energies of the nucleons provides means to distinguish between the axial-vector and tensor couplings.

The deuteron disintegration processes ν(ν̅ )+d→ν(ν̅ )+n+p have been studied at intermediate energies in impulse approximation using closure over the final dinucleon states. The disintegration cross section σ has been discussed as a function of neutrino (antineutrino) energy in various SU(2)×U(1) models for the helicity conserving weak neutral currents. A discussion on the helicity flipping weak neutral currents models of S, P, T couplings is also given.

NUCLEAR REACTIONS d(ν,ν)np: closure approximation, deuteron disintegration, neutral currents, helicity conserving theories, helicity flipping theories.

Several research groups have reported observations of scintillations in well-known ferroelectric crystals. Previously we reported similar activity occurring in unirradiated glycine (nonferroelectric) but did not report the optical spectrum associated with the scintillations. In this paper we present the optical-spectroscopy results for scintillations occurring in single crystals of glycine and L-alanine-doped triglycine sulfate (LATGS). Seven lines were identified in glycine and eight lines in LATGS, all belonging to the second positive system of N₂. The spectrum results from an electrical discharge from the crystal surface, which has a layer of N₂ adsorbed onto it, to ground.

Diffusion of silver into polyerystalline aluminum samples exposed to a temperature gradient demonstrates an activation energy comparable to isothermal results. This activation energy does not agree with previous single-crystal experiments using a temperature gradient.

Measurements have been performed of the Knight shift in cadmium as a function of field between 1 and 20 kG at 1.2 °K and as a function of temperature between 1.2 and 300 °K at 9.4kG. The field-dependent studies show an oscillatory behavior of the Knight shift at the de Haas-van Alphen frequency appropriate to the first band in the higher-field ranges. In the low-field region the average value of the Knight shift is found to exhibit a strong anisotropic field dependence. The temperature-dependent measurements show that the anisotropy in the Knight shift reverses sign between 1.2 and 300 °K. Measurements of the amplitude of the de Haas-van Alphen signals were performed in the same field range and orientation as the oscillatory Knight-shift measurements. All of the measurements reported are accounted for by the detailed field and temperature dependence of the topology of the cadmium Fermi surface.