Ital Al Research Articles

Interface dynamics and banding in rapid solidification.

Rapid-solidification experiments on metallic alloys in the last decade have provided widespread observations of a novel banded structure.'' We report the results of numerical and analytical studies of the interface dynamics underlying the formation of this structure in a model of directional solidification which includes both solute and heat diffusion and nonequilibrium effects. The thrust of these studies is on the unsteady dynamics of the planar interface and thermal effects. The main conclusion is that the origin of banding can be related to relaxation oscillations of the solidification front, characterized by large variations of the interface velocity, which are dramatically affected by latent-heat diffusion. Without the latter, the oscillations are found to be reasonably well approximated by the phenomenological model of Carrard [ital et] [ital al]. [Acta Metall. 40, 983 (1992)], and the band spacing is inversely proportional to the temperature gradient. In contrast, with latent-heat diffusion the band spacing is insensitive to the temperature gradient, but is controlled instead by the interplay of solute and heat diffusion. The smallness of the solutal diffusivity to thermal diffusivity ratio is exploited to explain analytically this effect and to derive considerably simpler equations of interface motion that provide an efficient numerical meansmore » to study the nonplanar interface dynamics expected to cause dark bands. A reasonable agreement with experiment is found for the spacing of banded structures dominated by light-band microsegregation-free regions in Al-Fe alloys.« less

Analysis of the compressibility of a finite symmetrical nucleus in terms of a simple, analytically tractable model.

The liquid-drop model type expansion of the finite nuclear compressibility coefficients {ital K}{sub {ital A}} is studied in an energy density formalism, using a leptodermous expansion of the energies. It is found that the effective curvature compressibility coefficient {ital K}{sub {ital c}} is always negative for Skyrme type forces. It is also shown that the unexpectedly large value of about {minus}800 MeV of the surface compressibility coefficient {ital K}{sub {ital s}} found by Sharma {ital et} {ital al}. is an artifact of their analysis procedure.

Dielectrons in pi -p collisions via the two-photon mechanism.

Dielectron production in 16 GeV {pi}{sup {minus}}{ital p} collisions is studied using the double equivalent photon method. Equivalent photon spectra of hadrons are calculated analytically in a fully quantal and semiclassical approach. The limitation of the latter approach and the influence of the proton spin-flip terms are studied. Conclusions at variance with those of a recent letter by Bottcher {ital et} {ital al}. are obtained.

Source-wave angular-momentum effects on electron-diffraction patterns

We use a simple model to predict when electron-emission diffraction patterns from surfaces will have peaks or dips along internuclear axes. A high angular-momentum electron wave emitted from an atom acts like an {ital s} wave ({ital l}=0) in an extra centrifugal potential. This extra potential changes the electron's effective wave number and phase shift, altering the conditions for constructive interference when this wave scatters from nearby atoms. We demonstrate that the difference in source-wave angular momentum between Cu {ital M}{sub 2,3}{ital M4,5}{ital M4,5} Auger and Cu 3{ital p} photoelectrons explains the difference between their emission angular distributions from surfaces: the Auger-electron emission has a predominantly {ital f}-like source wave ({ital l}=3) and destructive interference in the forward electron-scattering direction ( silhouette'') while the photoelectron has lower angular momentum and constructive interference ( peak''). As long as this effect is considered, Auger-electron emission patterns can be used to determine surface structures. The unusual Auger-electron emission patterns observed by Frank {ital et} {ital al}. (Science 247, 182 (1990)) can be explained as due to high source-wave angular momentum and low electron energy.

Multiple electron transfer in slow Ne9+-Ne collisions.

Multielectron transfer probabilities in 90-keV Ne{sup 9+}-Ne collisions have been measured with respect to the projectile scattering angle {theta} in a range between {theta}=12 and 75 mrad. The projectile final charge state was determined in coincidence with the target-ion final charge state using a time-of-flight technique. For {theta}{gt}45 mrad, projectile and target undergo a complete equilibration of their atomic shells (including the {ital K} shell) leading to the excitation of both collision partners and the autoionization of three electrons on average. A comparison of the measured final charge-state distributions of projectile and target with those obtained in a former experiment with the system 90-keV Ne{sup 7+}-Ne (H. Schmidt-Boecking {ital et} {ital al}., Phys. Rev. A 37, 4640 (1989)) leads to the conclusion that {ital L}- and {ital K}-shell charge transfer can be treated as independent processes. Interference structure in the {ital K}-{ital K} vacancy-transfer probabilities has been observed in the final charge-state probabilities of the heavy-ion''--atom system.

Magnetic-flux patterns on the surface of a type-II superconductor.

Various regimes exist where the pattern of magnetic flux through the surface of a type-II superconductor is predominantly determined by surface effects and not by the pattern of flux in the bulk. In particular, for zero pinning and small enough flux density, the flux lattice at a flat surface is isomorphic to the classical two-dimensional electron lattice and crystallizes at a much higher temperature than does the bulk flux lattice. Within uniaxially anisotropic London theory, the flux-line energy recently calculated by Sudbo and Brandt shows that for sufficiently strong anisotropy of the sign appropriate for layered superconductors an isolated straight flux line in the bulk is only stable when it is either nearly parallel or nearly perpendicular to the layers. The flux chains'' observed on BSCCO by Bolle {ital et} {ital al}. occur in a regime where both orientations of stable flux lines should be present and thus reflect the interactions between parallel and nearly perpendicular flux lines.

Comment on "Scalar Aharonov-Bohm experiment with neutrons"

A Comment on the Letter by B. E. Allman [ital et] [ital al]. Phys. Rev. Lett. 68, 2409 (1992).

Flux-flow resistivity in model high-temperature superconductors.

We calculate the resistivity of a model high-temperature superconductor'' consisting of a simple-cubic arrangement of superconducting grains'' coupled together by resistively shunted Josephson junctions. The effects of temperature are simulated by Langevin noise in each junction. We find a strong magnetoresistance for magnetic fields both parallel and perpendicular to the applied current, in agreement with the results of Kwok {ital et} {ital al}. (Phys. Rev. Lett. 64, 966 (1990)). When the magnetic field {bold B} and the current density {bold J} make an angle {phi}, the resistivity at sufficiently high temperatures roughly obeys the law {rho}({ital B},{ital T},{phi})={rho}{sub 0}({ital B},{ital T})+{Delta}{rho} sin{sup 2}({phi}) in agreement with experiment. The resistivity is strongly dependent on current density. At zero magnetic field it is found to satisfy the scaling relation {ital E}={xi}{sup {minus}1{minus}{ital z}F}{sub {plus minus}}({ital J}{xi}{sup {ital d}{minus}1}{Phi}{sub 0}/{ital ck}{sub {ital B}T}), where {ital E} is the electric field, {ital c} is the speed of light, {ital J} is the current density, {ital d} is the dimensionality, and {ital F}{sub {plus minus}} are scaling functions which apply above and below {ital T}{sub {ital c}}. The dynamical critical exponent is estimated for this model as 1.5{plus minus}0.5.

Search for the 2S-2P energy difference in muonic 4He ions.

We searched for laser-induced 2{ital S}{sub 1/2}-2{ital P}{sub 3/2} transitions in ({mu}{sup {minus}} {sup 4}He){sup +} ions at low pressure (4.1 kPa), where we determined a 2{ital S} lifetime of 1.014{plus minus}0.121 {mu}s from the simultaneously measured 2{ital S}{r arrow}1{ital S} two-photon transitions. In contrast to a similar experiment in He gas of 40 bar (4{times}10{sup 6} Pa) reported by Carboni {ital et} {ital al}. (Nucl. Phys. A278, 381 (1977)), which was based on the questionable metastability of the 2{ital S} state at that pressure, we found no resonance effect at the wavelength {lambda}=811.7 nm. We can reject this wavelength with a confidence level of less than 2{times}10{sup {minus}3}, and the interval 811.4{le}{lambda}{le}812.0 nm with a greater than 95% probability.

Differential cross sections for state-selective electron capture in 25-100-keV proton-helium collisions.

Cross sections differential in the scattering angle of the projectile are presented for electron capture summed over all states and to the 2{ital s}, 2{ital p}, 3{ital s}, 3{ital p}, 4{ital s}, and 4{ital p} states of hydrogen in 25-, 50-, and 100-keV proton-helium collisions. The classical-trajectory Monte Carlo (CTMC) technique was employed for these calculations as well as to compute total cross sections as a function of impact energy. The latter are compared with experiment to display the behavior of the integral state-selective cross sections in this energy regime. Detailed comparison is also made between the calculated angular differential cross sections and the experimental measurements of Martin {ital et} {ital al}. (Phys. Rev. A 23, 285 (1981)) for capture summed over all states and of Seely {ital et} {ital al}. (Phys. Rev. A 45, R1287 (1992)) for capture to the 2{ital p} state. Very good overall agreement is found. Regarding the cross section for capture summed over all states, an improved agreement is demonstrated by using an alternate representation of the initial state in the CTMC method, which improves the electronic radial distribution but which cannot presently be applied to state-selective determinations.

Model-independent connections between pion photoproduction and Compton scattering in the Delta (1232) region.

We exploit Grushin {ital et} {ital al}.'s {ital independent} {ital determinations} of the real and the imaginary parts of the pion photoproduction multipoles in the {Delta}(1232) region, without the use of an approximate form of Watson's theorem, and obtain model-independent estimates for the Compton amplitudes for the resonance excitation off nucleons. We compare these with the results obtained from other analyses of photoproduction multipoles. We find the lower bound for the differential cross section using unitarity, and compare our results with available experiments, both the older-generation experiments, primarily at Bonn, and the preliminary results from the recent studies at the Saskatchewan Accelerator Lab. We explore consistency of photoproduction multipoles with the forward Compton scattering amplitude extracted from the measured total hadronic cross sections of photons in hydrogen, using the optical theorem. Finally, we discuss implications for future precision Compton studies of the {Delta}(1232) excitation, in particular, attempts to measure the {ital E}2 to {ital M}1 amplitude ratio, in the nucleon-to-{Delta} electromagnetic transition, which will be feasible at new photon facilities such as the Brookhaven Laser Electron-Gamma Source, exploring the photon polarization observables. These amplitudes contain valuable information on the structure of nucleon and {Delta} baryons, of great topical interest. Wemore » vividly demonstrate here the inadequacy of the older generation of Compton scattering experiments, as their poor photon energy resolution and counting statistics limit the quality of physics extractable from the data. This urgently calls for newer-generation {ital high}{minus}{ital statistics} experiments with {ital high} photon energy resolution, using the photon polarization as a powerful tool.« less

Modified Steiner functional string action.

It has recently been suggested by Ambartzumian {ital et} {ital al}. that the modified Steiner functional has desirable properties as an action for random surfaces and hence string world sheets. We perform a simulation of this action on a dynamically triangulated random surface to investigate this claim and find that the surfaces are in a flat phase.

Four-band model for oxygen holes in copper oxide superconductors. II. Phase diagram.

Transport in the doped lamellar copper oxides is carried by quasiparticles, composed of holes which move on the oxygen ions on the CuO{sub 2} planes, with strong magnetic coupling to the neighboring Cu spins. In the preceding paper, we calculated the bands of these quasiparticles. In the present paper, we use the lowest of these bands to obtain the critical temperature {ital T}{sub {ital c}} as a function of the concentration of holes in these planes, using simple BCS equations. We find that the pairing interaction induced by the magnetic frustration model of Aharony {ital et} {ital al}. yields a phase diagram which is quite similar to the one observed experimentally, provided that one allows hopping between the planes, of order 0.03 eV. We discuss the variation of the phase diagram with the different hopping and interaction parameters. Some features should apply beyond the scope of our specific model.

Density effect in Cu K-shell ionization by 5.1-GeV electrons.

We have made an absolute measurement of the Cu {ital K}-shell impact ionization cross section by 5.1-GeV electrons, which demonstrates directly a density effect predicted by Fermi in 1940. By determining the ratio of the {ital K} x-ray yield from a thin front and back layer of the target by a novel grazing emission method, we have verified the effect of transition radiation on the x-ray production, suggested by Sorensen and reported by Bak {ital et} {ital al}.

Heavy flavor production at large transverse momentum through the boson-gluon fusion mechanism.

We present alternative calculations of the matrix elements for the process of heavy flavor production through the {gamma}/{ital W}/{ital Z}-gluon fusion mechanisms in hard hadronic interactions. The expressions for the matrix elements squared have been implemented in the EUROGET Monte Carlo event generator providing a flexible tool to study event distributions for both partonic and hadronic final states. In case of a charged mediator, we find complete agreement between our {ital O}({alpha}{sup 2}{alpha}{sub {ital s}}) calculations and recent calculations by Zerwas {ital et} {ital al}. Results on the factorization of the {ital O}({alpha}{sup 2}{alpha}{sub {ital s}}) matrix elements, which allows an independent treatment of {ital O}({alpha}{sup 2}) and {ital O}({alpha}{sup 2}{alpha}{sub {ital s}}) contributions to cross sections, are presented as well. We have performed an analysis of the main heavy flavor sources in hadronic interactions and have studied their relative importance as a function of accelerator energies. Heavy flavor masses have been varied over a wide range of possible values. The pure QCD contributions dominate for only part of the parameter combinations, while the distinct event topology arising in boson-gluon fusion processes enhances the observability of heavy flavors. We briefly discuss implications of the fusion mechanisms for production of heavymore » E{sub 6} fermions.« less

Asymmetry parameter for double photoionization

For the first time we calculate {ital ab} {ital initio} values of the asymmetry parameter {beta} for double photoionization and obtain agreement with experiment. The suggestion that the measurements do not agree with the Wannier predictions of Greene and Huetz {ital et} {ital al}. is shown not to be substantiated by our calculations.

K-shell ionization cross sections of Al, Si, S, Ca, and Zn for oxygen ions in the energy range 1.1-8 MeV.

{ital K}-shell ionization cross sections induced by 1.1--8-MeV oxygen ions in Al, Si, S, Ca, and Zn were measured using different target thicknesses. The cross sections for vanishingly thin and for charge-equilibrium targets were obtained by extrapolation. The experimental results are compared to the perturbed stationary-state approximation with energy-loss, Coulomb, and relativistic corrections (ECPSSR) cross sections (Brandt and Lapicki, Phys. Rev. A 23, 1717 (1981)), to the modification of the ECPSSR theory (MECPSSR) (Benka, Geretschlaeger, and Paul, J. Phys. (Paris) Colloq. Suppl. 12, C9-251 (1987)), to the theory for direct Coulomb ionization of the 1{ital s}{sigma} molecular orbital (Montenegro and Sigaud, J. Phys. B 18, 299 (1985)), and to several semiclassical approximation codes using either the united atom binding procedure or the variational approach of Andersen {ital et} {ital al}. (Nucl. Instrum. Methods 192, 79 (1982)). The cross sections were also compared to the statistical molecular-orbital theory of inner-shell ionization for (nearly) symmetric atomic collisions (Mittelman and Wilets, Phys. Rev. 154, 12 (1967)). For fast collisions ({xi}{similar to}1), the ionization cross sections are well reproduced by theories for direct Coulomb ionization. For slower collisions ({xi}{lt}1), the experimental cross sections are systematically higher than the direct-ionization values, but they agree satisfactorilymore » with the summed cross sections for direct Coulomb ionization and for molecular-orbital ionization. Best agreement (within a factor of 2) was found for the sums of MECPSSR and statistical cross sections.« less

Search for cluster radioactivity of 237Np.

The few odd-{ital A} nuclides known to exhibit cluster radioactivity decay at rates hindered with respect to their even-even neighbors. For {sup 237}Np, the most likely clusters are predicted to be {sup 30}Mg and {sup 32}Si. We have completed a rigorous search for cluster emission from {sup 237}Np, with a null result. Our 90% confidence limit on the branching ratio with respect to alpha emission, {ital B} (cluster/{alpha}) {lt}1.8{times}10{sup {minus}14}, is a factor 4.3 lower than the limit obtained by Tretyakova {ital et} {ital al}. The two experiments together place an upper limit of 1.5{times}10{sup {minus}14} on the branching ratio for emission of clusters with 10{le}{ital Z}{le}14 from {sup 237}Np.

Ductile-brittle transition in random porous Au.

We report experimental results describing the mechanical breakdown of random porous Au which is a new material specifically developed for this study. Digital image analysis was used to characterize the microstructures of the samples which varied by more than 2 orders of magnitude in length scale. The porous Au undergoes a microstructurally controlled ductile-brittle transition. Similar transitions have been found in numerical simulations by Kahng {ital et} {ital al}. (Phys. Rev. B 37, 7625 (1988)).

Comment on "Confinement degradation and enhanced microturbulence as long-time precursors to high-density-limit tokamak disruptions"

A Comment on the Letter by D. L. Brower {ital et} {ital al}., Phys. Rev. Lett. 67, 200 (1991).