Combined analyser-based and propagation-based phase-contrast imaging of weak objects

A new theoretical method combining analyser-based and propagation-based hard X-ray phase-contrast imaging is investigated. Unlike the previous theoretical model of the combined imaging method constructed under the assumption of slow variation of the individual transfer functions (large Fresnel numbers), a new model proposed in this paper uses the assumption of a weak scatterer (analogous to the first Born approximation). Consequently, the results are not limited to the case of short propagation distances or low-resolution imaging. An explicit expression for the combined transfer function is derived and analytical and numerical examples solving related inverse imaging problems are presented.     

New types of short-range orientational ordering in ortho-para hydrogen monolayers

Orientational ordering in monolayers of solid hydrogen is discussed in view of recent experimental findings in NMR studies of (ortho)_c-(para)_1−c-hydrogen mixtures on boron nitride substrate reported for ortho-H₂ concentrations 0.35≤c≤0.92 and temperatures 0.14≤T≤1.80 K. Analysis of the temperature-concentration behavior for the observed NMR frequency splitting is given on the basis of a two-dimension (J=1)_c-(J=0)_1−c-rotor model for which the fundamental parameters, quadrupolar coupling constant Γ₀=0.50±0.03 K and the crystal-field amplitude V₀=0.70±0.10 K, are derived from experiment. The two distinct para-rotational (PR) short-range ordered structures driven by positive and negative crystalline fields are described in terms of the local alignment and orientation of the polar principal axis. It is shown that they are rather different from the ferromagnetic-type ordering suggested earlier by Harris and Berlinsky as a unique PR structure. The common quadrupolar glass and a new hindered rotor phases found below the 2D site percolation threshold c_p=0.72 are also discussed.     

Molecular structure calculations via path integral simulations: Estimating finite-discretization errors

Path integral simulations are now recognized as a useful tool to determine theoretically the structure of complex molecules at finite temperatures including quantum effects. In addition to statistical errors due to incomplete sampling, also systematic errors are inherent in this procedure because of the finite discretization of the path integral. Here, useful “back of the envelope” estimates to assess the systematic errors of bond-length distribution functions are introduced. These analytical estimates are tested for two small molecules, HD⁺ and H₃ ⁺, where quasi-exact benchmark data are available. The accuracy of the formulae is shown to be sufficient in order to allow for a reliable assessment of the quality of the discretization in a given simulation. The estimates will also be applicable in condensed phase path integral simulations, and the basic idea can be generalized to other observables than those presented. Received 13 September 1999 and Received in final form 18 November 1999     

Investigation of structural phase transition in polycrystalline SrTiO3 thin films by Raman spectroscopy

Polycrystalline SrTiO₃ thin films were prepared by pulsed laser deposition technique. The phonon properties and structural phase transition were studied by Raman spectroscopy. The first-order Raman scattering, which is forbidden in SrTiO₃ single crystal, has been observed in the films, due to the structural distortion caused by strain effect and oxygen vacancies. The Fano-type line shape of TO₂ phonon reveals the existence of polar microregions in the STO thin films. The evolution of TO₂ and TO₃ phonons with temperature shows the occurrence of a structural phase transition at 120 K related to the formation of polar macroregions in the films.     

Formation of amorphous films by solid-state reaction in an immiscible Y-W system

Received: 26 November 1996/Accepted: 7 February 1997     

The Hume-Rothery rules and phase stabilities in noble metal alloys

Results from martensitic transformations are used to evaluate stabilities of the equilibrium phases in the Hume-Rothery electron compounds based on Cu, Ag and Au, and to give arguments why the electron concentration plays such an important role in the selection of the crystal structures. It is shown that the vibrational entropy difference observed for the martensitic transformation from ordered bcc to the close packed martensite and its e/a dependence can also account for the entropy difference ΔS ^α/β between the equilibrium α and β at high temperatures, and can be made largely responsible for the composition dependence of the (α+β) two phase field. The enthalpy of mixing can be decomposed into a small term which depends on the average periodic lattice, which is different in α and β but which is nearly the same in all alloys studied, and a contribution which is due to the difference in the properties of the atoms and which can be expressed by pair interchange energies. This contribution depends strongly on the specific alloy system, but is independent of structure, which is compatible with a pair interchange energy depending only on pair distance but not on structure, as suggested by simple pseudopotential theory. The same pair interchange energies account also for long range order and the critical ordering temperature. The evaluation for several alloy systems shows a surprisingly good agreement within this picture, and permits to understand better why the electron concentration plays such an important role also for other structures, although the energy contribution of the conduction electrons is only a small part of the total enthalpy of formation of any of the equilibrium structures.     

Kinetics of CoSi2 from evaporated silicon

2 MeV⁴He⁺ backscattering spectrometry and CuK x-ray diffraction were used to study CoSi₂ formed by annealing at temperatures between 405° and 500 °C from CoSi with evaporated Si films. A laterally uniform layer of CoSi₂ forms, in contrast to the laterally nonuniform CoSi₂ layer that is obtained on single crystal Si substrates. The thickness of the CoSi₂ film formed is proportional to the square root of time at a fixed temperature. The activation energy of this reaction is about 2.3 eV.     

Experimental observation of focal shifts in focused partially coherent beams

We report the experimental observation of focal shifts in partially coherent beams of Gaussian Schell-model (GSM) type focused by a circularly apertured thin lens. The relative focal shift increases as the aperture radius or the state of coherence of the GSM source decrease. The experimental results agree well with theoretical predictions.     

Disturbance growth in two-fluid channel flow: The role of capillarity

This work examines the role of capillarity in the non-modal linear stability properties of three-dimensional disturbances in sheared two-layer flow of immiscible fluids of similar density. Capillarity reduces the transient growth of energy that occurs due to the non-normality of the linear stability problem according to a scaling of peak energy with We^1/2 over a wide range of Weber number, viscosity ratio and wavenumber. More importantly, the participation of capillary modes in non-modal growth leads to oscillatory energy growth and to larger disturbance growth rates, features that are confirmed by computing the numerical range and numerical abscissa of the non-normal disturbance evolution operator. Examination of energy components and disturbance structure reveals that early rapid growth and subsequent oscillations are due to the coupling of streamwise vortices – the two-fluid analog of lift-up – to the displaced interface.     

Structural evolution and metastable phase formation in Ni-W multilayers upon ion irradiation

A crystal-to-amorphous structural transition was induced in the Ni₂₅W₇₅ and Ni₃₅W₆₅ multilayers by ion irradiation at room temperature. More interestingly, prior to complete amorphization, a sequential disordering of first Ni and then W crystalline lattices was observed in the Ni₂₅W₇₅ sample with increasing ion dose. Such sequence in disordering is attributed to the difference in melting points between the two constituent metals. In another two multilayered samples with overall compositions of Ni₆₀W₄₀ and Ni₇₈Nb₂₂, ion irradiation under similar conditions resulted in the formation of two Ni-based fcc solid solutions, respectively. In comparison, the same Ni-based fcc solid solution was formed in the Ni₃₅W₆₅ multilayered sample upon solid-state reaction at 500 °C. Solid-state reaction at 550 °C resulted in the formation of a new W-rich metastable hcp phase in the Ni₂₅W₇₅ multilayered sample and the bcc–hcp transition was thought to be realized through a shearing mechanism. A Gibbs free-energy diagram, including the free-energy curves of the newly formed metastable crystalline phases, of the Ni-W system was calculated based on Miedema’s model and it can give a reasonable explanation of the observed sequential disordering. The calculated results also showed that the free-energy difference between the amorphous and metastable crystalline phases was quite small, leading to a situation that the phase selection, namely which phase was more favored to be formed eventually than its competitors, was influenced or even determined by the kinetics involved in the respective processes. Besides, the growth kinetics of the MX phases was also discussed. Received: 26 January 1999 / Accepted: 8 March 1999 / Published online: 14 June 1999