152 research outputs found
Stability in the instantaneous Bethe-Salpeter formalism: harmonic-oscillator reduced Salpeter equation
A popular three-dimensional reduction of the Bethe-Salpeter formalism for the
description of bound states in quantum field theory is the Salpeter equation,
derived by assuming both instantaneous interactions and free propagation of all
bound-state constituents. Numerical (variational) studies of the Salpeter
equation with confining interaction, however, observed specific instabilities
of the solutions, likely related to the Klein paradox and rendering (part of
the) bound states unstable. An analytic investigation of this problem by a
comprehensive spectral analysis is feasible for the reduced Salpeter equation
with only harmonic-oscillator confining interactions. There we are able to
prove rigorously that the bound-state solutions correspond to real discrete
energy spectra bounded from below and are thus free of any instabilities.Comment: 23 pages, 3 figures, extended conclusions, version to appear in Phys.
Rev.
High-resolution transmission electron microscopy investigation of diffusion in metallic glass multilayer films
Lack of plasticity is one of the main disadvantages of metallic glasses. One of the solutions to this problem can be composite materials. Diffusion bonding is promising for composite fabrication. In the present work the diffusion process in glassy multilayer films was investigated. A combination of advanced transmission electron microscopy (TEM)methods and precision sputtering techniques allows visualization and study of diffusion in amorphous metallic layers with high resolution. Multilayered films were obtained by radio frequency sputter deposition of Zr-Cu and Zr-Pd. The multilayers were annealed under a high vacuum (10 −5 Pa)for 1 and 5 h at 400 °C, that is, well below the crystallization temperatures but very close to the glass-transition temperatures of both types of the glassy layer. The structural evolution in the deposited films was investigated by high-resolution transmission electron microscopy. It was observed that, despite the big differences in the atomic mass and size, Pd and Cu have similar diffusion coefficients. Surprisingly, 1 h of annealing results in formation of metastable copper nanocrystals in the Zr-Cu layers which, however, disappear after 5 h of annealing. This effect may be connected with nanovoid formation under a complex stress state evolving upon annealing, and is related to the exceptionally slow relaxation of the glassy layers sealed with a Ta overlayer.The authors acknowledge the financial support through the European Research Council under the ERC Advanced Grants INTELHYB (grant ERC-2013-ADG-340025) and ExtendGlass (grant ERC-2015-AdG-695487), the German Science Foundation (DFG) under the grant SO 1518/1-1, and the Ministry of Education and Science of the Russian Federation in the framework of the ‘Increase Competitiveness’ program of NUST ‘MISiS’ (№ К2-2014-013 and К2-2017-089)
Pion Generalized Dipole Polarizabilities by Virtual Compton Scattering
We present a calculation of the cross section and the event generator of the
reaction . This reaction is sensitive to the pion
generalized dipole polarizabilities, namely, the longitudinal electric
, the transverse electric , and the magnetic
which, in the real-photon limit, reduce to the ordinary electric
and magnetic polarizabilities and , respectively.
The calculation of the cross section is done in the framework of chiral
perturbation theory at . A pion VCS event generator has been
written which is ready for implementation in GEANT simulation codes or for
independent use.Comment: 33 pages, Revtex, 15 figure
Meson exchange and nucleon polarizabilities in the quark model
Modifications to the nucleon electric polarizability induced by pion and
sigma exchange in the q-q potentials are studied by means of sum rule
techniques within a non-relativistic quark model. Contributions from meson
exchange interactions are found to be small and in general reduce the quark
core polarizability for a number of hybrid and one-boson-exchange q-q models.
These results can be explained by the constraints that the baryonic spectrum
impose on the short range behavior of the mesonic interactions.Comment: 11 pages, 1 figure added, expanded discussio
Virtual reality surgery simulation: A survey on patient specific solution
For surgeons, the precise anatomy structure and its dynamics are important in the surgery interaction, which is critical for generating the immersive experience in VR based surgical training applications. Presently, a normal therapeutic scheme might not be able to be straightforwardly applied to a specific patient, because the diagnostic results are based on averages, which result in a rough solution. Patient Specific Modeling (PSM), using patient-specific medical image data (e.g. CT, MRI, or Ultrasound), could deliver a computational anatomical model. It provides the potential for surgeons to practice the operation procedures for a particular patient, which will improve the accuracy of diagnosis and treatment, thus enhance the prophetic ability of VR simulation framework and raise the patient care. This paper presents a general review based on existing literature of patient specific surgical simulation on data acquisition, medical image segmentation, computational mesh generation, and soft tissue real time simulation
Multigrid algorithms for hp-version interior penalty discontinuous Galerkin methods on polygonal and polyhedral meshes
In this paper we analyze the convergence properties of two-level and W-cycle multigrid solvers for the numerical solution of the linear system of equations arising from hp-version symmetric interior penalty discontinuous Galerkin discretizations of second-order elliptic partial differential equations on polygonal/polyhedral meshes. We prove that the two-level method converges uniformly with respect to the granularity of the grid and the polynomial approximation degree p, provided that the number of smoothing steps, which depends on p, is chosen sufficiently large. An analogous result is obtained for the W-cycle multigrid algorithm, which is proved to be uniformly convergent with respect to the mesh size, the polynomial approximation degree, and the number of levels, provided the latter remains bounded and the number of smoothing steps is chosen sufficiently large. Numerical experiments are presented which underpin the theoretical predictions; moreover, the proposed multilevel solvers are shown to be convergent in practice, even when some of the theoretical assumptions are not fully satisfied
The Role of Spatially Controlled Cell Proliferation in Limb Bud Morphogenesis
Oriented cell behaviors likely have a more important role in limb bud elongation during development than previously suggested by the “growth-based morphogenesis” hypothesis
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