343 research outputs found
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Fabrication of Bone Substitute Material by Rapid Prototyping
Bone tissue engineering has gained much attention in recent years. A key requirement in this
field is the development of scaffold structures, on which cells adhere. This can be done by
fabricating scaffolds by direct procedures like 3D-printing or by indirect procedures like casting.
With the 3D-printing process different structures were build up by using hydroxyapatite powder
(HA) and a special binder material. Afterwards the printed 3D structures were sintered.
For the casting process molds have been made of different resins by stereolithography and other
processes using polymers and waxes. These structures were filled by a suspension of HA. By
heating the resulting polymer/ceramic composite to a specific temperature it is possible to
combust the polymer or wax. By further heating the remaining body, the HA is sintered.
Compared to the 3D printing a better resolution can be obtained here. But there are restrictions
regarding the ratio of polymer and the HA ceramic during the heating process which means a
limitation for the level of porosity.Mechanical Engineerin
Exact Classical and Quantum Dynamics in Background Electromagnetic Fields
Analytic results for (Q)ED processes in external fields are limited to a few
special cases, such as plane waves. However, the strong focussing of intense
laser fields implies a need to go beyond the plane wave model. By exploiting
Poincar\'e symmetry and superintegrability we show how to construct, and solve
without approximation, new models of laser-particle interactions. We illustrate
the method with a model of a radially polarised (TM) laser beam, for which we
exactly determine the classical orbits and quantum wave functions. Including in
this way the effects of transverse field structure should improve predictions
and analyses for experiments at intense laser facilities.Comment: 6 pages, 3 pdf figure
A novel approach to light-front perturbation theory
We suggest a possible algorithm to calculate one-loop n-point functions
within a variant of light-front perturbation theory. The key ingredients are
the covariant Passarino-Veltman scheme and a surprising integration formula
that localises Feynman integrals at vanishing longitudinal momentum. The
resulting expressions are generalisations of Weinberg's infinite-momentum
results and are manifestly Lorentz invariant. For n = 2 and 3 we explicitly
show how to relate those to light-front integrals with standard energy
denominators. All expressions are rendered finite by means of transverse
dimensional regularisation.Comment: 10 pages, 5 figure
Classical and quantum particle dynamics in univariate background fields
We investigate deviations from the plane wave model in the interaction of
charged particles with strong electromagnetic fields. A general result is that
integrability of the dynamics is lost when going from lightlike to timelike or
spacelike field dependence. For a special scenario in the classical regime we
show how the radiation spectrum in the spacelike (undulator) case becomes
well-approximated by the plane wave model in the high energy limit, despite the
two systems being Lorentz inequivalent. In the quantum problem, there is no
analogue of the WKB-exact Volkov solution. Nevertheless, WKB and uniform-WKB
approaches give good approximations in all cases considered. Other approaches
that reduce the underlying differential equations from second to first order
are found to miss the correct physics for situations corresponding to barrier
transmission and wide-angle scattering.Comment: 12 pages, 8 figure
Spontaneous symmetry breaking of (1+1)-dimensional theory in light-front field theory (III)
We investigate (1+1)-dimensional field theory in the symmetric and
broken phases using discrete light-front quantization. We calculate the
perturbative solution of the zero-mode constraint equation for both the
symmetric and broken phases and show that standard renormalization of the
theory yields finite results. We study the perturbative zero-mode contribution
to two diagrams and show that the light-front formulation gives the same result
as the equal-time formulation. In the broken phase of the theory, we obtain the
nonperturbative solutions of the constraint equation and confirm our previous
speculation that the critical coupling is logarithmically divergent. We discuss
the renormalization of this divergence but are not able to find a satisfactory
nonperturbative technique. Finally we investigate properties that are
insensitive to this divergence, calculate the critical exponent of the theory,
and find agreement with mean field theory as expected.Comment: 21 pages; OHSTPY-HEP-TH-94-014 and DOE/ER/01545-6
Success in offshoring of application development – does culture matter?
Recently, offshoring of information systems (IS) services to external vendors has seen considerable growth. Outsourcing to vendors in foreign countries brings about unique challenges which need to be understood and managed effectively. This paper explores cultural differences in IS offshoring arrangements involving German client organizations that outsource application development activities to Indian vendors. For this purpose, a research framework is developed based on both theoretical considerations and specific empirical observations from multiple case studies. The goal is to (1) explore the nature of cultural differences in offshoring arrangements in depth and to (2) analyze the relationship between those cultural differences and offshoring success. Based on the case findings, implications and practices for the management of offshore development projects are outlined
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