1,384 research outputs found
Metal surface temperature induced by moving laser beams \ud
Whenever a metal is irradiated with a laser beam, electromagnetic energy is transformed into heat in a thin surface layer. The maximum surface temperature is the most important quantity which determines the processing result. Expressions for this maximum temperature are provided by the literature for stationary cases. In practice, however, moving beams are of more importance.\ud
Based on a fast numerical algorithm which allows calculation of the induced temperature profile, the maximum surface temperature for stationary and moving laser beams is calculated. Next, two types of approximating functions are presented relating the scanning speed to the maximum surface temperature. Using dimensionless numbers, the results can be applied to different material
Collision detection for rigid superellipsoids using the normal parameterization
The normal parameterization as an approach to describe geometries is introduced. The advantages of this description – as compared to other parameterizations or implicit functions – in the context of collision detection are: the possibility to explicitly calculate axis aligned bounding boxes for any convex geometry, an efficient iterative algorithm for collision detection between objects with arbitrary geometry that does not require any (analytical) derivatives. A system of several rigid superellipsoids is used to demonstrate the application and performance of the proposed approach in a multibody simulation
High-pressure phases and transitions of the layered alkaline earth nitridosilicates SrSiN2 and BaSiN2
We investigate the high-pressure phase diagram of SrSiN2 and BaSiN2 with density-functional calculation. Searching a manifold of possible candidate structures, we propose new structural modifications of SrSiN2 and BaSiN2 attainable in high-pressure experiments. The monoclinic ground state of SrSiN2 transforms at 3 GPa into an orthorhombic BaSiN2 type. At 14 GPa a CaSiN2-type structure becomes the most stable configuration of SrSiN2. A hitherto unknown Pbcm modification is adopted at 85 GPa and, finally, at 131 GPa a LiFeO2-type structure. The higher homologue BaSiN2 transforms to a CaSiN2 type at 41 GPa and further to a Pbcm modification at 105 GPa. Both systems follow the pressure-coordination rule: the coordination environment of Si increases from tetrahedral through trigonal bipyramidal to octahedral. Some high-pressure phases are related in structure through simple group–subgroup mechanisms, indicating displacive phase transformations with low activation barriers
Generalized Quantum Theory: Overview and Latest Developments
The main formal structures of Generalized Quantum Theory are summarized.
Recent progress has sharpened some of the concepts, in particular the notion of
an observable, the action of an observable on states (putting more emphasis on
the role of proposition observables), and the concept of generalized
entanglement. Furthermore, the active role of the observer in the structure of
observables and the partitioning of systems is emphasized.Comment: 14 pages, update in reference
Hamiltonian Multivector Fields and Poisson Forms in Multisymplectic Field Theory
We present a general classification of Hamiltonian multivector fields and of
Poisson forms on the extended multiphase space appearing in the geometric
formulation of first order classical field theories. This is a prerequisite for
computing explicit expressions for the Poisson bracket between two Poisson
forms.Comment: 50 page
A Brief Survey on Non-standard Constraints: Simulation and Optimal Control
In terms of simulation and control holonomic constraints are well documented and thus termed standard. As non-standard constraints, we understand non-holonomic and unilateral constraints. We limit this survey to mechanical systems with a finite number of degrees of freedom. The long-term behavior of non-
holonomic integrators as compared to structure-preserving integrators for holonomically constrained systems is briefly discussed. Some recent research regarding the treatment of unilaterally constrained systems by event-driven or time-stepping schemes for time integration and in the context of optimal control problems is outlined
On the concept of pressure in quantum mechanics
Heat and work are fundamental concepts for thermodynamical systems. When
these are scaled down to the quantum level they require appropriate embeddings.
Here we show that the dependence of the particle spectrum on system size giving
rise to a formal definition of pressure can, indeed, be correlated with an
external mechanical degree of freedom, modelled as a spatial coordinate of a
quantum oscillator. Under specific conditions this correlation is reminiscent
of that occurring in the classical manometer.Comment: 7 pages, 3 figure
Electron spin relaxation in bulk GaAs for doping densities close to the metal-to-insulator transition
We have measured the electron spin relaxation rate and the integrated spin
noise power in n-doped GaAs for temperatures between 4 K and 80 K and for
doping concentrations ranging from 2.7 x 10^{-15} cm^{-3} to 8.8 x 10^{-16}
cm^{-3} using spin noise spectroscopy. The temperature dependent measurements
show a clear transition from localized to free electrons for the lower doped
samples and confirm mainly free electrons at all temperatures for the highest
doped sample. While the sample at the metal-insulator-transition shows the
longest spin relaxation time at low temperatures, a clear crossing of the spin
relaxation rates is observed at 70 K and the highest doped sample reveals the
longest spin relaxation time above 70 K.Comment: 6 pages, 4 figure
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