8,592 research outputs found
Critical collapse of rotating radiation fluids
We present results from the first fully relativistic simulations of the
critical collapse of rotating radiation fluids. We observe critical scaling
both in subcritical evolutions, in which case the fluid disperses to infinity
and leaves behind flat space, and in supercritical evolutions that lead to the
formation of black holes. We measure the mass and angular momentum of these
black holes, and find that both show critical scaling with critical exponents
that are consistent with perturbative results. The critical exponents are
universal; they are not affected by angular momentum, and are independent of
the direction in which the critical curve, which separates subcritical from
supercritical evolutions in our two-dimensional parameter space, is crossed. In
particular, these findings suggest that the angular momentum decreases more
rapidly than the square of the mass, so that, as criticality is approached, the
collapse leads to the formation of a non-spinning black hole. We also
demonstrate excellent agreement of our numerical data with new closed-form
extensions of power-law scalings that describe the mass and angular momentum of
rotating black holes formed close to criticality.Comment: 5 pages, 4 figures; version accepted for publication in PR
Stability Boundary and Design Criteria for Haptic Rendering of Virtual Walls
This paper is about haptic simulations of virtual walls, which are represented by a discrete PD-control. A normalized discrete-time transfer function is used to derive the fundamental stability boundaries for this problem. Hereby, the case of direct action and the more often case of an one sampling step delayed action are addressed. Inside the stable region the set of all parameters was determined that result in real system poles. Furthermore, three dierent design criteria are compared to nd optimum control parameters for the virtual wall. Finally, important conclusions for haptic simulations are derived
Hexagonal High-Entropy Alloys
We report on the discovery of a high-entropy alloy with a hexagonal crystal
structure. Equiatomic samples in the alloy system Ho-Dy-Y-Gd-Tb were found to
solidify as homogeneous single-phase high-entropy alloys. The results of our
electron diffraction investigations and high-resolution scanning transmission
electron microscopy are consistent with a Mg-type hexagonal structure. The
possibility of hexagonal high-entropy alloys in other alloy systems is
discussed.Comment: Changes upon replacement: inserted submission date of manuscript to
journal. No other changes were mad
Impedance generalization for plasmonic waveguides beyond the lumped circuit model
We analytically derive a rigorous expression for the relative impedance ratio
between two photonic structures based on their electromagnetic interaction. Our
approach generalizes the physical meaning of the impedance to a measure for the
reciprocity-based overlap of eigenmodes. The consistence with known cases in
the radiofrequency and optical domain is shown. The analysis reveals where the
applicability of simple circuit parameters ends and how the impedance can be
interpreted beyond this point. We illustrate our approach by successfully
describing a Bragg reflector that terminates an insulator-metal-insulator
plasmonic waveguide in the near-infrared by our mpedance concept
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