7,806 research outputs found
Protoplanetary Disk Turbulence Driven by the Streaming Instability: Non-Linear Saturation and Particle Concentration
We present simulations of the non-linear evolution of streaming instabilities
in protoplanetary disks. The two components of the disk, gas treated with grid
hydrodynamics and solids treated as superparticles, are mutually coupled by
drag forces. We find that the initially laminar equilibrium flow spontaneously
develops into turbulence in our unstratified local model. Marginally coupled
solids (that couple to the gas on a Keplerian time-scale) trigger an upward
cascade to large particle clumps with peak overdensities above 100. The clumps
evolve dynamically by losing material downstream to the radial drift flow while
receiving recycled material from upstream. Smaller, more tightly coupled solids
produce weaker turbulence with more transient overdensities on smaller length
scales. The net inward radial drift is decreased for marginally coupled
particles, whereas the tightly coupled particles migrate faster in the
saturated turbulent state. The turbulent diffusion of solid particles, measured
by their random walk, depends strongly on their stopping time and on the
solids-to-gas ratio of the background state, but diffusion is generally modest,
particularly for tightly coupled solids. Angular momentum transport is too weak
and of the wrong sign to influence stellar accretion. Self-gravity and
collisions will be needed to determine the relevance of particle overdensities
for planetesimal formation.Comment: Accepted for publication in ApJ (17 pages). Movies of the simulations
can be downloaded at http://www.mpia.de/~johansen/research_en.ph
Dendritic flux patterns in MgB2 films
Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical
current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at
temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and
gradual flux penetration in an applied magnetic field, we observe an abrupt
invasion of complex dendritic structures. When the applied field subsequently
decreases, similar dendritic structures of the return flux penetrate the film.
The static and dynamic properties of the dendrites are discussed.Comment: Accepted to Supercond. Sci. Techno
Ray optics in flux avalanche propagation in superconducting films
Experimental evidence of wave properties of dendritic flux avalanches in
superconducting films is reported. Using magneto-optical imaging the
propagation of dendrites across boundaries between a bare NbN film and areas
coated by a Cu-layer was visualized, and it was found that the propagation is
refracted in full quantitative agreement with Snell's law. For the studied film
of 170 nm thickness and a 0.9 mkm thick metal layer, the refractive index was
close to n=1.4. The origin of the refraction is believed to be caused by the
dendrites propagating as an electromagnetic shock wave, similar to damped modes
considered previously for normal metals. The analogy is justified by the large
dissipation during the avalanches raising the local temperature significantly.
Additional time-resolved measurements of voltage pulses generated by segments
of the dendrites traversing an electrode confirm the consistency of the adapted
physical picture.Comment: 4 pages, 4 figure
Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes
We have performed time-resolved spectroscopy on InAs quantum dot ensembles in
photonic crystal membranes. The influence of the photonic crystal is
investigated by varying the lattice constant systematically. We observe a
strong slow down of the quantum dots' spontaneous emission rates as the
two-dimensional bandgap is tuned through their emission frequencies. The
measured band edges are in full agreement with theoretical predictions. We
characterize the multi-exponential decay curves by their mean decay time and
find enhancement of the spontaneous emission at the bandgap edges and strong
inhibition inside the bandgap in good agreement with local density of states
calculations.Comment: 9 pages (preprint), 3 figure
A new approach to the inverse problem for current mapping in thin-film superconductors
A novel mathematical approach has been developed to complete the inversion of
the Biot-Savart law in one- and two-dimensional cases from measurements of the
perpendicular component of the magnetic field using the well-developed
Magneto-Optical Imaging technique. Our approach, especially in the 2D case, is
provided in great detail to allow a straightforward implementation as opposed
to those found in the literature. Our new approach also refines our previous
results for the 1D case [Johansen et al., Phys. Rev. B 54, 16264 (1996)], and
streamlines the method developed by Jooss et al. [Physica C 299, 215 (1998)]
deemed as the most accurate if compared to that of Roth et al. [J. Appl. Phys.
65, 361 (1989)]. We also verify and streamline the iterative technique, which
was developed following Laviano et al. [Supercond. Sci. Technol. 16, 71 (2002)]
to account for in-plane magnetic fields caused by the bending of the applied
magnetic field due to the demagnetising effect. After testing on
magneto-optical images of a high quality YBa2Cu3O7 superconducting thin film,
we show that the procedure employed is effective
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