430 research outputs found
Molecular Dynamics of Comminution in Ball Mills
We investigate autogenous fragmentation of dry granular material in rotating
cylinders using two-dimensional molecular dynamics. By evaluation of spatial
force distributions achieved numerically for various rotation velocities we
argue that comminution occurs mainly due to the existence of force chains. A
statistical analysis of theses force chains explains the spatial distribution
of comminution efficiency in ball mills as measured experimentally by Rothkegel
and Rolf. For animated sequences of our simulations see
http://summa.physik.hu-berlin.de/~kies/Research/RotatingCylinder/rotatingcylind
er.htmlComment: 15 pages, 13 figure
Shadow on the wall cast by an Abrikosov vortex
At the surface of a d-wave superconductor, a zero-energy peak in the
quasiparticle spectrum can be observed. This peak appears due to Andreev bound
states and is maximal if the nodal direction of the d-wave pairing potential is
perpendicular to the boundary. We examine the effect of a single Abrikosov
vortex in front of a reflecting boundary on the zero-energy density of states.
We can clearly see a splitting of the low-energy peak and therefore a
suppression of the zero-energy density of states in a shadow-like region
extending from the vortex to the boundary. This effect is stable for different
models of the single Abrikosov vortex, for different mean free paths and also
for different distances between the vortex center and the boundary. This
observation promises to have also a substantial influence on the differential
conductance and the tunneling characteristics for low excitation energies.Comment: 5 pages, 5 figure
Crossing the Frontier: a Hypothesis for the Origins of Meristic Constraint in Mammalian Axial Patterning
Serially homologous systems with high internal differentiation frequently exhibit meristic constraints, although the developmental basis for constraint is unknown. Constraints in the counts of the cervical and lumbosacral vertebral series are unique to mammals, and appeared in the Triassic, early in their history. Concurrent adaptive modifications of the mammalian respiratory and locomotor systems involved a novel source of cells for muscularization of the diaphragm from cervical somites, and the loss of ribs from lumbar vertebrae. Each of these innovations increased the modularity of the somitic mesoderm, and altered somitic and lateral plate mesodermal interactions across the lateral somitic frontier. These developmental innovations are hypothesized here to constrain the anteroposterior transposition of the limbs along the column, and thus also cervical and thoracolumbar count. Meristic constraints are therefore regarded here as the nonadaptive, secondary consequences of adaptive respiratory and locomotor traits
Hands-On TAROT: Intercontinental use of the TAROT for Education and Public Outreach
The TAROT telescope has for primary goal the search for the prompt optical
counterpart of Cosmic Gamma-Ray Bursts. It is a completely autonomous 25cm
telescope installed near Nice (France), able to point any location of the sky
within 1-2 seconds. The control, scheduling, and data processing activities are
completely automated, so the instrument is completely autonomous. In addition
to its un-manned modes, we added recently the possibility to remotely control
the telescope, as a request of the "Hands-On Universe" (HOU) program for
exchange of time within automatic telescopes for the education and public
outreach. To this purpose we developed a simple control interface. A webcam was
installed to visualize the telescope. Access to the data is possible through a
web interface. The images can be processed by the HOU software, a program
specially suited for use within the classroom. We experienced these feature
during the open days of the University of California Berkeley and the Astronomy
Festival of Fleurance (France). We plan a regular use for an astronomy course
of the Museum of Tokyo, as well as for French schools. Not only does Hands-On
TAROT gives the general public an access to professional astronomy, but it is
also a more general tool to demonstrate the use of a complex automated system,
the techniques of data processing and automation. Last but not least, through
the use of telescopes located in many countries over the globe, a form of
powerful and genuine cooperation between teachers and children from various
countries is promoted, with a clear educational goal.Comment: 4 pages, Based on a demonstration presented at the ADASS X
Conference, Boston, MA, USA, October 2000, to appear in ASP Conf. Serie
Thermodynamic properties of thin films of superfluid 3He-A
The pairing correlations in superfluid He-3 are strongly modified by
quasiparticle scattering off a surface or an interface. We present theoretical
results and predictions for the order parameter, the quasiparticle excitation
spectrum and the free energy for thin films of superfluid He-3. Both specular
and diffuse scattering by a substrate are considered, while the free surface is
assumed to be a perfectly reflecting specular boundary. The results are based
on self-consistent calculations of the order parameter and quasiparticle
excitation spectrum at zero pressure. We obtain new results for the phase
diagram, free energy, entropy and specific heat of thin films of superfluid
He-3.Comment: Replaced with an updated versio
Exact microscopic analysis of a thermal Brownian motor
We study a genuine Brownian motor by hard disk molecular dynamics and
calculate analytically its properties, including its drift speed and thermal
conductivity, from microscopic theory.Comment: 4 pages, 5 figure
Low-energy quasiparticle states near extended scatterers in d-wave superconductors and their connection with SUSY quantum mechanics
Low-energy quasiparticle states, arising from scattering by single-particle
potentials in d-wave superconductors, are addressed. Via a natural extension of
the Andreev approximation, the idea that sign-variations in the superconducting
pair-potential lead to such states is extended beyond its original setting of
boundary scattering to the broader context of scattering by general
single-particle potentials, such as those due to impurities. The
index-theoretic origin of these states is exhibited via a simple connection
with Witten's supersymmetric quantum-mechanical model.Comment: 5 page
Odd-frequency pairing in normal metal/superconductor junctions
We study the induced odd-frequency pairing states in ballistic normal
metal/superconductor (N/S) junctions where a superconductor has even-frequency
symmetry in the bulk and a normal metal layer has an arbitrary length. Using
the quasiclassical Green's function formalism, we demonstrate that, quite
generally, the pair amplitude in the junction has an admixture of an
odd-frequency component due to the breakdown of translational invariance near
the N/S interface where the pair potential acquires spatial dependence. If a
superconductor has even-parity pair potential (spin-singlet s-wave state), the
odd-frequency pairing component with odd-parity is induced near the N/S
interface, while in the case of odd-parity pair potential (spin-triplet
-wave or spin-singlet -wave) the odd-frequency component with
even-parity is generated. We show that in conventional s-wave junctions, the
amplitude of the odd-frequency pairing state is enhanced at energies
corresponding to the peaks in the local density of states (LDOS). In - and
-wave junctions, the amplitude of the odd-frequency component on the S
side of the N/S interface is enhanced at zero energy where the midgap Andreev
resonant state (MARS) appears due to the sign change of the pair potential. The
odd-frequency component extends into the N region and exceeds the
even-frequency component at energies corresponding to the LDOS peak positions,
including the MARS.Comment: 27 pages, 12 figure
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