3,475 research outputs found
The shape evolution of cometary nuclei via anisotropic mass loss
Context. Breathtaking imagery recorded during the European Space Agency's
Rosetta mission confirmed the bilobate nature of comet
67P/Churyumov-Gerasimenko's nucleus. Its peculiar appearance is not unique
among comets. The majority of cometary cores imaged at high resolution exhibit
a similar build. Various theories have been brought forward as to how cometary
nuclei attain such peculiar shapes.
Aims. We illustrate that anisotropic mass loss and local collapse of
subsurface structures caused by non-uniform exposure of the nucleus to solar
irradiation can transform initially spherical comet cores into bilobed ones.
Methods. A mathematical framework to describe the changes in morphology
resulting from non-uniform insolation during a nucleus' spin-orbit evolution is
derived. The resulting partial differential equations that govern the change in
the shape of a nucleus subject to mass loss and consequent collapse of depleted
subsurface structures are solved analytically for simple insolation
configurations and numerically for more realistic scenarios.
Results. The here proposed mechanism is capable of explaining why a large
fraction of periodic comets appear to have peanut-shaped cores and why
light-curve amplitudes of comet nuclei are on average larger than those of
typical main belt asteroids of the same size.Comment: 4 pages of the main text, 2 pages of appendix, 4 figure
Random matrix models for phase diagrams
We describe a random matrix approach that can provide generic and readily
soluble mean-field descriptions of the phase diagram for a variety of systems
ranging from QCD to high-T_c materials. Instead of working from specific
models, phase diagrams are constructed by averaging over the ensemble of
theories that possesses the relevant symmetries of the problem. Although
approximate in nature, this approach has a number of advantages. First, it can
be useful in distinguishing generic features from model-dependent details.
Second, it can help in understanding the `minimal' number of symmetry
constraints required to reproduce specific phase structures. Third, the
robustness of predictions can be checked with respect to variations in the
detailed description of the interactions. Finally, near critical points, random
matrix models bear strong similarities to Ginsburg-Landau theories with the
advantage of additional constraints inherited from the symmetries of the
underlying interaction. These constraints can be helpful in ruling out certain
topologies in the phase diagram. In this Key Issue, we illustrate the basic
structure of random matrix models, discuss their strengths and weaknesses, and
consider the kinds of system to which they can be applied.Comment: 29 pages, 2 figures, uses iopart.sty. Author's postprint versio
Photocurrent, Rectification, and Magnetic Field Symmetry of Induced Current Through Quantum Dots
We report mesoscopic dc current generation in an open chaotic quantum dot
with ac excitation applied to one of the shape-defining gates. For excitation
frequencies large compared to the inverse dwell time of electrons in the dot
(i.e., GHz), we find mesoscopic fluctuations of induced current that are fully
asymmetric in the applied perpendicular magnetic field, as predicted by recent
theory. Conductance, measured simultaneously, is found to be symmetric in
field. In the adiabatic (i.e., MHz) regime, in contrast, the induced current is
always symmetric in field, suggesting its origin is mesoscopic rectification.Comment: related papers at http://marcuslab.harvard.ed
Status of the TRIUMF PIENU Experiment
The PIENU experiment at TRIUMF aims to measure the pion decay branching ratio
with precision % to provide a sensitive test of electron-muon
universality in weak interactions. The current status of the PIENU experiment
is presented.Comment: Talk presented CIPANP2015. 8 pages, LaTeX, 4 eps figure
Improved Search for Heavy Neutrinos in the Decay
A search for massive neutrinos has been made in the decay . No evidence was found for extra peaks in the positron energy spectrum
indicative of pion decays involving massive neutrinos (). Upper limits (90 \% C.L.) on the neutrino mixing matrix element
in the neutrino mass region 60--135 MeV/ were set, which are
%representing an order of magnitude improvement over previous results
Selecting the conditions of molding iron oxide catalysts for dehydrogenation
The effect of the conditions for molding iron oxide catalysts for dehydrogenation of isoamylenes to isoprene on their operation characteristics was studied. The investigation of laboratory samples allowed us to determine the optimum molding pressure (200-250 MPa) under which high mechanical strength (strength factor KG = 33.4-37.3 N/mm), the stability of the kinetic characteristics in the dehydrogenation reaction, and the formation of 15-50 nm pores, ensuring the occurrence of the process in the kinetic region, were attained. we propose using the extrudate density as an indirect criterion for estimating the molding pressure under industrial conditions of extrusion. It was shown that in order to develop high strength properties (KG ~ 29.1 N/mm) of the catalysts upon their production under industrial conditions and to ensure the occurrence of the reaction in the kinetic region, the extrudate density must be 2.40-2.46 g/cm3. The obtained results were verified via paste molding on various industrial extruders, thereby enabling us to recommend the type of molding extrusion equipment. © Pleiades Publishing, Ltd., 2011
Dephasing in the semiclassical limit is system-dependent
We investigate dephasing in open quantum chaotic systems in the limit of
large system size to Fermi wavelength ratio, . We
semiclassically calculate the weak localization correction to the
conductance for a quantum dot coupled to (i) an external closed dot and (ii) a
dephasing voltage probe. In addition to the universal algebraic suppression
with the dwell time through
the cavity and the dephasing rate , we find an exponential
suppression of weak localization by a factor , with a system-dependent . In the
dephasing probe model, coincides with the Ehrenfest time,
, for both perfectly and partially
transparent dot-lead couplings. In contrast, when dephasing occurs due to the
coupling to an external dot, depends on the
correlation length of the coupling potential instead of .Comment: 4 pages 3 figures (v2 contains numerous cosmetic changes
Counting statistics for arbitrary cycles in quantum pumps
Statistics of charge transport in an adiabatic pump are determined by the
dynamics of the scattering matrix S(t). We show that, up to an integer offset,
the statistics depend only on the corresponding path N(t)=S^\dagger\sigma_3 S
in the coset space (the sphere for a single channel). For a general loop S(t)
we solve for the noise-minimizing pumping strategy. The average current is
given by the area enclosed by N(t) in the coset space; its minimal noise by the
area of a minimal surface (soap film) spanned by N(t) in the space of all
matrices. We formulate conditions for quantization of the pumped charge.Comment: 4 pages, 2 figure
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