10,358 research outputs found
An efficient method for scattering problems in open billiards: Theory and applications
We present an efficient method to solve scattering problems in
two-dimensional open billiards with two leads and a complicated scattering
region. The basic idea is to transform the scattering region to a rectangle,
which will lead to complicated dynamics in the interior, but simple boundary
conditions. The method can be specialized to closed billiards, and it allows
the treatment of interacting particles in the billiard. We apply this method to
quantum echoes measured recently in a microwave cavity, and indicate, how it
can be used for interacting particles.Comment: 9 pages 6 figures submitted to PR
Characterisation of a three-dimensional Brownian motor in optical lattices
We present here a detailed study of the behaviour of a three dimensional
Brownian motor based on cold atoms in a double optical lattice [P. Sjolund et
al., Phys. Rev. Lett. 96, 190602 (2006)]. This includes both experiments and
numerical simulations of a Brownian particle. The potentials used are spatially
and temporally symmetric, but combined spatiotemporal symmetry is broken by
phase shifts and asymmetric transfer rates between potentials. The diffusion of
atoms in the optical lattices is rectified and controlled both in direction and
speed along three dimensions. We explore a large range of experimental
parameters, where irradiances and detunings of the optical lattice lights are
varied within the dissipative regime. Induced drift velocities in the order of
one atomic recoil velocity have been achieved.Comment: 8 pages, 14 figure
Polarization Swings Reveal Magnetic Energy Dissipation in Blazars
The polarization signatures of the blazar emissions are known to be highly
variable. In addition to small fluctuations of the polarization angle around a
mean value, sometimes large (> 180^o) polarization angle swings are observed.
We suggest that such p henomena can be interpreted as arising from
light-travel-time effects within an underlying axisymmetric emission region. We
present the first simultaneous fitting of the multi-wavelength spectrum,
variability and time-dependent polarization features of a correlated optical
and gamma-ray flaring event of the prominent blazar 3C279, which was
accompanied by a drastic change of its polarization signatures. This
unprecedented combination of spectral, variability, and polarization
information in a coherent physical model allows us to place stringent
constraints on the particle acceleration and magnetic-field topology in the
relativistic jet of a blazar, strongly favoring a scenario in which magnetic
energy dissipation is the primary driver of the flare event.Comment: Accepted for Publication in The Astrophysical Journa
Multiple transient memories in sheared suspensions: robustness, structure, and routes to plasticity
Multiple transient memories, originally discovered in charge-density-wave
conductors, are a remarkable and initially counterintuitive example of how a
system can store information about its driving. In this class of memories, a
system can learn multiple driving inputs, nearly all of which are eventually
forgotten despite their continual input. If sufficient noise is present, the
system regains plasticity so that it can continue to learn new memories
indefinitely. Recently, Keim & Nagel showed how multiple transient memories
could be generalized to a generic driven disordered system with noise, giving
as an example simulations of a simple model of a sheared non-Brownian
suspension. Here, we further explore simulation models of suspensions under
cyclic shear, focussing on three main themes: robustness, structure, and
overdriving. We show that multiple transient memories are a robust feature
independent of many details of the model. The steady-state spatial distribution
of the particles is sensitive to the driving algorithm; nonetheless, the memory
formation is independent of such a change in particle correlations. Finally, we
demonstrate that overdriving provides another means for controlling memory
formation and retention
Particle currents and the distribution of terrace sizes in unstable epitaxial growth
A solid-on-solid model of epitaxial growth in 1+1 dimensions is investigated
in which slope dependent upward and downward particle currents compete on the
surface. The microscopic mechanisms which give rise to these currents are the
smoothening incorporation of particles upon deposition and an Ehrlich-Schwoebel
barrier which hinders inter-layer transport at step edges. We calculate the
distribution of terrace sizes and the resulting currents on a stepped surface
with a given inclination angle. The cancellation of the competing effects leads
to the selection of a stable magic slope. Simulation results are in very good
agreement with the theoretical findings.Comment: 4 pages, including 3 figure
Molecular Basis for poly(A) RNP Architecture and Recognition by the Pan2-Pan3 Deadenylase
The stability of eukaryotic mRNAs is dependent on a ribonucleoprotein (RNP) complex of poly(A)-binding proteins (PABPC1/Pab1) organized on the poly(A) tail. This poly(A) RNP not only protects mRNAs from premature degradation but also stimulates the Pan2-Pan3 deadenylase complex to catalyze the first step of poly(A) tail shortening. We reconstituted this process in vitro using recombinant proteins and show that Pan2-Pan3 associates with and degrades poly(A) RNPs containing two or more Pab1 molecules. The cryo-EM structure of Pan2-Pan3 in complex with a poly(A) RNP composed of 90 adenosines and three Pab1 protomers shows how the oligomerization interfaces of Pab1 are recognized by conserved features of the deadenylase and thread the poly(A) RNA substrate into the nuclease active site. The structure reveals the basis for the periodic repeating architecture at the 3' end of cytoplasmic mRNAs. This illustrates mechanistically how RNA-bound Pab1 oligomers act as rulers for poly(A) tail length over the mRNAs' lifetime.We would like to thank ... the MPIB cryo-EM, and core facilities ..
- …