518 research outputs found
Exploiting the nonlinear impact dynamics of a single-electron shuttle for highly regular current transport
The nanomechanical single-electron shuttle is a resonant system in which a
suspended metallic island oscillates between and impacts at two electrodes.
This setup holds promise for one-by-one electron transport and the
establishment of an absolute current standard. While the charge transported per
oscillation by the nanoscale island will be quantized in the Coulomb blockade
regime, the frequency of such a shuttle depends sensitively on many parameters,
leading to drift and noise. Instead of considering the nonlinearities
introduced by the impact events as a nuisance, here we propose to exploit the
resulting nonlinear dynamics to realize a highly precise oscillation frequency
via synchronization of the shuttle self-oscillations to an external signal.Comment: 5 pages, 4 figure
Binary Capture Rates for Massive Protostars
The high multiplicity of massive stars in dense, young clusters is
established early in their evolution. The mechanism behind this remains
unresolved. Recent results suggest that massive protostars may capture
companions through disk interactions with much higher efficiency than their
solar mass counterparts. However, this conclusion is based on analytic
determinations of capture rates and estimates of the robustness of the
resulting binaries. We present the results of coupled n-body and SPH
simulations of star-disk encounters to further test the idea that disk-captured
binaries contribute to the observed multiplicity of massive stars.Comment: 4 pages, 3 figures, accepted to ApJ
Exact results for nonlinear ac-transport through a resonant level model
We obtain exact results for the transport through a resonant level model
(noninteracting Anderson impurity model) for rectangular voltage bias as a
function of time. We study both the transient behavior after switching on the
tunneling at time t = 0 and the ensuing steady state behavior. Explicit
expressions are obtained for the ac-current in the linear response regime and
beyond for large voltage bias. Among other effects, we observe current ringing
and PAT (photon assisted tunneling) oscillations.Comment: 7 page
Symmetry, bifurcation and stacking of the central configurations of the planar 1+4 body problem
In this work we are interested in the central configurations of the planar
1+4 body problem where the satellites have different infinitesimal masses and
two of them are diametrically opposite in a circle. We can think this problem
as a stacked central configuration too. We show that the configuration are
necessarily symmetric and the other sattelites has the same mass. Moreover we
proved that the number of central configuration in this case is in general one,
two or three and in the special case where the satellites diametrically
opposite have the same mass we proved that the number of central configuration
is one or two saying the exact value of the ratio of the masses that provides
this bifurcation.Comment: 9 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1103.627
New theoretical approaches for correlated systems in nonequilibrium
Abstract.: We review recent developments in the theory of interacting quantum many-particle systems that are not in equilibrium. We focus mainly on the nonequilibrium generalizations of the flow equation approach and of dynamical mean-field theory (DMFT). In the nonequilibrium flow equation approach one first diagonalizes the Hamiltonian iteratively, performs the time evolution in this diagonal basis, and then transforms back to the original basis, thereby avoiding a direct perturbation expansion with errors that grow linearly in time. In nonequilibrium DMFT, on the other hand, the Hubbard model can be mapped onto a time-dependent self-consistent single-site problem. We discuss results from the flow equation approach for nonlinear transport in the Kondo model, and further applications of this method to the relaxation behavior in the ferromagnetic Kondo model and the Hubbard model after an interaction quench. For the interaction quench in the Hubbard model, we have also obtained numerical DMFT results using quantum Monte Carlo simulations. In agreement with the flow equation approach they show that for weak coupling the system relaxes to a "prethermalized” intermediate state instead of rapid thermalization. We discuss the description of nonthermal steady states with generalized Gibbs ensemble
Chaos around a H\'enon-Heiles-inspired exact perturbation of a black hole
A solution of the Einstein's equations that represents the superposition of a
Schwarszchild black hole with both quadrupolar and octopolar terms describing a
halo is exhibited. We show that this solution, in the Newtonian limit, is an
analog to the well known H\'enon-Heiles potential. The integrability of orbits
of test particles moving around a black hole representing the galactic center
is studied and bounded zones of chaotic behavior are found.Comment: 7 pages Revte
Extrasolar Planet Eccentricities from Scattering in the Presence of Residual Gas Disks
Gravitational scattering between massive planets has been invoked to explain
the eccentricity distribution of extrasolar planets. For scattering to occur,
the planets must either form in -- or migrate into -- an unstable
configuration. In either case, it is likely that a residual gas disk, with a
mass comparable to that of the planets, will be present when scattering occurs.
Using explicit hydrodynamic simulations, we study the impact of gas disks on
the outcome of two-planet scattering. We assume a specific model in which the
planets are driven toward instability by gravitational torques from an outer
low mass disk. We find that the accretion of mass and angular momentum that
occurs when a scattered planet impacts the disk can strongly influence the
subsequent dynamics by reducing the number of close encounters. The
eccentricity of the innermost surviving planet at the epoch when the system
becomes Hill stable is not substantially altered from the gas-free case, but
the outer planet is circularized by its interaction with the disk. The
signature of scattering initiated by gas disk migration is thus a high fraction
of low eccentricity planets at larger radii accompanying known eccentric
planets. Subsequent secular evolution of the two planets in the presence of
damping can further damp both eccentricities, and tends to push systems away
from apsidal alignment and toward anti-alignment. We note that the late burst
of accretion when the outer planet impacts the disk is in principle observable,
probably via detection of a strong near-IR excess in systems with otherwise
weak disk and stellar accretion signatures.Comment: 7 pages, 7 figures. Accepted to Ap
Crossover from adiabatic to sudden interaction quenches in the Hubbard model: Prethermalization and nonequilibrium dynamics
The recent experimental implementation of condensed matter models in optical
lattices has motivated research on their nonequilibrium behavior. Predictions
on the dynamics of superconductors following a sudden quench of the pairing
interaction have been made based on the effective BCS Hamiltonian; however,
their experimental verification requires the preparation of a suitable excited
state of the Hubbard model along a twofold constraint: (i) a sufficiently
nonadiabatic ramping scheme is essential to excite the nonequilibrium dynamics,
and (ii) overheating beyond the critical temperature of superconductivity must
be avoided. For commonly discussed interaction ramps there is no clear
separation of the corresponding energy scales. Here we show that the matching
of both conditions is simplified by the intrinsic relaxation behavior of
ultracold fermionic systems: For the particular example of a linear ramp we
examine the transient regime of prethermalization [M. Moeckel and S. Kehrein,
Phys. Rev. Lett. 100, 175702 (2008)] under the crossover from sudden to
adiabatic switching using Keldysh perturbation theory. A real-time analysis of
the momentum distribution exhibits a temporal separation of an early energy
relaxation and its later thermalization by scattering events. For long but
finite ramping times this separation can be large. In the prethermalization
regime the momentum distribution resembles a zero temperature Fermi liquid as
the energy inserted by the ramp remains located in high energy modes. Thus
ultracold fermions prove robust to heating which simplifies the observation of
nonequilibrium BCS dynamics in optical lattices.Comment: 27 pages, 8 figures Second version with small modifications in
section
Using risk-ranking of metals to identify which poses the greatest threat to freshwater organisms in the UK
Freshwater aquatic organisms face the challenge of being exposed to a multitude of chemicals discharged by the human population. The objective of this study was to rank metals according to the threat they pose to aquatic organisms. This will contribute to a wider Chemical Strategy for freshwater which will risk rank all chemicals based on their potential risk to wildlife in a UK setting. The method involved comparing information on ecotoxicological thresholds with measured concentrations in rivers. The bioconcentration factor was also considered as a ranking method. The metals; Ag, Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn, were analysed using this approach. Triclosan and lindane were used as comparative organic pollutants. Using a range of ranking techniques, Cu, Al and Zn came top of the list of concern, with Cu coming first
Stellar Encounters with Massive Star-Disk Systems
The dense, clustered environment in which massive stars form can lead to
interactions with neighboring stars. It has been hypothesized that collisions
and mergers may contribute to the growth of the most massive stars. In this
paper we extend the study of star-disk interactions to explore encounters
between a massive protostar and a less massive cluster sibling using the
publicly available SPH code GADGET-2. Collisions do not occur in the parameter
space studied, but the end state of many encounters is an eccentric binary with
a semi-major axis ~ 100 AU. Disk material is sometimes captured by the
impactor. Most encounters result in disruption and destruction of the initial
disk, and periodic torquing of the remnant disk. We consider the effect of the
changing orientation of the disk on an accretion driven jet, and the evolution
of the systems in the presence of on-going accretion from the parent core.Comment: 11 pages, 10 figures, accepted to Ap
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