9,007 research outputs found
Phase Transitions and superuniversality in the dynamics of a self-driven particle
We study an active random walker model in which a particle's motion is
determined by a self-generated field. The field encodes information about the
particle's path history. This leads to either self-attractive or self-repelling
behavior. For self-repelling behavior, we find a phase transition in the
dynamics: when the coupling between the field and the walker exceeds a critical
value, the particle's behavior changes from renormalized diffusion to one
characterized by a diverging diffusion coefficient. The dynamical behavior for
all cases is surprisingly independent of dimension and of the noise amplitude.Comment: 14 pages, 4 figure
Migration of extrasolar planets to large orbital radii
Observations of structure in circumstellar debris discs provide
circumstantial evidence for the presence of massive planets at large (several
tens of au) orbital radii, where the timescale for planet formation via core
accretion is prohibitively long. Here, we investigate whether a population of
distant planets can be produced via outward migration subsequent to formation
in the inner disc. Two possibilities for significant outward migration are
identified. First, cores that form early at radii of around 10 au can be
carried to larger radii via gravitational interaction with the gaseous disc.
This process is efficient if there is strong mass loss from the disc - either
within a cluster or due to photoevaporation from a star more massive than the
Sun - but does not require the extremely destructive environment found, for
example, in the core of the Orion Nebula. We find that, depending upon the disc
model, gas disc migration can yield massive planets (several Jupiter masses) at
radii of around 20-50 au. Second, interactions within multiple planet systems
can drive the outer planet into a large, normally highly eccentric orbit. A
series of scattering experiments suggests that this process is most efficient
for lower mass planets within systems of unequal mass ratio. This mechanism is
a good candidate for explaining the origin of relatively low mass giant planets
in eccentric orbits at large radii.Comment: MNRAS, in pres
Tilted accretion discs in cataclysmic variables: tidal instabilities and superhumps
We investigate the growth of tidal instabilities in accretion discs in a
binary star potential, using three dimensional numerical simulations. As
expected from analytic work, the disc is prone to an eccentric instability
provided that it is large enough to extend to the 3:1 resonance. The eccentric
disc leads to positive superhumps in the light curve. It has been proposed that
negative superhumps might arise from a tilted disc, but we find no evidence
that the companion gravitational tilt instability can grow fast enough in a
fluid disc to create a measurable inclination. The origin of negative
superhumps in the light curves of cataclysmic variables remains a puzzle.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Charge Transfer and Charge Transport on the Double Helix
We present a short review of various experiments that measure charge transfer
and charge transport in DNA. Some general comments are made on the possible
connection between 'chemistry-style' charge transfer experiments that probe
fluorescence quenching and remote oxidative damage and 'physics-style'
measurements that measure transport properties as defined typically in the
solid-state. We then describe measurements performed by our group on the
millimeter wave response of DNA. By measuring over a wide range of humidity
conditions and comparing the response of single strand DNA and double strand
DNA, we show that the appreciable AC conductivity of DNA is not due to photon
assisted hopping between localized states, but instead due to dissipation from
dipole motion in the surrounding water helix.Comment: 7 pages, 3 figure
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