51,431 research outputs found
Out of equilibrium quantum field dynamics in external fields
The quantum dynamics of the symmetry broken \lambda (\Phi^2)^2 scalar field
theory in the presence of an homogeneous external field is investigated in the
large N limit. We consider an initial thermal state of temperature T for a
constant external field J. A subsequent sign flip of the external field, J to
-J, gives rise to an out of equilibrium nonperturbative quantum field dynamics.
We review here the dynamics for the symmetry broken lambda(\Phi^2)^2 scalar N
component field theory in the large N limit, with particular stress in the
comparison between the results when the initial temperature is zero and when it
is finite. The presence of a finite temperature modifies the dynamical
effective potential for the expectation value, and also makes that the
transition between the two regimes of the early dynamics occurs for lower
values of the external field. The two regimes are characterized by the presence
or absence of a temporal trapping close to the metastable equilibrium position
of the potential. In the cases when the trapping occurs it is shorter for
larger initial temperatures.Comment: LaTeX, 3 pages, 2 figures. Presented at the IVth International
Conference on Quarks and Nuclear Physics (QNP06). Selected to appear in Eur.
Phys. J.
Transport reversal in a delayed feedback ratchet
Feedback flashing ratchets are thermal rectifiers that use information on the
state of the system to operate the switching on and off of a periodic
potential. They can induce directed transport even with symmetric potentials
thanks to the asymmetry of the feedback protocol. We investigate here the
dynamics of a feedback flashing ratchet when the asymmetry of the ratchet
potential and of the feedback protocol favor transport in opposite directions.
The introduction of a time delay in the control strategy allows one to
nontrivially tune the relative relevance of the competing asymmetries leading
to an interesting dynamics. We show that the competition between the
asymmetries leads to a current reversal for large delays. For small ensembles
of particles current reversal appears as the consequence of the emergence of an
open-loop like dynamical regime, while for large ensembles of particles it can
be understood as a consequence of the stabilization of quasiperiodic solutions.
We also comment on the experimental feasibility of these feedback ratchets and
their potential applications.Comment: LaTeX, 7 pages, 6 figure
The interweaving of diaries and lives : diary-keeping behaviour in a diary-interview study of international students’ employability management
This article explores ‘diary-keeping behaviour’, or the ways in which participants conduct the completion and submission of diaries in diary research. There is a paucity of methodologically oriented literature on diary method and as such this article makes a contribution to extending the existing knowledge of this method. The primary aim of this article is to set out in detail the key issues relating to diary-keeping behaviour, in order to provide a foundation for future critical explorations of this facet of diary research. The research that this paper is based on involved a 12-month diary-interview study. This project explored the employability management of Chinese international Master’s students in social sciences studying in the UK during one academic year. The article sets out key facets of diary-keeping behaviour and explores specific considerations for diary studies in higher education contexts, where diary research has been particularly neglected
How occasional backstepping can speed up a processive motor protein
Fueled by the hydrolysis of ATP, the motor protein kinesin literally walks on
two legs along the biopolymer microtubule. The number of accidental backsteps
that kinesin takes appears to be much larger than what one would expect given
the amount of free energy that ATP hydrolysis makes available. This is puzzling
as more than a billion years of natural selection should have optimized the
motor protein for its speed and efficiency. But more backstepping allows for
the production of more entropy. Such entropy production will make free energy
available. With this additional free energy, the catalytic cycle of the kinesin
can be speeded up. We show how measured backstep percentages represent an
optimum at which maximal net forward speed is achieved.Comment: LaTeX, 5 pages, 3 figure
Arithmetic purity of strong approximation for homogeneous spaces
We prove that any open subset of a semi-simple simply connected quasi-split linear algebraic group with over a number field satisfies strong approximation by establishing a fibration of over a toric variety. We also prove a similar result of strong approximation with Brauer-Manin obstruction for a partial equivariant smooth compactification of a homogeneous space where all invertible functions are constant and the semi-simple part of the linear algebraic group is quasi-split. Some semi-abelian varieties of any given dimension where the complements of a rational point do not satisfy strong approximation with Brauer-Manin obstruction are given
Lacunar fractal photon sieves
We present a new family of diffractive lenses whose structure is based on the
combination of two concepts: photon sieve and fractal zone plates with variable
lacunarity. The focusing properties of different members of this family are
examined. It is shown that the sieves provide a smoothing effect on the higher
order foci of a conventional lacunar fractal zone plate. However, the
characteristic self-similar axial response of the fractal zone plates is always
preserved.Comment: 7 pages, 5 figure
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