43,539 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

### 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

### SUSY Dark Matter In Light Of CDMS/XENON Limits

In this talk we briefly review the current CDMS/XENON constraints on the
neutralino dark matter in three popular supersymmetric models: the minimal
(MSSM), the next-to-minimal (NMSSM) and the nearly minimal (nMSSM). The
constraints from the dark matter relic density and various collider experiments
are also taken into account. The conclusion is that for each model the current
CDMS/XENON limits can readily exclude a large part of the parameter space
allowed by other constraints and the future SuperCDMS or XENON100 can cover
most of the allowed parameter space. The implication for the Higgs search at
the LHC is also discussed. It is found that in the currently allowed parameter
space the MSSM charged Higgs boson is quite unlikely to be discovered at the
LHC while the neutral Higgs bosons $H$ and $A$ may be accessible at the LHC in
the parameter space with a large $\mu$ parameter.Comment: talk given at 2nd International Workshop on Dark Matter, Dark Energy
and Matter-Antimatter Asymmetry, Nov 5-6, 2010, Hsinchu, Taiwan (to appear in
Int. J. Mod. Phys. D

### Inflation from Tsunami-waves

We investigate inflation driven by the evolution of highly excited quantum
states within the framework of out of equilibrium field dynamics. These states
are characterized by a non-perturbatively large number of quanta in a band of
momenta but with vanishing expectation value of the scalar field.They represent
the situation in which initially a non-perturbatively large energy density is
localized in a band of high energy quantum modes and are coined tsunami-waves.
The self-consistent evolution of this quantum state and the scale factor is
studied analytically and numerically. It is shown that the time evolution of
these quantum states lead to two consecutive stages of inflation under
conditions that are the quantum analogue of slow-roll. The evolution of the
scale factor during the first stage has new features that are characteristic of
the quantum state. During this initial stage the quantum fluctuations in the
highly excited band build up an effective homogeneous condensate with a non-
perturbatively large amplitude as a consequence of the large number of quanta.
The second stage of inflation is similar to the usual classical chaotic
scenario but driven by this effective condensate.The excited quantum modes are
already superhorizon in the first stage and do not affect the power spectrum of
scalar perturbations. Thus, this tsunami quantum state provides a field
theoretical justification for chaotic scenarios driven by a classical
homogeneous scalar field of large amplitude.Comment: LaTex, 36 pages, 7 .ps figures. Improved version to appear in Nucl.
Phys.

### Threshold feedback control for a collective flashing ratchet: threshold dependence

We study the threshold control protocol for a collective flashing ratchet. In
particular, we analyze the dependence of the current on the values of the
thresholds. We have found analytical expressions for the small threshold
dependence both for the few and for the many particle case. For few particles
the current is a decreasing function of the thresholds, thus, the maximum
current is reached for zero thresholds. In contrast, for many particles the
optimal thresholds have a nonzero finite value. We have numerically checked the
relation that allows to obtain the optimal thresholds for an infinite number of
particles from the optimal period of the periodic protocol. These optimal
thresholds for an infinite number of particles give good results for many
particles. In addition, they also give good results for few particles due to
the smooth dependence of the current up to these threshold values.Comment: LaTeX, 10 pages, 7 figures, improved version to appear in Phys. Rev.

### Closed-loop control strategy with improved current for a flashing ratchet

We show how to switch on and off the ratchet potential of a collective
Brownian motor, depending only on the position of the particles, in order to
attain a current higher than or at least equal to that induced by any periodic
flashing. Maximization of instant velocity turns out to be the optimal protocol
for one particle but is nevertheless defeated by a periodic switching when a
sufficiently large ensemble of particles is considered. The protocol presented
in this article, although not the optimal one, yields approximately the same
current as the optimal protocol for one particle and as the optimal periodic
switching for an infinite number of them.Comment: 4 pages, 4 figure

### Freezing of Nonlinear Bloch Oscillations in the Generalized Discrete Nonlinear Schrodinger Equation

The dynamics in a nonlinear Schrodinger chain in an homogeneous electric
field is studied. We show that discrete translational invariant
integrability-breaking terms can freeze the Bloch nonlinear oscillations and
introduce new faster frequencies in their dynamics. These phenomena are studied
by direct numerical integration and through an adiabatic approximation. The
adiabatic approximation allows a description in terms of an effective potential
that greatly clarifies the phenomenon.Comment: LaTeX, 7 pages, 6 figures. Improved version to appear in Phys. Rev.

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