2,656 research outputs found
Dark matter density profiles: A comparison of nonextensive theory with N-body simulations
Density profiles of simulated galaxy cluster-sized dark matter haloes are
analysed in the context of a recently introduced nonextensive theory of dark
matter and gas density distributions. Nonextensive statistics accounts for
long-range interactions in gravitationally coupled systems and is derived from
the fundamental concept of entropy generalisation. The simulated profiles are
determined down to radii of ~1% of R_200. The general trend of the relaxed,
spherically averaged profiles is accurately reproduced by the theory. For the
main free parameter kappa, measuring the degree of coupling within the system,
and linked to physical quantities as the heat capacity and the polytropic index
of the self-gravitating ensembles, we find a value of -15. The significant
advantage over empirical fitting functions is provided by the physical content
of the nonextensive approach.Comment: 6 pages, 3 figures, accepted for publication in A&
Cooperative protein transport in cellular organelles
Compartmentalization into biochemically distinct organelles constantly
exchanging material is one of the hallmarks of eukaryotic cells. In the most
naive picture of inter-organelle transport driven by concentration gradients,
concentration differences between organelles should relax. We determine the
conditions under which cooperative transport, i.e. based on molecular
recognition, allows for the existence and maintenance of distinct organelle
identities. Cooperative transport is also shown to control the flux of material
transiting through a compartmentalized system, dramatically increasing the
transit time under high incoming flux. By including chemical processing of the
transported species, we show that this property provides a strong functional
advantage to a system responsible for protein maturation and sorting.Comment: 9 pages, 5 figure
Generalized Fokker-Planck equation, Brownian motion, and ergodicity
Microscopic theory of Brownian motion of a particle of mass in a bath of
molecules of mass is considered beyond lowest order in the mass ratio
. The corresponding Langevin equation contains nonlinear corrections to
the dissipative force, and the generalized Fokker-Planck equation involves
derivatives of order higher than two. These equations are derived from first
principles with coefficients expressed in terms of correlation functions of
microscopic force on the particle. The coefficients are evaluated explicitly
for a generalized Rayleigh model with a finite time of molecule-particle
collisions. In the limit of a low-density bath, we recover the results obtained
previously for a model with instantaneous binary collisions. In general case,
the equations contain additional corrections, quadratic in bath density,
originating from a finite collision time. These corrections survive to order
and are found to make the stationary distribution non-Maxwellian.
Some relevant numerical simulations are also presented
Fidelity of holonomic quantum computations in the case of random errors in the values of control parameters
We investigate the influence of random errors in external control parameters
on the stability of holonomic quantum computation in the case of arbitrary
loops and adiabatic connections. A simple expression is obtained for the case
of small random uncorrelated errors. Due to universality of mathematical
description our results are valid for any physical system which can be
described in terms of holonomies. Theoretical results are confirmed by
numerical simulations.Comment: 7 pages, 3 figure
Phenomenological approach to non-linear Langevin equations
In this paper we address the problem of consistently construct Langevin
equations to describe fluctuations in non-linear systems. Detailed balance
severely restricts the choice of the random force, but we prove that this
property together with the macroscopic knowledge of the system is not enough to
determine all the properties of the random force. If the cause of the
fluctuations is weakly coupled to the fluctuating variable, then the
statistical properties of the random force can be completely specified. For
variables odd under time-reversal, microscopic reversibility and weak coupling
impose symmetry relations on the variable-dependent Onsager coefficients. We
then analyze the fluctuations in two cases: Brownian motion in position space
and an asymmetric diode, for which the analysis based in the master equation
approach is known. We find that, to the order of validity of the Langevin
equation proposed here, the phenomenological theory is in agreement with the
results predicted by more microscopic models.Comment: LaTex file, 2 figures available upon request, to appear in Phys.Rev.
NOTES ON A SMALL COLLECTION OF AMPHIBIA FROM SUMBA
abstract not availabl
Hamiltonian for coupled flux qubits
An effective Hamiltonian is derived for two coupled three-Josephson-junction
(3JJ) qubits. This is not quite trivial, for the customary "free" 3JJ
Hamiltonian is written in the limit of zero inductance L. Neglecting the
self-flux is already dubious for one qubit when it comes to readout, and
becomes untenable when discussing inductive coupling. First, inductance effects
are analyzed for a single qubit. For small L, the self-flux is a "fast
variable" which can be eliminated adiabatically. However, the commonly used
junction phases are_not_ appropriate "slow variables", and instead one
introduces degrees of freedom which are decoupled from the loop current to
leading order. In the quantum case, the zero-point fluctuations (LC
oscillations) in the loop current diverge as L->0. Fortunately, they merely
renormalize the Josephson couplings of the effective (two-phase) theory.
In the coupled case, the strong zero-point fluctuations render the full
(six-phase) wave function significantly entangled in leading order. However, in
going to the four-phase theory, this uncontrollable entanglement is integrated
out completely, leaving a computationally usable mutual-inductance term of the
expected form as the effective interaction.Comment: REVTeX4, 16pp., one figure. N.B.: "Alec" is my first, and "Maassen
van den Brink" my family name. Informal note. v2: completely rewritten;
correction of final result and major expansion. v3: added numerical
verification plus a discussion of Ref. [2
Coarse-graining a restricted solid-on-solid model
A procedure suggested by Vvedensky for obtaining continuum equations as the
coarse-grained limit of discrete models is applied to the restricted
solid-on-solid model with both adsorption and desorption. Using an expansion of
the master equation, discrete Langevin equations are derived; these agree
quantitatively with direct simulation of the model. From these, a continuum
differential equation is derived, and the model is found to exhibit either
Edwards-Wilkinson or Kardar-Parisi-Zhang exponents, as expected from symmetry
arguments. The coefficients of the resulting continuum equation remain
well-defined in the coarse-grained limit.Comment: Accepted for pubication in PR
Markov Chain Modeling of Polymer Translocation Through Pores
We solve the Chapman-Kolmogorov equation and study the exact splitting
probabilities of the general stochastic process which describes polymer
translocation through membrane pores within the broad class of Markov chains.
Transition probabilities which satisfy a specific balance constraint provide a
refinement of the Chuang-Kantor-Kardar relaxation picture of translocation,
allowing us to investigate finite size effects in the evaluation of dynamical
scaling exponents. We find that (i) previous Langevin simulation results can be
recovered only if corrections to the polymer mobility exponent are taken into
account and that (ii) the dynamical scaling exponents have a slow approach to
their predicted asymptotic values as the polymer's length increases. We also
address, along with strong support from additional numerical simulations, a
critical discussion which points in a clear way the viability of the Markov
chain approach put forward in this work.Comment: 17 pages, 5 figure
Conservation, Dissipation, and Ballistics: Mesoscopic Physics beyond the Landauer-Buettiker Theory
The standard physical model of contemporary mesoscopic noise and transport
consists in a phenomenologically based approach, proposed originally by
Landauer and since continued and amplified by Buettiker (and others).
Throughout all the years of its gestation and growth, it is surprising that the
Landauer-Buettiker approach to mesoscopics has matured with scant attention to
the conservation properties lying at its roots: that is, at the level of actual
microscopic principles. We systematically apply the conserving sum rules for
the electron gas to clarify this fundamental issue within the standard
phenomenology of mesoscopic conduction. Noise, as observed in quantum point
contacts, provides the vital clue.Comment: 10 pp 3 figs, RevTe
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