53,219 research outputs found
Quantization of the Nonlinear Sigma Model Revisited
We revisit the subject of perturbatively quantizing the nonlinear sigma model
in two dimensions from a rigorous, mathematical point of view. Our main
contribution is to make precise the cohomological problem of eliminating
potential anomalies that may arise when trying to preserve symmetries under
quantization. The symmetries we consider are twofold: (i) diffeomorphism
covariance for a general target manifold; (ii) a transitive group of isometries
when the target manifold is a homogeneous space. We show that there are no
anomalies in case (i) and that (ii) is also anomaly-free under additional
assumptions on the target homogeneous space, in agreement with the work of
Friedan. We carry out some explicit computations for the -model. Finally,
we show how a suitable notion of the renormalization group establishes the
Ricci flow as the one loop renormalization group flow of the nonlinear sigma
model.Comment: 51 page
Gate voltage tuned quantum superconductor to insulator transition in an ultrathin bismuth film revisited
We explore the implications of Berezinskii-Kosterlitz-Thouless (BKT) critical
behavior and variable-range hopping on the two dimensional (2D) quantum
superconductor-insulator (QSI) transition driven by tuning the gate voltage. To
illustrate the potential and the implications of this scenario we analyze sheet
resistance data of Parendo et al. taken on a gate voltage tuned ultrathin
amorphous bismuth film. The finite size scaling analysis of the BKT-transition
uncovers a limiting length preventing the correlation length to diverge and to
enter the critical regime deeply. Nevertheless the attained BKT critical regime
reveals consistency with two parameter quantum scaling and an explicit quantum
scaling function determined by the BKT correlation length. The two parameter
scaling yields for the zero temperature critical exponents of the
QSI-transition the estimates zn = 3/2, z = 3, and n = 1/2, revealing that
hyperscaling is violated and in contrast to finite temperature disorder is
relevant at zero temperature. Furthermore, zn = 3/2 is also consistent with the
two variable quantum scaling form associated with a variable-range hopping
controlled insulating ground state
Scaling Invariance in a Time-Dependent Elliptical Billiard
We study some dynamical properties of a classical time-dependent elliptical
billiard. We consider periodically moving boundary and collisions between the
particle and the boundary are assumed to be elastic. Our results confirm that
although the static elliptical billiard is an integrable system, after to
introduce time-dependent perturbation on the boundary the unlimited energy
growth is observed. The behaviour of the average velocity is described using
scaling arguments
Casimir force between two ideal-conductor walls revisited
The high-temperature aspects of the Casimir force between two neutral
conducting walls are studied. The mathematical model of "inert" ideal-conductor
walls, considered in the original formulations of the Casimir effect, is based
on the universal properties of the electromagnetic radiation in the vacuum
between the conductors, with zero boundary conditions for the tangential
components of the electric field on the walls. This formulation seems to be in
agreement with experiments on metallic conductors at room temperature. At high
temperatures or large distances, at least, fluctuations of the electric field
are present in the bulk and at the surface of a particle system forming the
walls, even in the high-density limit: "living" ideal conductors. This makes
the enforcement of the inert boundary conditions inadequate. Within a hierarchy
of length scales, the high-temperature Casimir force is shown to be entirely
determined by the thermal fluctuations in the conducting walls, modelled
microscopically by classical Coulomb fluids in the Debye-H\"{u}ckel regime. The
semi-classical regime, in the framework of quantum electrodynamics, is studied
in the companion letter by P.R.Buenzli and Ph.A.Martin, cond-mat/0506363,
Europhys.Lett.72, 42 (2005).Comment: 7 pages.One reference updated. Domain of validity of eq.(11)
correcte
The Bosma effect revisited - I. HI and stellar disc scaling models
The observed proportionality between the centripetal contribution of the
dynamically insignificant HI gas in the discs of spiral galaxies and the
dominant contribution of DM - the "Bosma effect" - has been repeatedly
mentioned in the literature but largely ignored. We have re-examined the
evidence for the Bosma effect by fitting Bosma effect models for 17 galaxies in
the THINGS data set, either by scaling the contribution of the HI gas alone or
by using both the observed stellar disc and HI gas as proxies. The results are
compared with two models for exotic cold DM: internally consistent cosmological
NFW models with constrained compactness parameters, and URC models using fully
unconstrained Burkert density profiles. The Bosma models that use the stellar
discs as additional proxies are statistically nearly as good as the URC models
and clearly better than the NFW ones. We thus confirm the correlation between
the centripetal effects of DM and that of the interstellar medium of spiral
galaxies. The edificacy of "maximal disc" models is explained as the natural
consequence of "classic" Bosma models which include the stellar disc as a proxy
in regions of reduced atomic gas. The standard explanation - that the effect
reflects a statistical correlation between the visible and exotic DM - seems
highly unlikely, given that the geometric forms and hence centripetal
signatures of spherical halo and disc components are so different. A literal
interpretation of the Bosma effect as being due to the presence of significant
amounts of disc DM requires a median visible baryon to disc DM ratio of about
40%.Comment: Accepted by A&A (Paper I
Semigroup approach to birth-and-death stochastic dynamics in continuum
We describe a general approach to the construction of a state evolution
corresponding to the Markov generator of a spatial birth-and-death dynamics in
. We present conditions on the birth-and-death intensities which
are sufficient for the existence of an evolution as a strongly continuous
semigroup in a proper Banach space of correlation functions satisfying the
Ruelle bound. The convergence of a Vlasov-type scaling for the corresponding
stochastic dynamics is considered.Comment: 35 page
Fermi acceleration and suppression of Fermi acceleration in a time-dependent Lorentz Gas
We study some dynamical properties of a Lorentz gas. We have considered both
the static and time dependent boundary. For the static case we have shown that
the system has a chaotic component characterized with a positive Lyapunov
Exponent. For the time-dependent perturbation we describe the model using a
four-dimensional nonlinear map. The behaviour of the average velocity is
considered in two situations (i) non-dissipative and (ii) dissipative. Our
results show that the unlimited energy growth is observed for the
non-dissipative case. However, when dissipation, via damping coefficients, is
introduced the senary changes and the unlimited engergy growth is suppressed.
The behaviour of the average velocity is described using scaling approach
The Central Charge of the Warped AdS^3 Black Hole
The AdS/CFT conjecture offers the possibility of a quantum description for a
black hole in terms of a CFT. This has led to the study of general AdS^3 type
black holes with a view to constructing an explicit toy quantum black hole
model. Such a CFT description would be characterized by its central charge and
the dimensions of its primary fields. Recently the expression for the central
charges (C_L, C_R) of the CFT dual to the warped AdS^3 have been determined
using asymptotic symmetry arguments. The central charges depend, as expected,
on the warping factor. We show that topological arguments, used by Witten to
constrain central charges for the BTZ black hole, can be generalized to deal
with the warped AdS^3 case. Topology constrains the warped factor to be
rational numbers while quasinormal modes are conjectured to give the dimensions
of primary fields. We find that in the limit when warping is large or when it
takes special rational values the system tends to Witten's conjectured unique
CFT's with central charges that are multiples of 24.Comment: 6 pages, Latex fil
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