873 research outputs found
Stationary Properties of a Randomly Driven Ising Ferromagnet
We consider the behavior of an Ising ferromagnet obeying the Glauber dynamics
under the influence of a fast switching, random external field. Analytic
results for the stationary state are presented in mean-field approximation,
exhibiting a novel type of first order phase transition related to dynamic
freezing. Monte Carlo simulations performed on a quadratic lattice indicate
that many features of the mean field theory may survive the presence of
fluctuations.Comment: 5 pages in RevTex format, 7 eps/ps figures, send comments to
"mailto:[email protected]", submitted to PR
Phase diagram of the random field Ising model on the Bethe lattice
The phase diagram of the random field Ising model on the Bethe lattice with a
symmetric dichotomous random field is closely investigated with respect to the
transition between the ferromagnetic and paramagnetic regime. Refining
arguments of Bleher, Ruiz and Zagrebnov [J. Stat. Phys. 93, 33 (1998)] an exact
upper bound for the existence of a unique paramagnetic phase is found which
considerably improves the earlier results. Several numerical estimates of
transition lines between a ferromagnetic and a paramagnetic regime are
presented. The obtained results do not coincide with a lower bound for the
onset of ferromagnetism proposed by Bruinsma [Phys. Rev. B 30, 289 (1984)]. If
the latter one proves correct this would hint to a region of coexistence of
stable ferromagnetic phases and a stable paramagnetic phase.Comment: Article has been condensed and reorganized; Figs 3,5,6 merged; Fig 4
omitted; Some discussion added at end of Sec. III; 9 pages, 5 figs, RevTeX4,
AMSTe
A two-qubit Bell inequality for which POVM measurements are relevant
A bipartite Bell inequality is derived which is maximally violated on the
two-qubit state space if measurements describable by positive operator valued
measure (POVM) elements are allowed rather than restricting the possible
measurements to projective ones. In particular, the presented Bell inequality
requires POVMs in order to be maximally violated by a maximally entangled
two-qubit state. This answers a question raised by N. Gisin.Comment: 7 pages, 1 figur
A New Method for Computing Topological Pressure
The topological pressure introduced by Ruelle and similar quantities describe
dynamical multifractal properties of dynamical systems. These are important
characteristics of mesoscopic systems in the classical regime. Original
definition of these quantities are based on the symbolic description of the
dynamics. It is hard or impossible to find symbolic description and generating
partition to a general dynamical system, therefore these quantities are often
not accessible for further studies. Here we present a new method by which the
symbolic description can be omitted. We apply the method for a mixing and an
intermittent system.Comment: 8 pages LaTeX with revtex.sty, the 4 postscript figures are included
using psfig.tex to appear in PR
Limitations of squeezing due to collisional decoherence in Bose-Einstein condensates
We study the limitations for entanglement due to collisional decoherence in a
Bose-Einstein condensate. Specifically we consider relative number squeezing
between photons and atoms coupled out from a homogeneous condensate. We study
the decay of excited quasiparticle modes due to collisions, in condensates of
atoms with one or two internal degrees of freedom. The time evolution of these
modes is determined in the linear response approximation to the deviation from
equilibrium. We use Heisenberg-Langevin equations to derive equations of motion
for the densities and higher correlation functions which determine the
squeezing. In this way we can show that decoherence due to quasiparticle
interactions imposes an important limit on the degree of number squeezing which
may be achieved. Our results are also relevant for the determination of
decoherence times in other experiments based on entanglement, e.g. the slowing
and stopping of light in condensed atomic gases using dark states.Comment: 16 pages RevTeX, 3 figure
The Cosmological Probability Density Function for Bianchi Class A Models in Quantum Supergravity
Nicolai's theorem suggests a simple stochastic interpetation for
supersymmetric Euclidean quantum theories, without requiring any inner product
to be defined on the space of states. In order to apply this idea to
supergravity, we first reduce to a one-dimensional theory with local
supersymmetry by the imposition of homogeneity conditions. We then make the
supersymmetry rigid by imposing gauge conditions, and quantise to obtain the
evolution equation for a time-dependent wave function. Owing to the inclusion
of a certain boundary term in the classical action, and a careful treatment of
the initial conditions, the evolution equation has the form of a Fokker-Planck
equation. Of particular interest is the static solution, as this satisfies all
the standard quantum constraints. This is naturally interpreted as a
cosmological probability density function, and is found to coincide with the
square of the magnitude of the conventional wave function for the wormhole
state.Comment: 22 pages, Late
Supersymmetric minisuperspace with non-vanishing fermion number
The Lagrangean of supergravity is dimensionally reduced to one
(time-like) dimension assuming spatial homogeneity of any Bianchi type within
class A of the classification of Ellis and McCallum. The algebra of the
supersymmetry generators, the Lorentz generators, the diffeomorphism generators
and the Hamiltonian generator is determined and found to close. In contrast to
earlier work, infinitely many physical states with non-vanishing even fermion
number are found to exist in these models, indicating that minisuperspace
models in supergravity may be just as useful as in pure gravity.Comment: 4 page
Quantum Cosmology of Generalized Two--Dimensional Dilaton Gravity Models
The quantum cosmology of two-dimensional dilaton-gravity models is
investigated. A class of models is mapped onto the constrained
oscillator-ghost-oscillator model. A number of exact and approximate solutions
to the corresponding Wheeler-DeWitt equation are presented. A wider class of
minisuperspace models that can be solved in this fashion is identified.
Supersymmetric extensions to the induced gravity theory and the bosonic string
theory are then considered and closed-form solutions to the associated quantum
constraints are derived. The possibility of applying the third-quantization
procedure to two-dimensional dilaton-gravity is briefly discussed.Comment: 28 pages, late
Resilience as a policy narrative: potentials and limits in the context of urban planning
The aim of this paper is to analyse the emergence of the concept of ‘urban resilience’ in the literature and to assess its potentials and limitations as an element of policy planning. Using a systematic literature review covering the period 2003–2013 and a combination of techniques derived from narrative analysis, we show that diverse views of what urban resilience means and how it is best used (as a goal or as a conceptual/analytical framework) compete in the literature. Underlying these views are various (and sometimes diverging) interpretations of what the main issues are and what forms of policies or interventions are needed to address these issues. Urban planners need to be better aware of these different interpretations if they want to be in a position to use resilience appropriately and spell out what resilience can bring to their work. The review also highlights that the notion of urban resilience often lacks adequate acknowledgement of the political economy of urbanization and consequently does not challenge the status quo which, some argue, is socially unjust and environmentally unsustainable. As such it runs the risk to be seen as simply making marginalized urban communities more resilient to the shocks and inequity created by the current dominant paradigm
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