27,057 research outputs found
Bures metric over thermal state manifolds and quantum criticality
We analyze the Bures metric over the manifold of thermal density matrices for
systems featuring a zero temperature quantum phase transition. We show that the
quantum critical region can be characterized in terms of the temperature
scaling behavior of the metric tensor itself. Furthermore, the analysis of the
metric tensor when both temperature and an external field are varied, allows to
complement the understanding of the phase diagram including cross-over regions
which are not characterized by any singular behavior. These results provide a
further extension of the scope of the metric approach to quantum criticality.Comment: 9 pages, 4 figures, LaTeX problems fixed, references adde
Liquid relaxation: A new Parodi-like relation for nematic liquid crystals
We put forward a hydrodynamic theory of nematic liquid crystals that includes
both anisotropic elasticity and dynamic relaxation. Liquid remodeling is
encompassed through a continuous update of the shear-stress free configuration.
The low-frequency limit of the dynamical theory reproduces the classical
Ericksen-Leslie theory, but it predicts two independent identities between the
six Leslie viscosity coefficients. One replicates Parodi's relation, while the
other-which involves five Leslie viscosities in a nonlinear way-is new. We
discuss its significance, and we test its validity against evidence from
physical experiments, independent theoretical predictions, and
molecular-dynamics simulations.Comment: 6 pages, 1 figure, 2 table
TMsim : an algorithmic tool for the parametric and worst-case simulation of systems with uncertainties
This paper presents a general purpose, algebraic tool—named TMsim—for the combined parametric and worst-case analysis of systems with bounded uncertain parameters.The tool is based on the theory of Taylor models and represents uncertain variables on a bounded domain in terms of a Taylor polynomial plus an interval remainder accounting for truncation and round-off errors.This representation is propagated from inputs to outputs by means of a suitable redefinition of the involved calculations, in both scalar and matrix form. The polynomial provides a parametric approximation of the variable, while the remainder gives a conservative bound of the associated error. The combination between the bound of the polynomial and the interval remainder provides an estimation of the overall (worst-case) bound of the variable. After a preliminary theoretical background, the tool (freely available online) is introduced step by step along with the necessary theoretical notions. As a validation, it is applied to illustrative examples as well as to real-life problems of relevance in electrical engineering applications, specifically a quarter-car model and a continuous time linear equalizer
Time Dependent Modeling of the Markarian 501 X-ray and TeV Gamma-Ray Data Taken During March and April, 1997
If the high-energy emission from TeV blazars is produced by the Synchrotron
Self-Compton (SSC) mechanism, then simultaneous X-ray and Gamma-ray
observations of these objects are a powerful probe of the electron (and/or
positron) populations responsible for this emission. Understanding the emitting
particle distributions and their evolution in turn allow us to probe physical
conditions in the inner blazar jet and test, for example, various acceleration
scenarios. By constraining the SSC emission model parameters, such observations
also allow us to predict the intrinsic (unabsorbed) Gamma-ray spectra of these
sources, a major uncertainty in current attempts to use the observed Gamma-ray
spectra to constrain the intensity of the extragalactic background at
optical/infrared wavelengths. As a next step in testing the SSC model and as a
demonstration of the potential power of coordinated X-ray and Gamma-ray
observations, we attempt to model in detail the X-ray and Gamma-ray light
curves of the TeV Blazar Mrk 501 during its April-May 1997 outburst using a
time dependent SSC emission model. Extensive, quasi-simultaneous X-ray and
gamma-ray coverage exists for this period. We discuss and explore
quantitatively several of the flare scenarios presented in the literature. We
show that simple two-component models (with a soft, steady X-ray component plus
a variable SSC component) involving substantial pre-acceleration of electrons
to Lorentz factors on the order of 1E+5 describe the data train surprisingly
well. All considered models imply an emission region that is strongly out of
equipartition and low radiative efficiencies (ratio between kinetic jet
luminosity and comoving radiative luminosity) of 1 per-mill and less.Comment: 16 pages, Refereed Manuscript. Minor changes to previous versio
Analytic Hypoellipticity in the Presence of Lower Order Terms
We consider a second order operator with analytic coefficients whose
principal symbol vanishes exactly to order two on a symplectic real analytic
manifold. We assume that the first (non degenerate) eigenvalue vanishes on a
symplectic submanifold of the characteristic manifold. In the
framework this situation would mean a loss of 3/2 derivatives. We prove that
this operator is analytic hypoelliptic. The main tool is the FBI transform. A
case in which hypoellipticity fails is also discussed.Comment: 40 page
World Heritage: Where are we? An empirical analysis
A statistical analysis of the UNESCO World Heritage List is presented. The World Heritage Convention intends to protect global heritage of outstanding value to mankind, but there has been great concern about the missing representativity of the member countries. There is a strongly biased distribution of Sites according to a country’s population, area or per capita income. The paper reveals the facts but refrains from judging whether the existing distribution is appropriate or not. This task must be left to the discussion in the World Heritage Convention.Global public goods, world heritage, international organizations, international political economy, culture, UNESCO
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