From thermal to excited-state quantum phase transitions ---the Dicke model

We study the thermodynamics of the full version of the Dicke model, including all the possible values of the total angular momentum $j$, with both microcanonical and canonical ensembles. We focus on how the excited-state quantum phase transition, which only appears in the microcanonical description of the maximum angular momentum sector, $j=N/2$, change to a standard thermal phase transition when all the sectors are taken into account. We show that both the thermal and the excited-state quantum phase transitions have the same origin; in other words, that both are two faces of the same phenomenon. Despite all the logarithmic singularities which characterize the excited-state quantum phase transition are ruled out when all the $j$-sectors are considered, the critical energy (or temperature) still divides the spectrum in two regions: one in which the parity symmetry can be broken, and another in which this symmetry is always well defined.Comment: Submitted to PRE. Comments are welcome. V2: Updated to match published versio

Multivariate Orthogonal Polynomials and Modified Moment Functionals

Multivariate orthogonal polynomials can be introduced by using a moment functional defined on the linear space of polynomials in several variables with real coefficients. We study the so-called Uvarov and Christoffel modifications obtained by adding to the moment functional a finite set of mass points, or by multiplying it times a polynomial of total degree 2, respectively. Orthogonal polynomials associated with modified moment functionals will be studied, as well as the impact of the modification in useful properties of the orthogonal polynomials. Finally, some illustrative examples will be given

Searching for a Time Ontology for Semantic Web Applications

We present our experience on reusing time ontologies in Fund Finder, a semantic web application of the EU Esperonto project. On the one hand, we show a set of time ontologies implemented in different machine readable languages. Such ontologies are analyzed considering a series of features typical of time models (e.g., if they consider different granularities or different time zones). On the other hand, we present the specification of time modeling necessities for Fund Finder. Finally, we choose the ontology which fits best with the specifications