The Two-Dimensional Bose-Einstein Condensate

We study the Hartree-Fock-Bogoliubov mean-field theory as applied to a two-dimensional finite trapped Bose gas at low temperatures and find that, in the Hartree-Fock approximation, the system can be described either with or without the presence of a condensate; this is true in the thermodynamic limit as well. Of the two solutions, the one that includes a condensate has a lower free energy at all temperatures. However, the Hartree-Fock scheme neglects the presence of phonons within the system, and when we allow for the possibility of phonons we are unable to find condensed solutions; the uncondensed solutions, on the other hand, are valid also in the latter, more general scheme. Our results confirm that low-energy phonons destabilize the two-dimensional condensate.Comment: 8 pages, 3 figures, REVTeX 4. To appear in J. Low Temp. Phys. Corrected a mistake in a calculation and changed the conclusions accordingl

Economic Crises, High Public Pension Spending and Blame-avoidance Strategies : Pension Policy Retrenchments in 14 Social-insurance Countries, 1981–2005

This paper examines the determinants of the timing of public pension policy retrenchments in 14 affluent democracies. Available research does not satisfactorily capture the multidimensionality of these legislative events, because it relies on indicators of pension policy provisions for current pensioners even though recent retrenchment pension reforms have been characterized by phased-in or grandfathering measures. Instead, this paper identifies these events by considering the individual long-term implications of each pension reform passed in 14 OECD social-insurance countries between 1981 and 2005. Based on a synthetic review of the pension policy literature, data from financial projections, and principles from the economics of welfare programs, I identify 62 pension retrenchments passed in these countries. My argument is that macroeconomic conditions, the size of the public pension system, and the stage in the electoral cycle shape the likelihood of pension retrenchments. Results obtained from conditional frailty models for recurrent and sequential events support this argument. The interval between pension retrenchments is shorter in countries with low economic growth and high public pension spending, as well as in countries in a post-election year.Dieses Papier betrachtet die zeitlichen Muster von Rentenkürzungen und deren Determinanten in wohlhabenden Demokratien. Die derzeitige Forschung berücksichtigt die Multidimensionalität dieser legislativen Maßnahmen nur unzureichend, da sie sich auf die Indikatoren für die aktuelle Rentnerpopulation konzentriert, obwohl diese in Zusammenhang mit bereits eingeleiteten oder früheren gesetzlichen Maßnahmen stehen. Die vorliegende Studie hingegen bezieht die Langzeitfolgen der Rentenreformen und deren Entwicklung in vierzehn OECD Ländern im Zeitraum von 1981 bis 2005 in die Analyse ein. Auf der Grundlage einer zusammenfassenden Bestandsaufnahme der Literatur zur Rentenpolitik, von Daten aus finanziellen Hochrechnungen sowie der ökonomischen Prinzipien von Wohlfahrtsprogrammen werden in diesen Ländern zunächst insgesamt 62 Rentenkürzungsmaßnahmen identifiziert. Zur Erklärung der zeitlichen Abfolge der Maßnahmen werden die makroökonomischen Bedingungen, die Größe des Rentensystems sowie die Zeitpunkte der Anpassungen im Wahlzyklus herangezogen. Die unter Anwendung konditionaler Frailty-Modelle erzielten Resultate stützen das Argument, dass die häufigsten Rentenkürzungen sich in Ländern im Jahr nach der Wahl sowie in Ländern mit geringem Wirtschaftswachstum und hohen Rentenausgaben finden

A, B, C's (and D)'s for Understanding VARs

The dynamics of a linear (or linearized) dynamic stochastic economic model can be expressed in terms of matrices (A,B,C,D) that define a state space system. An associated state space system (A,K,C,Sigma) determines a vector autoregression for observables available to an econometrician. We review circumstances under which the impulse response of the VAR resembles the impulse response associated with the economic model. We give four examples that illustrate a simple condition for checking whether the mapping from VAR shocks to economic shocks is invertible. The condition applies when there are equal numbers of VAR and economic shocks.

Joint effect of ageing and multilayer structure prevents ordering in the voter model

The voter model rules are simple, with agents copying the state of a random neighbor, but they lead to non-trivial dynamics. Besides opinion processes, the model has also applications for catalysis and species competition. Inspired by the temporal inhomogeneities found in human interactions, one can introduce ageing in the agents: the probability to update decreases with the time elapsed since the last change. This modified dynamics induces an approach to consensus via coarsening in complex networks. Additionally, multilayer networks produce profound changes in the dynamics of models. In this work, we investigate how a multilayer structure affects the dynamics of an ageing voter model. The system is studied as a function of the fraction of nodes sharing states across layers (multiplexity parameter q ). We find that the dynamics of the system suffers a notable change at an intermediate value q*. Above it, the voter model always orders to an absorbing configuration. While, below, a fraction of the realizations falls into dynamical traps associated to a spontaneous symmetry breaking in which the majority opinion in the different layers takes opposite signs and that due to the ageing indefinitely delay the arrival at the absorbing state.Comment: 10 pages, 8 figure

A, B, C’s (And D’s) For Understanding VARS

The dynamics of a linear (or linearized) dynamic stochastic economic model can be expressed in terms of matrices (A,B,C,D) that define a state space system. An associated state space system (A,K,C, Sigma) determines a vector autoregression for observables available to an econometrician. We review circumstances under which the impulse response of the VAR resembles the impulse response associated with the economic model. We give four examples that illustrate a simple condition for checking whether the mapping from VAR shocks to economic shocks is invertible. The condition applies when there are equal numbers of VAR and economic shocks.VARs , Invertibility, Estimation of Dynamic Equilibrium Models, economic shocks, innovations

Magnetic-field control of near-field radiative heat transfer and the realization of highly tunable hyperbolic thermal emitters

We present a comprehensive theoretical study of the magnetic field dependence of the near-field radiative heat transfer (NFRHT) between two parallel plates. We show that when the plates are made of doped semiconductors, the near-field thermal radiation can be severely affected by the application of a static magnetic field. We find that irrespective of its direction, the presence of a magnetic field reduces the radiative heat conductance, and dramatic reductions up to 700% can be found with fields of about 6 T at room temperature. We show that this striking behavior is due to the fact that the magnetic field radically changes the nature of the NFRHT. The field not only affects the electromagnetic surface waves (both plasmons and phonon polaritons) that normally dominate the near-field radiation in doped semiconductors, but it also induces hyperbolic modes that progressively dominate the heat transfer as the field increases. In particular, we show that when the field is perpendicular to the plates, the semiconductors become ideal hyperbolic near-field emitters. More importantly, by changing the magnetic field, the system can be continuously tuned from a situation where the surface waves dominate the heat transfer to a situation where hyperbolic modes completely govern the near-field thermal radiation. We show that this high tunability can be achieved with accessible magnetic fields and very common materials like n-doped InSb or Si. Our study paves the way for an active control of NFRHT and it opens the possibility to study unique hyperbolic thermal emitters without the need to resort to complicated metamaterials.Comment: 21 pages, 10 figure

Mechanics of Bistable Two-Shelled Composite Booms

The phenomenon of bistability in single-walled composite cylindrical shells or slit tubes has been extensively studied with detailed models that represent the mechanics of these structures as they undergo large deformations from the extended to the stored state and vice versa. This study focuses on the mechanics of bistable composite booms that are formed by coupling or bonding two thin shells. A two-parameter inextensional analytical model is used to describe the behavior of the various two-shelled structures and find laminates and shell geometries of interest that induce bistability. The natural coiled diameters of all boom types are predicted analytically and compared with preliminary experimental data. Using the derived model, parametric analysis is conducted to determine optimal boom geometries that maximize stiffnesses and meet system requirements while retaining bistability

A study of some genetically determined features in Swedish, Spanish and North English populations

A surrey has been conducted with the assistance of the reflectophotometric techniques within the populations of three different countries - Sweden, Spain and England - in order to study the pigmentary differences between and within them, and to what extent they obey to environmental instead of genetic diversity. In order to illustrate the differences between accurately determining the colour of a skin and looking at its adaptive role, a detailed exposition of the physics of colour has been undertaken in the introductory chapter. In recognition of the the effects of human behaviour and the physical milieu in moulding the phenotypical manifestations of the genetical structure of a population, extensive information was gathered about the ethnic origins of people and about their leisure and working habits. That information - together with other about non- cutaneous pigmentation, stature, blood groups and their reflectance readings - was codified and transformed into variables, in order to study their associations (if any) with each other. That task has been undertaken in chapters III to VII, where the combined effect of the behavioural variables was in certain cases found capable of explaining as much as 35% of the internal variability. Some significant differences were also found to parallel climatic factors such as insolation and precipitation. In chapter V11 a number of significant associations were found between pigmentation, blood groups and stature - and were attempted to be put in relation with certain racial strains. Chapter VIII constitutes an enquiry into the subject of mating preferences and its potential effect on the genetic structure of the populations under study. The author’s samples have, in the last chapter, been compared to other Caucasoid populations, arriving to the conclusion that a non-negligible part of the differences is probably due to plain instrumental error