Nature of the spin-glass phase in dense packings of Ising dipoles with random anisotropy axes

By Monte Carlo simulations, we study the character of the spinglass (SG) phase in dense disordered packings of magnetic nanoparticles (NPs). We focus on NPs which have large uniaxial anisotropies and can be well represented as Ising dipoles. Dipoles are placed on SC lattices and point along randomly oriented axes. From the behaviour of a SG correlation length we determine the transition temperature Tc between the paramagnetic and a SG phase. For temperatures well below Tc we find distributions of the SG overlap parameter q that are strongly sample-dependent and exhibit several spikes. We find that the average width of spikes, and the fraction of samples with spikes higher than a certain threshold does not vary appreciably with the system sizes studied. We compare these results with the ones found previously for 3D site-diluted systems of parallel Ising dipoles and with the behaviour of the Sherrington-Kirkpatrick model.We thank financial support from MINECO FIS2013-43201-P Gran

Low-temperature spin-glass behaviour in a diluted dipolar Ising system

Using Monte Carlo simulations, we study the character of the spin-glass (SG) state of a site-diluted dipolar Ising model. We consider systems of dipoles randomly placed on a fraction x of all L^3 sites of a simple cubic lattice that point up or down along a given crystalline axis. For x < 0.65 these systems are known to exhibit an equilibrium spin-glass phase below a temperature T_sg proportional to x. At high dilution and very low temperatures, well deep in the SG phase, we find spiky distributions of the overlap parameter q that are strongly sample-dependent. We focus on spikes associated with large excitations. From cumulative distributions of q and a pair correlation function averaged over several thousands of samples we find that, for the system sizes studied, the average width of spikes, and the fraction of samples with spikes higher than a certain threshold does not vary appreciably with L. This is compared with the behaviour found for the Sherrington-Kirkpatrick model.Comment: 9 LaTeX pages, 9 pdf figures, 2 table

A MATCHING MODEL OF CROWDING-OUT AND ON-THE- JOB SEARCH (WITH AN APPLICATION TO SPAIN)

This paper considers a matching model of heterogenous and jobs which includes on-the-job search. High-educated workers transitorily accept unskilled jobs and continue to search for skilled jobs. We study the implications of this model for the unemployment rates of high and low-educated workers, for the share of mismatched workers and wage nequality both within and between skill groups. The model is used to shed light on the Spanish experience following a large educational upgrading since the mid-eighties.

Unemployment and Inflation Persistence in Spain: Are There Phillips Trade-Offs?

This paper studies the dynamic behavior of inflation and unemployment in Spain during the period 1964?1997. In particular, we analyze the implications of high persistence in both unemployment and inflation dynamics for inference regarding the size of Phillips trade-offs and sacrifice ratios in the Spanish economy, in response to a demand shock. To do so we use a Stuctural VAR approach with several identification outlines which give rise to alternative interpretations of the joint unemployment-inflation dynamics. When using a bivariate VAR we cannot reject the existence of a permanent output loss of one-half of one percentage point for each percentage point of permanent disinflation. However, when the VAR is augmented with a third variable, in order to disentangle monetary from non-monetary shocks within the demand class, the evidence favours a lower and marginally permanent trade-off with an output loss of about one-fourth of one percentage point.Publicad

Competition and cost overruns. Optimal misspecification of procurement contracts

Most cases of cost overruns in public procurement are related to important changes in the initial project design. This paper deals with the problem of design specification in public procurement and provides a rationale for design misspecification. We propose a model in which the sponsor decides how much to invest in design specification and awards competitively the project to a contractor. After the project has been awarded the sponsor engages in bilateral renegotiation with the contractor, in order to accommodate changes in the initial project’s design that new information makes desirable. When procurement takes place in the presence of horizontally differentiated contractors, the design’s specification level is seen to affect the resulting degree of competition. The paper highlights this interaction between market competition and design specification and shows that the sponsor’s optimal strategy, when facing an imperfectly competitive market supply, is to underinvest in design specification so as to make significant cost overruns likely. Since no such misspecification occurs in a perfectly competitive market, cost overruns are seen to arise as a consequence of lack of competition in the procurement market.Cost overruns, procurement contracts, strategic ignorance

The Influence of Religious Beliefs and Ethnic Diversity on the HIV/AIDS Epidemic in Latin America and Muslim Countries

In the absence of substantial differences in the growth rates of per capita GDP across countries, what are the factors that explain the rapid spread of the HIV infection within a nation? This is an empirical question that needs to be explained. In order to elucidate the correlation between HIV prevalence and economic growth in a sample of 74 low- and middle-income countries, being Muslim and ethnic diversity can be used as main instruments to produce estimates of the effect of HIV prevalence on the growth rate of real GDP per capita that are not affected by the presence of simultaneity.HIV/AIDS, economic growth, religious belief, ethnic diversity, Latin America, Muslim Countries