The dominant spin relaxation mechanism in compound organic semiconductors

Despite the recent interest in "organic spintronics", the dominant spin relaxation mechanism of electrons or holes in an organic compound semiconductor has not been conclusively identified. There have been sporadic suggestions that it might be hyperfine interaction caused by background nuclear spins, but no confirmatory evidence to support this has ever been presented. Here, we report the electric-field dependence of the spin diffusion length in an organic spin-valve structure consisting of an Alq3 spacer layer, and argue that this data, as well as available data on the temperature dependence of this length, contradict the notion that hyperfine interactions relax spin. Instead, they suggest that the Elliott-Yafet mechanism, arising from spin-orbit interaction, is more likely the dominant spin relaxing mechanism.Comment: Accepted for publication in Physical Review

A survey of max-type recursive distributional equations

In certain problems in a variety of applied probability settings (from probabilistic analysis of algorithms to statistical physics), the central requirement is to solve a recursive distributional equation of the form X =^d g((\xi_i,X_i),i\geq 1). Here (\xi_i) and g(\cdot) are given and the X_i are independent copies of the unknown distribution X. We survey this area, emphasizing examples where the function g(\cdot) is essentially a ``maximum'' or ``minimum'' function. We draw attention to the theoretical question of endogeny: in the associated recursive tree process X_i, are the X_i measurable functions of the innovations process (\xi_i)?Comment: Published at http://dx.doi.org/10.1214/105051605000000142 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org

How to Combine Fast Heuristic Markov Chain Monte Carlo with Slow Exact Sampling

Use each of n exact samples as the initial state for a MCMC sampler run for m steps. We give confidence intervals for accuracy of estimators which are always valid and which, in certain settings, are almost as good as the intervals one would obtain if the (unknown) mixing time of the chain were known.Comment: 14 page

A latent factor model for spatial data with informative missingness

A large amount of data is typically collected during a periodontal exam. Analyzing these data poses several challenges. Several types of measurements are taken at many locations throughout the mouth. These spatially-referenced data are a mix of binary and continuous responses, making joint modeling difficult. Also, most patients have missing teeth. Periodontal disease is a leading cause of tooth loss, so it is likely that the number and location of missing teeth informs about the patient's periodontal health. In this paper we develop a multivariate spatial framework for these data which jointly models the binary and continuous responses as a function of a single latent spatial process representing general periodontal health. We also use the latent spatial process to model the location of missing teeth. We show using simulated and real data that exploiting spatial associations and jointly modeling the responses and locations of missing teeth mitigates the problems presented by these data.Comment: Published in at http://dx.doi.org/10.1214/09-AOAS278 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

The determinants of aid in the post-cold war era

The authors estimate the responsiveness of aid to recipient countries' economic and physical needs, civil/political rights, and government effectiveness. They look exclusively at the post-Cold War era and use fixed effects to control for the political, strategic, and other considerations of donors. They find that aid and per capita income have been negatively related, while aid has been positively related to infant mortality, rights, and government effectiveness.Foreign aid program

Is there too little immigration? an analysis of temporary skilled migration

This paper presents a model of legal migration of temporary skilled workers from one source country to two host countries, both of which can control their levels of such immigration. Because of complementarities between capital and labor, the return on capital is positively related to the level of immigration. Consequently, when capital is immobile, host nations’ optimal levels of immigration are positively related to their capital endowments. Further, when capital is mobile between the host nations, the common return on capital is a function of the levels of immigration in both countries, meaning that immigration is a public good. As a result, when immigration imposes costs on host countries, the Nash equilibrium results in free riding and less immigration than would occur in the cooperative equilibrium. These results are qualitatively unaltered when capital mobility extends to the source nation.Emigration and immigration

Immigration and outsourcing: a general equilibrium analysis

We analyze the effects of outsourcing in the presence of a minimum wage by presenting a general-equilibrium model with an oligopolistic export sector and a competitive import-competing sector. An outsourcing tax is politically popular because it switches jobs to unemployed natives. It is also economically sound because it raises national income. An export subsidy may or may not be justified on welfare grounds. Increased international competition has no effect on the level of outsourcing, but the direction of its effect on unemployment and national income depends on the relative factor intensities of the two sectors.Immigrants ; Labor market ; Contracting out

Complex small-scale structure in the infrared extinction towards the Galactic Centre

A high level of complex structure, or ``granularity'', has been observed in the distribution of infrared-obscuring material towards the Galactic Centre (GC), with a characteristic scale of 5arcsec - 15arcsec, corresponding to 0.2 - 0.6pc at a GC distance of 8.5kpc. This structure has been observed in ISAAC images which have a resolution of 0.6arcsec, significantly higher than that of previous studies of the GC. We have discovered granularity throughout the GC survey region, which covers an area of 1.6deg x 0.8deg in longitude and latitude respectively (300pc x 120pc at 8.5kpc) centred on Sgr A*. This granularity is variable over the whole region, with some areas exhibiting highly structured extinction in one or more wavebands and other areas displaying no structure and a uniform stellar distribution in all wavebands. The granularity does not appear to correspond to longitude, latitude or radial distance from Sgr A*. We find that regions exhibiting high granularity are strongly associated with high stellar reddening.Comment: 5 pages, 3 figures, accepted for publication in ApJ