Interatomic potentials for ionic systems with density functional accuracy based on charge densities obtained by a neural network

Based on an analysis of the short range chemical environment of each atom in a system, standard machine learning based approaches to the construction of interatomic potentials aim at determining directly the central quantity which is the total energy. This prevents for instance an accurate description of the energetics of systems where long range charge transfer is important as well as of ionized systems. We propose therefore not to target directly with machine learning methods the total energy but an intermediate physical quantity namely the charge density, which then in turn allows to determine the total energy. By allowing the electronic charge to distribute itself in an optimal way over the system, we can describe not only neutral but also ionized systems with unprecedented accuracy. We demonstrate the power of our approach for both neutral and ionized NaCl clusters where charge redistribution plays a decisive role for the energetics. We are able to obtain chemical accuracy, i.e. errors of less than a milli Hartree per atom compared to the reference density functional results. The introduction of physically motivated quantities which are determined by the short range atomic environment via a neural network leads also to an increased stability of the machine learning process and transferability of the potential.Comment: 4 figure

Unemployment in Latin America and the Caribbean

This study constructs a new data set on unemployment rates in Latin America and the Caribbean and then explores the determinants of unemployment. We compare different countries, finding that unemployment is influenced by the size of the rural population and that the effects of government regulations are generally weak. We also examine large, persistent increases in unemployment over time, finding that they are caused by contractions in aggregate demand. These demand contractions result from either disinflationary monetary policy or the defense of an exchange-rate peg in the face of capital flight. Our evidence supports hysteresis theories in which short-run changes in unemployment influence the natural rate.

Magnetism and domain formation in SU(3)-symmetric multi-species Fermi mixtures

We study the phase diagram of an SU(3)-symmetric mixture of three-component ultracold fermions with attractive interactions in an optical lattice, including the additional effect on the mixture of an effective three-body constraint induced by three-body losses. We address the properties of the system in $D \geq 2$ by using dynamical mean-field theory and variational Monte Carlo techniques. The phase diagram of the model shows a strong interplay between magnetism and superfluidity. In the absence of the three-body constraint (no losses), the system undergoes a phase transition from a color superfluid phase to a trionic phase, which shows additional particle density modulations at half-filling. Away from the particle-hole symmetric point the color superfluid phase is always spontaneously magnetized, leading to the formation of different color superfluid domains in systems where the total number of particles of each species is conserved. This can be seen as the SU(3) symmetric realization of a more general tendency to phase-separation in three-component Fermi mixtures. The three-body constraint strongly disfavors the trionic phase, stabilizing a (fully magnetized) color superfluid also at strong coupling. With increasing temperature we observe a transition to a non-magnetized SU(3) Fermi liquid phase.Comment: 36 pages, 17 figures; Corrected typo

Reciprocal interactions between the bark beetle-associated yeast Ogataea pini and host plant phytochemistry

Here we report the first experiments testing reciprocal effects between the bark beetle-associated yeast, Ogataea pini, and phytochemicals present in tree tissues (Pinus ponderosa). We tested two hypotheses: (i) tree phytochemicals mediate O. pin,i growth and (ii) O. pini affects chemical composition of plant tissues. We tested six monoterpenes on O. pins biomass growth in vitro and found that most monoterpenes inhibited O. pini growth; however mean O. pini biomass increased 21.5% when treated with myrcene and 75.5% when treated with terpinolene, relative to control. Ogataea pini was grown on phloem tissue ex vivo to determine whether O. pini affected phloem chemistry. Monoterpene concentrations declined in phloem over time, but phloem colonized by O. pini had significantly different concentrations of monoterpenes at two periods than phloem with no yeast. After 7 d, when O. pini was present, concentrations of the monoterpene Delta-3-carene was 42.9% lower than uncolonized phloem and concentrations of the monoterpene terpinolene was 345.0% higher than uncolonized phloem. After 15 d phloem colonized by O. pini had 505.4% higher concentrations of a-pinene than uncolonized phloem. These experiments suggest that O. pini responds to phytochemicals present in host tissues and the presence of O. pini might alter the chemical environment of phloem tissues during the early stages of beetle development. The interactions between O. pini and phytochemicals in pine vascular tissues might have consequences for the bark beetle that vectors O. pini, Dendroctonus brevicomis

Does a magnetic field modify the critical behaviour at the metal-insulator transition in 3-dimensional disordered systems?

The critical behaviour of 3-dimensional disordered systems with magnetic field is investigated by analyzing the spectral fluctuations of the energy spectrum. We show that in the thermodynamic limit we have two different regimes, one for the metallic side and one for the insulating side with different level statistics. The third statistics which occurs only exactly at the critical point is {\it independent} of the magnetic field. The critical behaviour which is determined by the symmetry of the system {\it at} the critical point should therefore be independent of the magnetic field.Comment: 10 pages, Revtex, 4 PostScript figures in uuencoded compressed tar file are appende

Numerical Studies of Fano Resonance in Quantum dots Embedded in AB Rings

The Fano resonance in quantum dots embedded in Aharonov-Bohm rings is examined theoretically, using two models of non-interacting electrons. The first model yields an analytical expression for the conductance G. G is written in an extended Fano form with a complex parameter. The shape of the resonance can be asymmetric or symmetric, depending on the magnetic flux enclosed in the ring. The "phase" of the resonance is changed continuously with increasing the flux in two-terminal situations. These are in accordance with recent experimental results. In the second model, we consider the dephasing effect on the Fano resonance by numerical calculations.Comment: 2 pages, 4 figures, to appear in J. Phys. Soc. Jpn., proceedings of International Conference on Quantum Transport and Quantum Coherence (Localisation 2002, Tokyo

Multiscale coupling and the maximum of P(ϕ)2\mathcal{P}(\phi)_2 models on the torus

We establish a coupling between the $\mathcal{P}(\phi)_2$ measure and the Gaussian free field on the two-dimensional unit torus at all spatial scales, quantified by probabilistic regularity estimates on the difference field. Our result includes the well-studied $\phi^4_2$ measure. The proof uses an exact correspondence between the Polchinski renormalisation group approach, which is used to define the coupling, and the Bou\'e-Dupuis stochastic control representation for $\mathcal{P}(\phi)_2$. More precisely, we show that the difference field is obtained from a specific minimiser of the variational problem. This allows to transfer regularity estimates for the small-scales of minimisers, obtained using discrete harmonic analysis tools, to the difference field. As an application of the coupling, we prove that the maximum of the $\mathcal{P}(\phi)_2$ field on the discretised torus with mesh-size $\epsilon > 0$ converges in distribution to a randomly shifted Gumbel distribution as $\epsilon \rightarrow 0$.Comment: 45 page