Felipe Torres

A Quantum Exchange Bias Model for Coupling Across a Nonmagnetic Interlayer

Exchange bias (EB) has been observed in systems where a paramagnetic (PM) spacer separates an antiferromagnet (AFM) from a ferromagnet. This coupling extends to PM spacer thicknesses of up to 100Å in Py/(Ag, Au, Cu)/CoO systems. We develop a quantum fluctuation model in which a dipole field is created at the AFM surface due to the breaking of translational invariance symmetry. The uncompensated spins of the semiinfinite AFM induce a field that extends all the way to the CoO/(Ag, Au, Cu) interface and thus generate EB. The magnitude of the EB field that the present model yields is of the same order of magnitude as the one measured in recent experiments.

Dzyaloshinskii-Moriya interaction and magnetic ordering in 1D and 2D at nonzero T

The inclusion of a Dzyaloshinskii-Moriya short-range antisymmetric interaction in the Heisenberg Hamiltonian induces spontaneous magnetization, at nonzero temperatures, in one and two dimensions. It is shown that quantum fluctuations are reduced by the Dzyaloshinskii-Moriya interaction, but short-range correlations are increased, thereby allowing the existence of long-range magnetic order in these low-dimensional systems.

The Stochastic Transport Dynamics of a Conserved Quantity on a Complex Network

The stochastic dynamics of conserved quantities is an emergent phenomena observed in many complex systems, ranging from social and to biological networks. Using an extension of the Ehrenfest urn model on a complex network, over which a conserved quantity is transported in a random fashion, we study the dynamics of many elementary packets transported through the network by means of a master equation approach and compare with the mean field approximation and stochastic simulations. By use of the mean field theory, it is possible to compute an approximation to the ensemble average evolution of the number of packets in each node which, in the thermodynamic limit, agrees quite well with the results of the master equation. However, the master equation gives a more complete description of the stochastic system and provides a probabilistic view of the occupation number at each node. Of particular relevance is the standard deviation of the occupation number at each node, which is not uniform for a complex network. We analyze and compare different network topologies (small world, scale free, Erdos-Renyi, among others). Given the computational complexity of directly evaluating the asymptotic, or equilibrium, occupation number probability distribution, we propose a scaling relation with the number of packets in the network, that allows to construct the asymptotic probability distributions from the network with one packet. The approximation, which relies on the same matrix found in the mean field approach, becomes increasingly more accurate for a large number of packets.

Nucleation of superfluid-light domains in a quenched dynamics

Strong correlation effects emerge from light-matter interactions in coupled resonator arrays, such as the Mott-insulator to superfluid phase transition of atom-photon excitations. We demonstrate that the quenched dynamics of a finite-sized complex array of coupled resonators induces a first-order like phase transition. The latter is accompanied by domain nucleation that can be used to manipulate the photonic transport properties of the simulated superfluid phase; this in turn leads to an empirical scaling law. This universal behavior emerges from the light-matter interaction and the topology of the array. The validity of our results over a wide range of complex architectures might lead to a promising device for use in scaled quantum simulations.

Author Correction: Nucleation of superfluid-light domains in a quenched dynamics

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Nucleation of superfluid-light domains

Electromagnetic coupled resonator arrays (CRAs) doped with a quantum two-level system allow for the quantum simulation of a Mott-insulator to superfluid phase transition. We demonstrate that the order of this simulated phase transition depends on the type of dynamics. Thus, a first order like phase transition can be induced by a quench dynamics, while a second order like phase transition is produced by an adiabatic dynamics. In addition, we show that the underlying macroscopic behavior of the phase transition in other many body systems, such as domain nucleation and phase coexistence, can also be observed in CRAs. This universal behavior emerges from the light-matter interaction and the topology of the array. Therefore, the latter can be used to manipulate the photonic transport properties of the simulated super fluid phase.

Updated molecular phylogeny of the squid family Ommastrephidae: Insights into the evolution of spawning strategies

Two types of spawning strategy have been described for ommastrephid squids: coastal and oceanic. It has been suggested that ancestral ommastrephids inhabited coastal waters and expanded their distribution into the open ocean during global changes in ocean circulation in the Oligocene. This hypothesis could explain the different reproductive strategies in oceanic squids, but has never been tested in a phylogenetic context. In the present study, we assess the coastal-to-open-ocean hypothesis through inferring the evolution of reproductive traits (spawning type) of ommastrephid squids using the phylogenetic comparative method to estimate ancestral states and divergence times. This analysis was performed using a robust molecular phylogeny with three mitochondrial genes (COI, CYTB and 16S) and two nuclear genes (RHO and 18S) for nearly all species of ommastrephid squid. Our results support dividing the Ommastrephidae into the three traditional subfamilies, plus the monotypic subfamily Todaropsinae as proposed previously. Divergence times were found to be older than those suggested. Our analyses strongly suggest that early ommastrephid squids spawned in coastal areas, with some species subsequently switching to spawn in oceanic areas, supporting previous non-tested hypotheses. We found evidence of gradual evolution change of spawning type in ommastrephid squids estimated to have occurred since the Cretaceous.

Neuropsychological assessment of Chilean children with a history of extreme prematurity: An exploratory study

Abstract This study was conducted to explore the neuropsychological abilities of premature Chilean children. Two groups (Premature and Control, 10 children each, age ranging from 5 to 7.11) were established based on weeks of gestation and/or weight at birth. Relevant variables such as age, gender, schooling, and socioeconomic level were matched considering Chile’s particular demographic context. Children were assessed by means of the Evaluación Neuropsicológica Infantil (ENI-2) battery, measuring nine cognitive domains encompassing 23 subscales. In turn, subscales are grouped in two scales: Cognitive Functions and Executive Functions. Since the ENI-2 battery provides norms for Spanish-speaking children, obtained data were inspected both for possible between-group differences and either adjustment or deviance from average range. Results show that premature children perform within typical ranges in all subscales except for Visual attention and Graphic fluency. When comparing both groups, some differences emerged. These differences are most prominent in subscales related to visuoperceptual skills. Interestingly, between-group linguistic performance is very similar. The point is made that early linguistic interventions conducted on premature children seem to positively impact on oral language expression and comprehension. On the contrary, early interventions focused on visuospatial abilities did not seem to attain the same impact. This may be a consequence of visual-information processing problems derived from cortical dorsal stream’s vulnerability, which literature correlates with prematurity.

Topological phase transition of a fractal spin system: The relevance of the network complexity

A new type of collective excitations, due to the topology of a complex random network that can be characterized by a fractal dimension DF, is investigated. We show analytically that these excitations generate phase transitions due to the non-periodic topology of the DF > 1 complex network. An Ising system, with long range interactions, is studied in detail to support the claim. The analytic treatment is possible because the evaluation of the partition function can be decomposed into closed factor loops, in spite of the architectural complexity. The removal of the infrared divergences leads to an unconventional phase transition, with spin correlations that are robust against thermal fluctuations.