Pairing instabilities of Dirac composite fermions

Recently, a Dirac (particle-hole symmetric) description of composite fermions in the half-filled quantum Hall system was proposed [D. T. Son, Phys. Rev. X 5, 031027 (2015)], and we study its possible consequences on BCS (Cooper) pairing of composite fermions (CF's). One of the main consequences is the existence of anisotropic states in single and bilayer systems, which was previously suggested in Ref. [J. S. Jeong and K. Park, Phys. Rev. B 91, 195119 (2015)]. We argue that in the half-filled single layer the gapped states may sustain anisotropy, because isotropic pairings may coexist with anisotropic ones. Furthermore, anisotropic pairings with addition of a particle-hole (PH) symmetry breaking mass term may evolve into rotationally symmetric states, i.e. Pfaffian states of Halperin-Lee-Read (HLR) ordinary CF's. On the basis of the Dirac formalism, we argue that in the quantum Hall bilayer at total filling one, with decreasing distance between layers weak pairing of p-wave paired CF's is gradually transformed from Dirac to ordinary, HLR-like, with concomitant decrease in the CF number. Global characterization of low-energy spectrum based on the Dirac CF's agrees well with previous calculations performed by exact diagonalization on a torus. Finally, we discuss features of Dirac formalism when applied in this context.Comment: 8 pages, no figures, published versio

Magnetic susceptibility anisotropies in a two-dimensional quantum Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions

The magnetic and thermodynamic properties of the two-dimensional quantum Heisenberg antiferromagnet that incorporates both a Dzyaloshinskii-Moriya and pseudo-dipolar interactions are studied within the framework of a generalized nonlinear sigma model (NLSM). We calculate the static uniform susceptibility and sublattice magnetization as a function of temperature and we show that: i) the magnetic-response is anisotropic and differs qualitatively from the expected behavior of a conventional easy-axis QHAF; ii) the Neel second-order phase transition becomes a crossover, for a magnetic field B perpendicular to the CuO(2) layers. We provide a simple and clear explanation for all the recently reported unusual magnetic anisotropies in the low-field susceptibility of La(2)CuO(4), L. N. Lavrov et al., Phys. Rev. Lett. 87, 017007 (2001), and we demonstrate explicitly why La(2)CuO(4) can not be classified as an ordinary easy-axis antiferromagnet.Comment: 6 pages, 3 figures, Revtex4, accepted for publication in Phys. Rev.

Dynamics of topological defects in a spiral: a scenario for the spin-glass phase of cuprates

We propose that the dissipative dynamics of topological defects in a spiral state is responsible for the transport properties in the spin-glass phase of cuprates. Using the collective-coordinate method, we show that topological defects are coupled to a bath of magnetic excitations. By integrating out the bath degrees of freedom, we find that the dynamical properties of the topological defects are dissipative. The calculated damping matrix is related to the in-plane resistivity, which exhibits an anisotropy and linear temperature dependence in agreement with experimental data.Comment: 4 pages, as publishe

Derivation of the generalized Non Linear Sigma Model in the presence of the Dzyaloshinskii-Moriya interaction

We derive the long-wavelength non-linear sigma model for a two-dimensional Heisenberg system in the presence of the Dzyaloshinskii-Moriya and pseudodipolar interactions. We show that the system is a non-conventional easy-axis antiferromagnet, displaying an anomalous coupling between the magnetic field and the staggered order parameter. Our results are in good agreement with recent experimental data for undoped La2CuO4 compounds.Comment: Proceedings of SCES05, to appear on Physica

Optimización del comportamiento

El comportamiento satisface u optimiza la obtención de un recurso en función de su abundancia y de la capacidad de utilizarlo por parte del individuo. La optimización del comportamiento se puede conseguir maximizando la tasa neta de obtención y manejo del recurso, la eficiencia o un valor intermedio entre la tasa neta y la eficiencia, dependiendo de las circunstancias en las que el individuo ejecuta una pauta de comportamiento. Los tipos de comportamiento más frecuentes en los análisis de optimización son la composición de la dieta, el tiempo de permanencia en un lugar y cualquier decisión que incluya el retorno a un lugar central. En todos ellos se debe estimar la diferencia entre el valor del recurso que el individuo ha seleccionado con respecto al valor promedio de ese recurso en el ambiente donde el animal puede desplazarse. La variabilidad temporal y en cantidad del recurso que se puede obtener favorece a los animales que son sensibles a la variabilidad o riesgo, los cuales maximizan la tasa de obtención del recurso a corto plazo. Dependiendo de sus reservas y expectativas de obtención del recurso, los individuos sensibles al riesgo evitan las situaciones con alta variabilidad temporal cuando sus reservas son altas y las expectativas bajas. Cuando la variabilidad afecta a la cantidad de recurso, los animales con pocas reservas pueden optar por el riesgo para la obtención del recurso, pero este fenómeno de sensibilidad al riesgo en cantidad de recurso es menos frecuente a la sensibilidad al riesgo temporal. La optimización del comportamiento en presencia de otros individuos se puede clasificar en dos grandes categorías: economías de agregación (tasa de obtención de recursos aumenta en ciertos tamaños de grupo) y economías de dispersión (tasa de obtención de recursos disminuye con el tamaño de grupo). En una economía de agregación, la tasa de obtención de recursos suele tener un pico máximo a un tamaño de grupo óptimo, aunque los grupos pueden agrandarse hasta llegar al tamaño estable. Sin embargo, grupos mayores que el tamaño estable se consideran dentro de un equilibrio inestable debido a que la tasa de obtención de recursos es menor que si el individuo se alimentara solitariamente. En una economía de dispersión, la presencia de otros individuos induce cambios en la selección del lugar de obtención del recurso, de manera que en circunstancias de renovación constante del recurso y tiempo suficiente para el cambio de lugar se produce una distribución de los individuos entre zonas hasta alcanzar un equilibrio en, por ejemplo, la maximización de la tasa neta de obtención del recurso, lo que se denomina una distribución libre-ideal de los individuos. En general, la optimización del comportamiento no suele ser perfecta, lo que se traduce en una distribución subóptima de los individuos, que ocupan en mayor proporción las zonas con menor cantidad de recurso. Con independencia de las circunstancias en los que los animales optimizan el comportamiento y de las reglas que maximizan el beneficio obtenido, los modelos de optimización se han revelado como una herramienta útil para investigar el comportamiento. La posibilidad de que el individuo maximice la tasa de obtención de un recurso induce al investigador a plantear tal posibilidad como una hipótesis nula en su trabajo, de manera que es preciso estimar el valor del recurso que el animal intenta conseguir, los tiempo de obtención y manejo y los costes de su obtención, así como su valor marginal y el efecto que puede tener la competencia o simplemente la interferencia con otros individuos del grupo en la optimización del comportamiento. El rechazo de la optimización como hipótesis nula puede llevar a nuevos descubrimientos sobre las restricciones, límites e incluso nuevas reglas de comportamiento, que de manera contraintuitiva pueden revelar pautas de comportamiento subóptimas pero adaptativasPeer reviewe

Dissipative dynamics of topological defects in frustrated Heisenberg spin systems

We study the dynamics of topological defects of a frustrated spin system displaying spiral order. As a starting point we consider the SO(3) nonlinear sigma model to describe long-wavelength fluctuations around the noncollinear spiral state. Besides the usual spin-wave magnetic excitations, the model allows for topologically non-trivial static solutions of the equations of motion, associated with the change of chirality (clockwise or counterclockwise) of the spiral. We consider two types of these topological defects, single vortices and vortex-antivortex pairs, and quantize the corresponding solutions by generalizing the semiclassical approach to a non-Abelian field theory. The use of the collective coordinates allows us to represent the defect as a particle coupled to a bath of harmonic oscillators, which can be integrated out employing the Feynman-Vernon path-integral formalism. The resulting effective action for the defect indicates that its motion is damped due to the scattering by the magnons. We derive a general expression for the damping coefficient of the defect, and evaluate its temperature dependence in both cases, for a single vortex and for a vortex-antivortex pair. Finally, we consider an application of the model for cuprates, where a spiral state has been argued to be realized in the spin-glass regime. By assuming that the defect motion contributes to the dissipative dynamics of the charges, we can compare our results with the measured inverse mobility in a wide range of temperature. The relatively good agreement between our calculations and the experiments confirms the possible relevance of an incommensurate spiral order for lightly doped cuprates.Comment: 22 pages, 7 figures, final published versio

Antiferromagnetic Ising spin glass competing with BCS pairing interaction in a transverse field

The competition among spin glass (SG), antiferromagnetism (AF) and local pairing superconductivity (PAIR) is studied in a two-sublattice fermionic Ising spin glass model with a local BCS pairing interaction in the presence of an applied magnetic transverse field $\Gamma$. In the present approach, spins in different sublattices interact with a Gaussian random coupling with an antiferromagnetic mean $J_0$ and standard deviation $J$. The problem is formulated in the path integral formalism in which spin operators are represented by bilinear combinations of Grassmann variables. The saddle-point Grand Canonical potential is obtained within the static approximation and the replica symmetric ansatz. The results are analysed in phase diagrams in which the AF and the SG phases can occur for small $g$ ($g$ is the strength of the local superconductor coupling written in units of $J$), while the PAIR phase appears as unique solution for large $g$. However, there is a complex line transition separating the PAIR phase from the others. It is second order at high temperature that ends in a tricritical point. The quantum fluctuations affect deeply the transition lines and the tricritical point due to the presence of $\Gamma$.Comment: 16 pages, 6 figures, accepted Eur. Phys. J.

Physiology, behavior, and conservation

Many animal populations are in decline as a result of human activity. Conservation practitioners are attempting to prevent further declines and loss of biodiversity as well as to facilitate recovery of endangered species, and they often rely on interdisciplinary approaches to generate conservation solutions. Two recent interfaces in conservation science involve animal behavior (i.e., conservation behavior) and physiology (i.e., conservation physiology). To date, these interfaces have been considered separate entities, but from both pragmatic and biological perspectives, there is merit in better integrating behavior and physiology to address applied conservation problems and to inform resource management. Although there are some institutional, conceptual, methodological, and communication-oriented challenges to integrating behavior and physiology to inform conservation actions, most of these barriers can be overcome. Through outlining several successful examples that integrate these disciplines, we conclude that physiology and behavior can together generate meaningful data to support animal conservation and management actions. Tangentially, applied conservation and management problems can, in turn, also help advance and reinvigorate the fundamental disciplines of animal physiology and behavior by providing advanced natural experiments that challenge traditional frameworks

Multi-boson effects and the normalization of the two-pion correlation function

The two-pion correlation function can be defined as a ratio of either the measured momentum distributions or the normalized momentum space probabilities. We show that the first alternative avoids certain ambiguities since then the normalization of the two-pion correlator contains important information on the multiplicity distribution of the event ensemble which is lost in the second alternative. We illustrate this explicitly for specific classes of event ensembles.Comment: 6 pages, three figures,submit to PR