On the stability in phase-lag heat conduction with two temperatures

We investigate the well-posedness and the stability of the solutions for several Taylor approximations of the phase-lag two-temperature equations.We give conditions on the parameters which guarantee the existence and uniqueness of solutions as well as the stability and the instability of the solutions for each approximationPeer ReviewedPostprint (author's final draft

Servicios de salud pública en San Salvador: diseño de instrumento de medición

El presente estudio tenía como objetivo hacer una evaluación de la calidad de atención en los servicios de salud en la ciudad de San Salvador. Para ello, se diseñó un instrumento a partir del modelo teórico sugerido por Donabedian (1966) que contiene las dimensiones de estructura, proceso-resultado y accesibilidad. Es un estudio descriptivo mediante encuesta (León y Montero, 2007), un diseño retrospectivo; la muestra es no probabilística de tipo incidental, constituida por 1,210 personas entre hombres (42.1%) y mujeres (57.9%) con edad media de 32.5 años. Entre los resultados se tienen que el cuestionario (ECASS) posee adecuadas propiedades psicométricas de fiabilidad y validez. Así mismo, se encontró que existen diferencias significativas en la calidad de atención en salud entre los centros de atención privada y los públicos, indicando que es de mejor calidad la atención en los centros privados. Algunos indicadores considerados como negativos son: mala atención en el sistema de salud público salvadoreño (63.4%), los tiempos de espera entre una cita y otra, son muy largos (70.8%), en el sistema de salud público el acceso al médico especialista es sumamente difícil (68.4%). Estos resultados pueden ayudar al sistema de salud ha implementar mejoras en el proceso de atención en salud pública.La presente investigación fue subvencionada por la Universidad Tecnológica de El Salvador. Agradezco sinceramente a los estudiantes de psicología de la asignatura Seminario de Investigación por su valioso apoyo y participación en la recolección y tabulación de los datos. Las solicitudes de información, separatas y otros documentos relativos al presente estudio pueden hacerlos a la dirección postal: calle Arce, 1020, Universidad Tecnológica de El Salvador, Vicerrectoría de Investigación y Proyección Social, Dirección de Investigaciones, edificio José Martí, 2ª planta, o al correo electrónico [email protected] Tecnológica de El Salvado

Scaling relation for determining the critical threshold for continuum percolation of overlapping discs of two sizes

We study continuum percolation of overlapping circular discs of two sizes. We propose a phenomenological scaling equation for the increase in the effective size of the larger discs due to the presence of the smaller discs. The critical percolation threshold as a function of the ratio of sizes of discs, for different values of the relative areal densities of two discs, can be described in terms of a scaling function of only one variable. The recent accurate Monte Carlo estimates of critical threshold by Quintanilla and Ziff [Phys. Rev. E, 76 051115 (2007)] are in very good agreement with the proposed scaling relation.Comment: 4 pages, 3 figure

Condensation in an Economic Model with Brand Competition

We present a linear agent based model on brand competition. Each agent belongs to one of the two brands and interacts with its nearest neighbors. In the process the agent can decide to change to the other brand if the move is beneficial. The numerical simulations show that the systems always condenses into a state when all agents belong to a single brand. We study the condensation times for different parameters of the model and the influence of different mechanisms to avoid condensation, like anti monopoly rules and brand fidelity.Comment: Accepted in: International Journal of Modern Physics

Single-ion anisotropy and magnetic field response in the spin-ice materials Ho2Ti2 O7 and Dy2Ti2 O7

Motivated by its role as a central pillar of current theories of dynamics of spin ice in and out of equilibrium, we study the single-ion dynamics of the magnetic rare earth ions in their local environments, subject to the effective fields set up by the magnetic moments they interact with. This effective field has a transverse component with respect to the local easy-axis of the crystal electric field, which can induce quantum tunnelling. We go beyond the projective spin-1/2 picture and use instead the full crystal-field Hamiltonian. We find that the Kramers vs non-Kramers nature, as well as the symmetries of the crystal-field Hamiltonian, result in different perturbative behaviour at small fields ($\lesssim 1$ T), with transverse field effects being more pronounced in Ho$_{2}$Ti$_{2}$O$_{7}$ than in Dy$_{2}$Ti$_{2}$O$_{7}$. Remarkably, the energy splitting range we find is consistent with time scales extracted from experiments. We also present a study of the static magnetic response which highlights the anisotropy of the system in the form of an off-diagonal $g$ tensor and we investigate the effects of thermal fluctuations in the temperature regime of relevance to experiments. We show that there is a narrow yet accessible window of experimental parameters where the anisotropic response can be observed.EPSRC, STFC, HEFCE, SEPnetThis is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevB.92.15512

The Approximate Invariance of the Average Number of Connections for the Continuum Percolation of Squares at Criticality

We perform Monte Carlo simulations to determine the average excluded area $$of randomly oriented squares, randomly oriented widthless sticks and aligned squares in two dimensions. We find significant differences between our results for randomly oriented squares and previous analytical results for the same. The sources of these differences are explained. Using our results for$$ and Monte Carlo simulation results for the percolation threshold, we estimate the mean number of connections per object $B_c$ at the percolation threshold for squares in 2-D. We study systems of squares that are allowed random orientations within a specified angular interval. Our simulations show that the variation in $B_c$ is within 1.6% when the angular interval is varied from 0 to $\pi/2$

Effect of spatial inhomogeneity on the mapping between strongly interacting fermions and weakly interacting spins

A combined analytical and numerical study is performed of the mapping between strongly interacting fermions and weakly interacting spins, in the framework of the Hubbard, t-J and Heisenberg models. While for spatially homogeneous models in the thermodynamic limit the mapping is thoroughly understood, we here focus on aspects that become relevant in spatially inhomogeneous situations, such as the effect of boundaries, impurities, superlattices and interfaces. We consider parameter regimes that are relevant for traditional applications of these models, such as electrons in cuprates and manganites, and for more recent applications to atoms in optical lattices. The rate of the mapping as a function of the interaction strength is determined from the Bethe-Ansatz for infinite systems and from numerical diagonalization for finite systems. We show analytically that if translational symmetry is broken through the presence of impurities, the mapping persists and is, in a certain sense, as local as possible, provided the spin-spin interaction between two sites of the Heisenberg model is calculated from the harmonic mean of the onsite Coulomb interaction on adjacent sites of the Hubbard model. Numerical calculations corroborate these findings also in interfaces and superlattices, where analytical calculations are more complicated.Comment: 7 pages, 6 figure