Optimal Routes to Ultrafast Polarization Reversal in Ferroelectric LiNbO3

We use the frozen phonon method to calculate the anharmonic potential energy surface and to model the ultrafast ferroelectric polarization reversal in LiNbO3 driven by intense pulses of THz light. Before stable switching of the polarization occurs, there exists a region of excitation field-strengths where transient switching can occur, as observed experimentally [Physical Review Letters 118, 197601 (2017)]. By varying the excitation frequency from 4 to 20 THz, our modeling suggests that more efficient, permanent polarization switching can occur by directly exciting the soft mode at 7 THz, compared to nonlinear phononic-induced switching driven by exciting a high frequency mode at 18 THz. We also show that neglecting anharmonic coupling pathways in the modeled experiment can lead to significant differences in the modeled switching field strengths.Comment: 8 pages, 3 figure

Adaptive hybrid optimization strategy for calibration and parameter estimation of physical models

A new adaptive hybrid optimization strategy, entitled squads, is proposed for complex inverse analysis of computationally intensive physical models. The new strategy is designed to be computationally efficient and robust in identification of the global optimum (e.g. maximum or minimum value of an objective function). It integrates a global Adaptive Particle Swarm Optimization (APSO) strategy with a local Levenberg-Marquardt (LM) optimization strategy using adaptive rules based on runtime performance. The global strategy optimizes the location of a set of solutions (particles) in the parameter space. The LM strategy is applied only to a subset of the particles at different stages of the optimization based on the adaptive rules. After the LM adjustment of the subset of particle positions, the updated particles are returned to the APSO strategy. The advantages of coupling APSO and LM in the manner implemented in squads is demonstrated by comparisons of squads performance against Levenberg-Marquardt (LM), Particle Swarm Optimization (PSO), Adaptive Particle Swarm Optimization (APSO; the TRIBES strategy), and an existing hybrid optimization strategy (hPSO). All the strategies are tested on 2D, 5D and 10D Rosenbrock and Griewank polynomial test functions and a synthetic hydrogeologic application to identify the source of a contaminant plume in an aquifer. Tests are performed using a series of runs with random initial guesses for the estimated (function/model) parameters. Squads is observed to have the best performance when both robustness and efficiency are taken into consideration than the other strategies for all test functions and the hydrogeologic application

Competition of crystal field splitting and Hund's rule coupling in two-orbital magnetic metal-insulator transitions

Competition of crystal field splitting and Hund's rule coupling in magnetic metal-insulator transitions of half-filled two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of Coulomb correlation, the system firstly transits from a paramagnetic (PM) metal to a {\it N\'{e}el} antiferromagnetic (AFM) Mott insulator, or a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund's rule coupling. The different AFM Mott insulator, PM metal and orbital insulating phase are none, partially and fully orbital polarized, respectively. For a small $J_{H}$ and a finite crystal field, the orbital insulator is robust. Although the system is nonmagnetic, the phase boundary of the orbital insulator transition obviously shifts to the small $U$ regime after the magnetic correlations is taken into account. These results demonstrate that large crystal field splitting favors the formation of the orbital insulating phase, while large Hund's rule coupling tends to destroy it, driving the low-spin to high-spin transition.Comment: 4 pages, 4 figure

An improved parameter estimation and comparison for soft tissue constitutive models containing an exponential function

Motivated by the well-known result that stiffness of soft tissue is proportional to the stress, many of the constitutive laws for soft tissues contain an exponential function. In this work, we analyze properties of the exponential function and how it affects the estimation and comparison of elastic parameters for soft tissues. In particular, we find that as a consequence of the exponential function there are lines of high covariance in the elastic parameter space. As a result, one can have widely varying mechanical parameters defining the tissue stiffness but similar effective stress–strain responses. Drawing from elementary algebra, we propose simple changes in the norm and the parameter space, which significantly improve the convergence of parameter estimation and robustness in the presence of noise. More importantly, we demonstrate that these changes improve the conditioning of the problem and provide a more robust solution in the case of heterogeneous material by reducing the chances of getting trapped in a local minima. Based upon the new insight, we also propose a transformed parameter space which will allow for rational parameter comparison and avoid misleading conclusions regarding soft tissue mechanics

A periodic boundary value problem for nonlinear singular differential systems with ‘maxima’

décembre 20052005/12 (N254)-2005/12.Appartient à l’ensemble documentaire : UnivJeun

Estudio de la calidad del agua de bebida para aves en granjas avícolas de la región centro-oeste de la provincia de Entre Ríos. Granjas de postura comercial

En Argentina existe una población de 41.000.000 de gallinas en postura, de las cuales el 20% aproximadamente están localizadas en la Provincia de Entre Ríos, representando la segunda provincia en importancia del país en producción de huevos. El agua utilizada en las granjas es de origen subterráneo. Existe información sobre sus características por los múltiples usos a que se destina. Sin embargo, el nivel de conocimientos actual no nos sirve a la hora de tomar decisiones en particular. Por este motivo y con el fin de caracterizar el agua que se destina a bebida aviar, se realizó un relevamiento de granjas de postura en los departamentos Paraná y Diamante de la provincia de Entre Ríos, donde se encuentra la mayor concentración de aves destinadas a la producción de huevos. El mismo estuvo enmarcado en el proyecto de investigación “Estudio de la calidad del agua de bebida para aves en granjas avícolas de la región centro-oeste de la provincia de Entre Ríos”, llevado a cabo por las Cátedras de Química General y Avicultura FCA-UNER. Se analizó el agua de 29 granjas de postura, realizándose análisis físico-químico y bacteriológico, los resultados muestran pH dentro de lo recomendado, elevada dureza, altos valores de sodio y sulfatos así como importantes variaciones de los componentes aun en predios cercanos y como consecuencia la necesidad realizar correcciones de los aportes minerales a fin de lograr un balance electrolítico adecuado, para el mejor rendimiento de la explotación. &nbsp