Magnon-polaron and Spin-polaron Signatures in the Specific Heat and Electrical Resistivity of La0.6Y0.1Ca0.3MnO3La_{0.6}Y_{0.1}Ca_{0.3}MnO_3 in Zero Magnetic Field, and the Effect of Mn−O−MnMn-O-Mn Bond Environment

$La_{0.6}Y_{0.1}Ca_{0.3}MnO_{3}$, an $ABO_{3}$ perovskite manganite oxide, exhibits a non trivial behavior in the vicinity of the sharp peak found in the resistivity $\rho$ as a function of temperature $T$ in zero magnetic field. The various features seen on $d\rho/dT$ are discussed in terms of competing phase transitions. They are related to the $Mn-O-Mn$ bond environment depending on the content of the $A$ crystallographic site. A Ginzburg-Landau type theory is presented for incorporating concurrent phase transitions. The specific heat $C$ of such a compound is also examined from 50 till 200 K. A log-log analysis indicates different regimes. In the low temperature conducting ferromagnetic phase, a collective magnon signature ($C \simeq T^{3/2}$) is found as for what are called magnon-polaron excitations. A $C \simeq T^{2/3}$ law is found at high temperature and discussed in terms of the fractal dimension of the conducting network of the weakly conducting (so-called insulating) phase and Orbach estimate of the excitation spectral behaviors. The need of considering both independent spin scattering and collective spin scattering is thus emphasized. The report indicates a remarkable agreement for the Fisher-Langer formula, i.e. $C$ $\sim$ $d\rho/dT$ at second order phase transitions. Within the Attfield model, we find an inverse square root relationship between the critical temperature(s) and the total local $Mn-O-Mn$ strain.Comment: 19 pages, 5 figures; to be published in Phys Rev

Separation of Long-Crested Nonlinear Bichromatic Waves into Incident and Reflected Components

Methods for the separation of long-crested linear waves into incident and reflected waves have existed for more than 40 years. The present paper presents a new method for the separation of nonlinear bichromatic long-crested waves into incident and reflected components, as well as into free and bound components. The new method is an extension of a recently proposed method for the separation of nonlinear regular waves. The new methods include both bound and free higher harmonics, which is important for nonlinear waves. The applied separation method covers interactions to the third order, but can easily be extended to a higher orders. Synthetic tests, as well as physical model tests, showed that the method accurately predict the bound amplitudes and incident and reflected surface elevations of nonlinear bichromatic waves. The new method is important in order to be able to describe the detailed characteristics of nonlinear bichromatic waves and their reflection

Control de malas hierbas: Densidades de siembra y rotaciones de cultivo

11 páginas, tablas estadísticas, y figuras.La rotación de cultivos ha sido uno de los métodos tradicionales para el control de las malas hierbas, que se ha visto desplazada por factores económicos, monocultivo de cereal y uso de herbicidas. Por otra parte no siempre son evidentes las ventajas de una aplicación de herbicidas, particularmente en densidades bajas de infestación de malas hierbas (Pardo et al., 2002 y Chao et al., 2002). Aumentar las densidades de siembra en los cereales también han sido empleadas como forma de aumentar la competencia del cultivo frente a las hierbas acompañantes y disminuir de esa forma el daño que estas causan en los rendimientos de los cereales. Actualmente sigue empleándose la expresión de “sembrar con mucho pan” para justificar está practica tradicional. Determinar la densidad optima de siembra ha sido objeto de varios trabajos con desigual resultado (Moreno et al., 2002 y Lacasta et al., 2004), las causas son las diferentes condiciones experimentales. En este trabajo se pretende evaluar la interacción de estas dos formas tradicionales de control de malas hierbas (rotación de cultivos y densidades de siembra) y su efecto sobre la productividad del cereal en las condiciones de semiáridez de los secanos y en un experimento de larga duración (14 años).Peer reviewe

Perceived quality of life and the specific physical activities by the elderly

Se analizaron indicadores de salud con respecto a la percepción de la calidad de vida (CVRS) mediante el cuestionario Short Form-36 Health Survey (SF-36) y parámetros cinemáticos manifestados en el movimiento press banca (12 kilos) como predictores de las adaptaciones neurofisiológicas que se producen con la actividad física. Se trataron valores de potencia, de velocidad, aceleraciones, fuerza y variables temporales. Se utilizaron dos grupos, uno activo, de práctica física habitual (12H, 6M, 68,4±5,6 años, 1,65±0,074m, 74,57±15,41kg, BMI 26,93±4,02; act/sem 4,5±1,65h.), y otro sedentario (16H, 7M, 69 ± 7,07 años, 1,67 ± 0,072m, 74,95 ± 7,4kg, BMI 26,84±2,78) Se encontraron diferencias significativas (p<0,05;gl=1,39;dz=0,5). El grupo considerado activo se percibe con mejor calidad de vida y manifiesta de manera más rápida y con mayor fuerza el movimiento solicitado. Las conclusiones son relevantes para posteriores estudios que se centren en la pérdida de propiedades neuromusculares acompañada a la falta de práctica y al efecto del envejecimiento.Quality of life related to health indicators (QLRH) were analyzed by means of the Short Form-36 Health Survey (SF-36) questionnaire and the specified kinetic parameters of the bench press movement (12 kg) as predictors of the neurophysiological adaptations produced by physical activity. These were the values of power, velocity, acceleration, strength and temporal variables. Two groups were studied: the first group was active, undertaking regular physical activity (12 male; 6 female; 68,4±5,6 years; 1,65±0,074m; 74,57±15,41kg; BMI 26,93±4,02; weekly activity 4,5±1,65h.); the second group was sedentary (16 male; 7 female; 69±7,07 years; 1,67±0,072m; 74,95±7,4kg; BMI 26,84±2,78). Significant differences were found (p<0,05;df=1,39;dz=0,5). The group regarded as active considered itself as having a better quality of life and was able to perform the requested movement with greater speed and strength. The conclusions are relevant for future studies that centre on the loss of neuromuscular properties accompanied by a lack of physical activity and the effects of aging

Shapiro steps in a superconducting film with an antidot lattice

Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in the voltage-current characteristics of a superconducting film with a square lattice of perforating microholes (antidots)in the presence of radiofrequent radiation. These equidistant steps appear at the second matching field H_2 when the flow of the interstitial vortex lattice in the periodic potential created by the antidots and the vortices trapped by them, is in phase with the applied rf frequency. Therefore, the observation of Shapiro steps clearly reveals the presence of mobile intersitial vortices in superconducting films with regular pinning arrays. The interstitial vortices, moved by the driving current, coexist with immobile vortices strongly pinned at the antidots.Comment: 6 pages text, 3 EPS figures, RevTeX, accepted for publication in PRB Rapid Communication

"Actual" does not imply "feasible"

The familiar complaint that some ambitious proposal is infeasible naturally invites the following response: Once upon a time, the abolition of slavery and the enfranchisement of women seemed infeasible, yet these things were actually achieved. Presumably, then, many of those things that seem infeasible in our own time may well be achieved too and, thus, turn out to have been perfectly feasible after all. The Appeal to History, as we call it, is a bad argument. It is not true that if some desirable state of affairs was actually achieved, then it was feasible that it was achieved. “Actual” does not imply “feasible,” as we put it. Here is our objection. “Feasible” implies “not counterfactually fluky.” But “actual” does not imply “not counterfactually fluky.” So, “actual” does not imply “feasible.” While something like the Flukiness Objection is sometimes hinted at in the context of the related literature on abilities, it has not been developed in any detail, and both premises are inadequately motivated. We offer a novel articulation of the Flukiness Objection that is both more precise and better motivated. Our conclusions have important implications, not only for the admissible use of history in normative argument, but also by potentially circumscribing the normative claims that are applicable to us

Restructuring of colloidal aggregates in shear flow: Coupling interparticle contact models with Stokesian dynamics

A method to couple interparticle contact models with Stokesian dynamics (SD) is introduced to simulate colloidal aggregates under flow conditions. The contact model mimics both the elastic and plastic behavior of the cohesive connections between particles within clusters. Owing to this, clusters can maintain their structures under low stress while restructuring or even breakage may occur under sufficiently high stress conditions. SD is an efficient method to deal with the long-ranged and many-body nature of hydrodynamic interactions for low Reynolds number flows. By using such a coupled model, the restructuring of colloidal aggregates under stepwise increasing shear flows was studied. Irreversible compaction occurs due to the increase of hydrodynamic stress on clusters. Results show that the greater part of the fractal clusters are compacted to rod-shaped packed structures, while the others show isotropic compaction.Comment: A simulation movie be found at http://www-levich.engr.ccny.cuny.edu/~seto/sites/colloidal_aggregates_shearflow.htm