General technique of calculating drift velocity and diffusion coefficient in arbitrary periodic systems

We develop a practical method of computing the stationary drift velocity V and the diffusion coefficient D of a particle (or a few particles) in a periodic system with arbitrary transition rates. We solve this problem both in a physically relevant continuous-time approach as well as for models with discrete-time kinetics, which are often used in computer simulations. We show that both approaches yield the same value of the drift, but the difference between the diffusion coefficients obtained in each of them equals V*V/2. Generalization to spaces of arbitrary dimension and several applications of the method are also presented.Comment: 12 pages + 2 figures, RevTeX. Submitted to J. Phys. A: Math. Ge

Hopping motion of lattice gases through nonsymmetric potentials under strong bias conditions

The hopping motion of lattice gases through potentials without mirror-reflection symmetry is investigated under various bias conditions. The model of 2 particles on a ring with 4 sites is solved explicitly; the resulting current in a sawtooth potential is discussed. The current of lattice gases in extended systems consisting of periodic repetitions of segments with sawtooth potentials is studied for different concentrations and values of the bias. Rectification effects are observed, similar to the single-particle case. A mean-field approximation for the current in the case of strong bias acting against the highest barriers in the system is made and compared with numerical simulations. The particle-vacancy symmetry of the model is discussed.Comment: 8 pages (incl. 6 eps figures); RevTeX 3.

Absence of self-averaging in the complex admittance for transport through random media

A random walk model in a one dimensional disordered medium with an oscillatory input current is presented as a generic model of boundary perturbation methods to investigate properties of a transport process in a disordered medium. It is rigorously shown that an admittance which is equal to the Fourier-Laplace transform of the first-passage time distribution is non-self-averaging when the disorder is strong. The low frequency behavior of the disorder-averaged admittance, $-1 \sim \omega^{\mu}$ where $\mu < 1$, does not coincide with the low frequency behavior of the admittance for any sample, $\chi - 1 \sim \omega$. It implies that the Cole-Cole plot of $$ appears at a different position from the Cole-Cole plots of $\chi$ of any sample. These results are confirmed by Monte-Carlo simulations.Comment: 7 pages, 2 figures, published in Phys. Rev.

Characteristics of ferroelectric-ferroelastic domains in N{\'e}el-type skyrmion host GaV4_4S8_8

GaV$_4$S$_8$ is a multiferroic semiconductor hosting N{\'e}el-type magnetic skyrmions dressed with electric polarization. At T$_s$ = 42K, the compound undergoes a structural phase transition of weakly first-order, from a non-centrosymmetric cubic phase at high temperatures to a polar rhombohedral structure at low temperatures. Below T$_s$, ferroelectric domains are formed with the electric polarization pointing along any of the four $\left< 111 \right>$ axes. Although in this material the size and the shape of the ferroelectric-ferroelastic domains may act as important limiting factors in the formation of the N{\'e}el-type skyrmion lattice emerging below T$_C$=13\:K, the characteristics of polar domains in GaV$_4$S$_8$ have not been studied yet. Here, we report on the inspection of the local-scale ferroelectric domain distribution in rhombohedral GaV$_4$S$_8$ using low-temperature piezoresponse force microscopy. We observed mechanically and electrically compatible lamellar domain patterns, where the lamellae are aligned parallel to the (100)-type planes with a typical spacing between 100 nm-1.2 $\mu$m. We expect that the control of ferroelectric domain size in polar skyrmion hosts can be exploited for the spatial confinement and manupulation of N{\'e}el-type skyrmions

A Salt Metathesis Route To Ruthenium Carbene Complex Isomers With Pyridine Dicarboxamide-Derived Chelate Pincer Ligands

Reaction of the doubly deprotonated pyridine 2,6-dicarboxamido ligand (1) with (PCy_3)_2Cl_2 Ru=CHPh (3a) in THF gave a mixture of (lig)(PCy_3)Ru=CHPh isomers (4). The pentane soluble N,N,O-4 isomer was isolated by extraction and characterized by X-ray diffraction. The O,N, O-4 isomer was identified in the residue. Single crystals of the closely related complex (lig)(NHC) Ru=CHPh, O,N,O-5, were obtained from the reaction of 1 with (NHC)(PCy_3)Cl_2Ru=CHPh (3b) and used for the X-ray crystal structure analysis of the system

Comparación entre nutrición enteral precoz y nutrición enteral tardía en el estado nutricional de pacientes gastrectomizados

Indexación: ScieloLa nutrición enteral (NE) es un método efectivo para cubrir los requerimientos nutricionales en pacientes que presentan un estado nutricional deteriorado. Objetivos: Comparar la Nutrición Enteral Precoz (NEP) versus Nutrición Enteral Tardía (NET) en la evolución clínica y nutricional de pacientes sometidos a gastrectomía total por Cáncer Gástrico. Material y Método: 18 pacientes con cáncer gástrico resecable, fueron estudiados con parámetros antropométricos, funcionales y bioquímicos que evalúan el estado nutricional, en el período preoperatorio y postoperatorio. Recibieron una fórmula enteral polimérica (1 kcal/ml) en el período postoperatorio. De manera aleatoria fueron asignados al tipo de nutrición (precoz o tardía). Resultados: El grupo con NEP presentó mejoría significativa del porcentaje de adecuación del pliegue bicipital y dinamometría. El grupo con NET presentó disminución significativa de la albuminemia. La distensión abdominal fue más frecuente en grupo con NEP. Conclusión: La Nutrición Enteral Precoz es un soporte nutricional seguro, eficaz y que trae consigo ventajas nutricionales en comparación con la Nutrición Enteral Tardía en el grupo de pacientes gastrectomizados totales por presentar cáncer gástrico.Enteral nutrition (EN) is an effective method to meet the nutritional requirements in patients who have a deteriorated nutritional status. Objectives: To compare clinical and nutritional performance oftwo groups: Early Enteral Nutrition (EEN) versus Late Enteral Nutrition (LEN) of patients undergoing to total gastrectomy for gastric cancer. Material and Methods: 18 patients with resectable gastric cancer were studied with anthropometric, functional and biochemical parameters to assess nutritional status in the preoperative and postoperative period. They received a polimeric enteral formula (1 kcal/ml) in the postoperative period. They were randomly assigned to the type of nutrition (early or late). Results: The group with EEN had a significant improvement in the bicipital fold adequacy percentage and dynamometry. The LEN group had a significant decrease of albumin. The bloating was more fre quent in the group with EEN. Conclusion: Early enteral nutrition is a safe nutritional support, effective and that brings nutritional benefits compared with late enteral nutrition in patients undergoing to total gastrectomy for gastric cancer.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-75182009000100002&nrm=is

Critical dimensions for random walks on random-walk chains

The probability distribution of random walks on linear structures generated by random walks in $d$-dimensional space, $P_d(r,t)$, is analytically studied for the case $\xi\equiv r/t^{1/4}\ll1$. It is shown to obey the scaling form $P_d(r,t)=\rho(r) t^{-1/2} \xi^{-2} f_d(\xi)$, where $\rho(r)\sim r^{2-d}$ is the density of the chain. Expanding $f_d(\xi)$ in powers of $\xi$, we find that there exists an infinite hierarchy of critical dimensions, $d_c=2,6,10,\ldots$, each one characterized by a logarithmic correction in $f_d(\xi)$. Namely, for $d=2$, $f_2(\xi)\simeq a_2\xi^2\ln\xi+b_2\xi^2$; for $3\le d\le 5$, $f_d(\xi)\simeq a_d\xi^2+b_d\xi^d$; for $d=6$, $f_6(\xi)\simeq a_6\xi^2+b_6\xi^6\ln\xi$; for $7\le d\le 9$, $f_d(\xi)\simeq a_d\xi^2+b_d\xi^6+c_d\xi^d$; for $d=10$, $f_{10}(\xi)\simeq a_{10}\xi^2+b_{10}\xi^6+c_{10}\xi^{10}\ln\xi$, {\it etc.\/} In particular, for $d=2$, this implies that the temporal dependence of the probability density of being close to the origin $Q_2(r,t)\equiv P_2(r,t)/\rho(r)\simeq t^{-1/2}\ln t$.Comment: LATeX, 10 pages, no figures submitted for publication in PR

Calidad microbiológica de una fórmula enteral lista para usar

Indexación: ScieloTo determinate and compare the microbiological quality of a ready to use enteral formula (EF): liquid ADN™ during different periods of time. Methods: The study was developed in the Hospital formula-preparing room. Twenty liters of EF were delivered in 40 plastic sterile bottles using aseptic technique, and were maintained at room temperature during 24 hours. Feed samples of 50 ml at time 0 and at 24 hours were obtained and frozen at _70º C, until they were investigated (40 feed samples of EF were cultivated at preparation time 0, and 24 hours). Mesophile count (Me), total coliform (TC) and faecal coliform (FC) bacteria were investigated. The feed samples were analized at the Microbiologic Laboratory of CESMEC. The microbial quality standards (MQS) were at time 0: < 10² UFC/ml of (Me), and no (TC) and (FC). At 24 hours: < 10³ UFC/ml of (Me), < 10 UFC/ml of (TC) and no (FC).The statistical data analysis was done using Stata program and Z test was used for proportions. The level of p < 0,05 was considered statistically significant. Results: The average of fulfilment (MQS) for liquid ADN™ at time 0, and 24 hours was 100% and 95% (p = 0,3) for Me. TC got 100% fulfilment (MQS) at any time. FC were not detected at any time. Conclusions: Liquid ADN™can be hung during 24 hours at room temperature.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0716-10182004000400005&nrm=is

Force-velocity relation and density profiles for biased diffusion in an adsorbed monolayer

In this paper, which completes our earlier short publication [Phys. Rev. Lett. 84, 511 (2000)], we study dynamics of a hard-core tracer particle (TP) performing a biased random walk in an adsorbed monolayer, composed of mobile hard-core particles undergoing continuous exchanges with a vapor phase. In terms of an approximate approach, based on the decoupling of the third-order correlation functions, we obtain the density profiles of the monolayer particles around the TP and derive the force-velocity relation, determining the TP terminal velocity, V_{tr}, as the function of the magnitude of external bias and other system's parameters. Asymptotic forms of the monolayer particles density profiles at large separations from the TP, and behavior of V_{tr} in the limit of small external bias are found explicitly.Comment: Latex, 31 pages, 3 figure

Near-field examination of perovskite-based superlenses and superlens-enhanced probe-object coupling

A planar slab of negative index material works as a superlens with sub-diffraction-limited imaging resolution, since propagating waves are focused and, moreover, evanescent waves are reconstructed in the image plane. Here, we demonstrate a superlens for electric evanescent fields with low losses using perovskites in the mid-infrared regime. The combination of near-field microscopy with a tunable free-electron laser allows us to address precisely the polariton modes, which are critical for super-resolution imaging. We spectrally study the lateral and vertical distributions of evanescent waves around the image plane of such a lens, and achieve imaging resolution of wavelength/14 at the superlensing wavelength. Interestingly, at certain distances between the probe and sample surface, we observe a maximum of these evanescent fields. Comparisons with numerical simulations indicate that this maximum originates from an enhanced coupling between probe and object, which might be applicable for multifunctional circuits, infrared spectroscopy, and thermal sensors.Comment: 20 pages, 6 figures, published as open access article in Nature Communications (see http://www.nature.com/ncomms/