Effects of semiclassical spiral fluctuations on hole dynamics

We investigate the dynamics of a single hole coupled to the spiral fluctuations related to the magnetic ground states of the antiferromagnetic J_1-J_2-J_3 Heisenberg model on a square lattice. Using exact diagonalization on finite size clusters and the self consistent Born approximation in the thermodynamic limit we find, as a general feature, a strong reduction of the quasiparticle weight along the spiral phases of the magnetic phase diagram. For an important region of the Brillouin Zone the hole spectral functions are completely incoherent, whereas at low energies the spectral weight is redistributed on several irregular peaks. We find a characteristic value of the spiral pitch, Q=(0.7,0.7)\pi, for which the available phase space for hole scattering is maximum. We argue that this behavior is due to the non trivial interference of the magnon assisted and the free hopping mechanism for hole motion, characteristic of a hole coupled to semiclassical spiral fluctuations.Comment: 6 pages, 5 figure

Selfconsistent hybridization expansions for static properties of the Anderson impurity model

By means of a projector-operator formalism we derive an approximation based on a self consistent hybridization expansion to study the ground state properties of the Anderson Impurity model. We applied the approximation to the general case of finite Coulomb repulsion $U$, extending previous work with the same formalism in the infinite-$U$ case. The treatment provides a very accurate calculation of the ground state energy and their related zero temperature properties in the case in which $U$ is large enough, but still finite, as compared with the rest of energy scales involved in the model. The results for the valence of the impurity are compared with exact results that we obtain from equations derived using the Bethe ansatz and with a perturbative approach. The magnetization and magnetic susceptibility is also compared with Bethe ansatz results. In order to do this comparison, we also show how to regularize the Bethe ansatz integral equations necessary to calculate the impurity valence, for arbitrary values of the parameters.Comment: 8 pages, 5 figure

Spin polaron in the J1-J2 Heisenberg model

We have studied the validity of the spin polaron picture in the frustrated J1-J2 Heisenberg model. For this purpose, we have computed the hole spectral functions for the Neel, collinear, and disordered phases of this model, by means of the self-consistent Born approximation and Lanczos exact diagonalization on finite-size clusters. We have found that the spin polaron quasiparticle excitation is always well defined for the magnetically ordered Neel and collinear phases, even in the vicinity of the magnetic quantum critical points, where the local magnetization vanishes. As a general feature, the effect of frustration is to increase the amplitude of the multimagnon states that build up the spin polaron wave function, leading to the reduction of the quasiparticle coherence. Based on Lanczos results, we discuss the validity of the spin polaron picture in the disordered phase.Comment: 9 pages, 12 figure

Cl electrosorption on Ag(100): Lateral interactions and electrosorption valency from comparison of Monte Carlo simulations with chronocoulometry experiments

We present Monte Carlo Simulations using an equilibrium lattice-gas model for the electrosorption of Cl on Ag(100) single-crystal surfaces. Fitting the simulated isotherms to chronocoulometry experiments, we extract parameters such as the electrosorption valency gamma and the next-nearest-neighbor lateral interaction energy phi_nnn. Both coverage-dependent and coverage independent gamma were previously studied assuming a constant phi_nnn [I. Abou Hamad, Th. Wandlowski, G. Brown, P.A. Rikvold, J. Electroanal. Chem. 554-555 (2003) 211]. Here, a self-consistent, entirely electrostatic picture of the lateral interactions with a coverage-dependent phi_nnn is developed, and a relationship between phi_nnn and gamma is investigated for Cl on Ag(100).Comment: Accepted for publication in Electrochimica Acta, 10 pages, 7 figures, 2 tables and an appendi

Comparison Between the Effects of Different Sources of Dietary Fiber on Blood Lipid Profile in Rats

This study was conducted to investigate the effect of different sources of dietary fiber on serum lipids and lipoproteins in Sprague-Dawley rats, namely total cholesterol (TC), Low-density lipoprotein cholesterol (LDL-C), high–density lipoprotein cholesterol (HDL-C) triglyceride (TG), and (HDL-C/LDL-C) ratio. The experimental diets included casein diet, untreated wheat bran diet, soaked wheat bran diet, Arabic white bread diet, lupine diet, chickpea and pectin diet. Each group of rats (6/group) was fed one of the seven prepared diets for 6 weeks. Untreated wheat bran has hypercholesterolemic effect since it significantly (p<0.05) increased TC, LDL-C and decreased HDL-C values as compared with treated bran diets. The soaking process lowered significantly (p<0.05) TC and LDL-C levels in comparison with casein diet. Chickpea diet had higher significant value of HDL-C than soaked wheat bran and lupine diets. However, it was not significantly different from casein and untreated wheat bran diet.  In addition, Chickpea diet had a significantly higher HDL-C value than pectin diet. The values of HDL-C of the Lupine diet are not significantly different (p>0.05) from casein and pectin diets, but it has also decreased LDL-C. White bread was found to have no hypocholesterolemic effect in comparison with treated brans groups and control group; it raised TG, TC and LDL-C levels. Pectin had a similar behavior in a remarkable decrease blood TC and LDL-C cholesterol. White bread was found to have  no hypocholesterolemic effect in comparison with treated brans groups and control group; it raised TG, TC and LDL-C  levels. It is concluded that the behavior of cereals and legumes varies in its effect on cholesterol–lowering ability. Preparation of wheat bran foods by soaking improve some physiological characteristics of insoluble fibers, particularly lowering total cholesterol TC and low-density lipoprotein cholesterol LDL-C). Keywords: Dietary fiber, Arabic bread, Wheat bran, Soaking, Chickpea, Lupine, Pectin, Lipoproteins, Rat

Understanding Nanopore Window Distortions in the Reversible Molecular Valve Zeolite RHO

Molecular valves are becoming popular for potential biomedical applications. However, little is known concerning their performance in energy and environmental areas. Zeolite RHO shows unique pore deformations upon changes in hydration, cation siting, cation type, or temperature-pressure conditions. By varying the level of distortion of double eight-rings, it is possible to control the adsorption properties, which confer a molecular valve behavior to this material. We have employed interatomic potentials-based simulations to obtain a detailed atomistic view of the structural distortion mechanisms of zeolite RHO, in contrast with the averaged and space group restricted information provided by diffraction studies. We have modeled four aluminosilicate structures, containing Li$^+$, Na$^+$, K$^+$, Ca$^{2+}$, and Sr$^{2+}$ cations. The distortions of the three different zeolite rings are coupled, and the six- and eight-membered rings are largely flexible. A large dependence on the polarizing power of the extra-framework cations and with the loading of water has been found for the minimum aperture of the eight-membered rings that control the nanovalve effect. The calculated energy barriers for moving the cations across the eight-membered rings are very high, which explains the experimentally observed slow kinetics of the phase transition as well as the appearance of metastable phases

Effects of lateral diffusion on morphology and dynamics of a microscopic lattice-gas model of pulsed electrodeposition

The influence of nearest-neighbor diffusion on the decay of a metastable low-coverage phase (monolayer adsorption) in a square lattice-gas model of electrochemical metal deposition is investigated by kinetic Monte Carlo simulations. The phase-transformation dynamics are compared to the well-established Kolmogorov-Johnson-Mehl-Avrami theory. The phase transformation is accelerated by diffusion, but remains in accord with the theory for continuous nucleation up to moderate diffusion rates. At very high diffusion rates the phase-transformation kinetic shows a crossover to instantaneous nucleation. Then, the probability of medium-sized clusters is reduced in favor of large clusters. Upon reversal of the supersaturation, the adsorbate desorbs, but large clusters still tend to grow during the initial stages of desorption. Calculation of the free energy of subcritical clusters by enumeration of lattice animals yields a quasi-equilibrium distribution which is in reasonable agreement with the simulation results. This is an improvement relative to classical droplet theory, which fails to describe the distributions, since the macroscopic surface tension is a bad approximation for small clusters.Comment: Minor corrections and modifications. 15 pages with 10 figures. Accepted for publication in the Journal of Chemical Physics, see http://jcp.aip.org/jcp

Efficiency Of Using Steel End Caps In Improving The Post-Fire Flexural Behavior Of Frp Reinforced Concrete Beams

The use of Fiber Reinforced Polymer (FRP) bars as an alternative to traditional steel reinforcement helps overcoming durability problems in reinforced concrete structures. The behavior of FRP-RC structures is satisfactory at only low temperatures, hence the application of combustible FRP materials in commercial, industrial and residential buildings, where the possibility of fire occurrence is relatively high, can be dangerous. Further research to evaluate and enhance the performance of FRP-RC structures under fire conditions is required. In this study, the effect of high temperatures on the mechanical properties of FRP/Steel bars, bond behavior between FRP/Steel bars and concrete, and the flexural response of concrete beams with different types of FRP bar reinforcement was investigated in much details. A new steel-end-caps technique was proposed aiming to improve anchorage of embedded FRP bars in concrete. For that FRP/Steel bars, plain concrete, pullout and beam specimens (with and without steel end caps) were prepared and then cured for 28 days and later tested before and after subjected to elevated temperatures of up to 500°C. Concrete and FRP bars suffered significant reductions in their mechanical properties due to exposure to high temperatures. Bond strength between FRP bars and concrete had decreased upon exposure to temperature in the range of 125 to 325°C, with the reduction reaching as high as 85%. These reductions were reflected negatively in the behavior of heated FRP-RC beams hence cracking load, ultimate load capacity, stiffness and total absorbed energy were reduced by as high as 89%, 81%, 79%, and 70%, respectively whereas mid-span deflections and ductility indices were increased noticeably by as high as 50% and 94%, respectively. Attaching steel end caps to the ends of FRP bars had improved their bond strength with concrete before and after exposure to high temperatures of up to 325oC. Consequently, the flexural performance of FRP-RC beams with end-cap anchorage was improved where the cracking load, ultimate load capacity, stiffness, deflection at ultimate load, and total absorbed energy were increased to reach as high as (124%, 208%, 225%, 196%, and 453%) and (33%, 123%, 58%, 216% and 215%) before and after heating up to 500°C, respectively, compared with that of control beams without end anchorage. Based on the experimental results, an analytical model was proposed to predict the behavior of the ascending part of bondslip relation between the different FRP bars and concrete under high temperatures. Another theoretical method was also proposed to predict the theoretical ultimate load capacity of FRP-RC beams. The predictions of the two models were in an excellent agreement with the experimental results