Enhancement of photoacoustic detection of inhomogeneities in polymers

We report a series of experiments on laser pulsed photoacoustic excitationin turbid polymer samples addressed to evaluate the sound speed in the samples and the presence of inhomogeneities in the bulk. We describe a system which allows the direct measurement of the speed of the detected waves by engraving the surface of the piece under study with a fiduciary pattern of black lines. We also describe how this pattern helps to enhance the sensitivity for the detection of an inhomogeneity in the bulk. These two facts are useful for studies in soft matter systems including, perhaps, biological samples. We have performed an experimental analysis on Grilon(R) samples in different situations and we show the limitations of the method.Comment: 8 pages, 7 figure

Interaction energy functional for lattice density functional theory: Applications to one-, two- and three-dimensional Hubbard models

The Hubbard model is investigated in the framework of lattice density functional theory (LDFT). The single-particle density matrix $\gamma_{ij}$ with respect the lattice sites is considered as the basic variable of the many-body problem. A new approximation to the interaction-energy functional $W[\gamma]$ is proposed which is based on its scaling properties and which recovers exactly the limit of strong electron correlations at half-band filling. In this way, a more accurate description of $W$ is obtained throughout the domain of representability of $\gamma_{ij}$, including the crossover from weak to strong correlations. As examples of applications results are given for the ground-state energy, charge-excitation gap, and charge susceptibility of the Hubbard model in one-, two-, and three-dimensional lattices. The performance of the method is demonstrated by comparison with available exact solutions, with numerical calculations, and with LDFT using a simpler dimer ansatz for $W$. Goals and limitations of the different approximations are discussed.Comment: 25 pages and 8 figures, submitted to Phys. Rev.

Effect of Different Urine Cow Nitrogen Rates and Different Moisture Conditions on Nitrogen Mineralization in an Andisol from Southern Chile

Urea present in cattle urine contributes large amounts of nitrogen (N) to grazed pastures, which can be an equivalent to approximately 1000 kg N ha−1. However, there are no studies in volcanic soils of southern Chile on the effect of different concentrations of urinary N deposited in the soil, nor of different wetting and drying conditions mimicking the variation of weather conditions on the nitrification process, from urea to NH4+ and total oxidized nitrogen (TON), throughout time. In addition, the inhibition on nitrification driven by the accumulation of NH3 at high rates of N applied to an Andisol have not been evaluated. Fresh cattle urine was applied at three different rates of N equivalent to 247 kg N ha−1 (Low N), 461 kg N ha−1 (Medium N), 921 kg N ha−1 (High N), and de-ionized water as Control. Further, three moisture conditions were imposed: constant moisture (CM), drying-rewetting (DRW) cycles at 7 days interval, and soil drying (SD). Destructive soil cores samples were evaluated for top and bottom halves individually every 7 days over a 36-d period to measure changes on inorganic N and pH. There were no interaction effects for N rates and soil moisture. The main effect of the different rates of N on mineralization was significant throughout the incubation period, while the effect of the different moisture conditions was var-iable over time. The High N was associated with elevated NH3 concentrations and could explain that total N mineralization was partially inhibited. These results suggest that the presence of different nitrifying microorganisms in the soil under different chemical and physical conditions determines nitrification, thus, the oxidation of ammonia should be studied in more detail as the first step of nitrification, specifically in volcanic soils

A study of relative accommodative responses: The Apple Macintosh computer vs hard copy

A study of relative accommodative responses: The Apple Macintosh computer vs hard cop