Spectral weight function for the half-filled Hubbard model: a singular value decomposition approach

The singular value decomposition technique is used to reconstruct the electronic spectral weight function for a half-filled Hubbard model with on-site repulsion $U=4t$ from Quantum Monte Carlo data. A two-band structure for the single-particle excitation spectrum is found to persist as the lattice size exceeds the spin-spin correlation length. The observed bands are flat in the vicinity of the $(0,\pi),(\pi,0)$ points in the Brillouin zone, in accordance with experimental data for high-temperature superconducting compounds.Comment: 4 pages, Revtex

Overproduction and Characterization of the Bacillus subtilis Anti-sigma Factor FlgM

FlgM is an anti-sigma factor of the flagellar-specific sigma (sigma) subunit of RNA polymerase in Bacillus subtilis, and it is responsible of the coupling of late flagellar gene expression to the completion of the hook-basal body structure. We have overproduced the protein in soluble form and characterized it. FlgM forms dimers as shown by gel exclusion chromatography and native polyacrylamide gel electrophoresis and interacts in vitro with the cognate sigmaD factor. The FlgM.sigmaD complex is a stable heterodimer as demonstrated by gel exclusion chromatography, chemical cross-linking, native polyacrylamide gel electrophoresis, and isoelectric focusing. sigmaD belongs to the group of sigma factors able to bind to the promoter sequence even in the absence of core RNA polymerase. The FlgM.sigmaD complex gave a shift in a DNA mobility shift assay with a probe containing a sigmaD-dependent promoter sequence. Limited proteolysis studies indicate the presence of two structural motifs, corresponding to the N- and C-terminal regions, respectively

Respuesta a la fertilización postergada con fósforo y azufre en pasturas perennes con base de alfalfa

Las pasturas perennes con base alfalfa ocupan una gran superficie en la denominada llanura pampeana de la República Argentina, tienen una doble función en el sistema de producción, por un lado son la base forrajera de las líneas de producción ganadera y por otro cumplen la función de recuperar parte de la fertilidad perdida durante el ciclo agrícola de la rotación. Estas pasturas son grandes consumidoras de nutrientes. Los nutrientes que con más frecuencia limitan el crecimiento de dichas pasturas son el P, N Yen menor frecuencia el S, solo cuando están cubiertas las necesidades de los dos primeros. Existen algunos antecedentes locales de fertilizaciones de pasturas, pero a la siembra e incorporados. Para la realización del presente trabajo de fertilización postergada con N, P Y S se establecieron ensayos con lotes de pasturas como para tener elementos para poder justificar los resultados. Se realizó una fertilización al voleo con fertilizadora de péndulo a principios de primavera, utilizando un formulado comercial que contienen una mezcla N, P Y S. Para la evaluación de los resultados se hicieron cortes de dichas pasturas, se midió la producción de materia seca, analizando las diferencias entre medias a través de análisis de varianza y se realizó un pequeño análisis económico de la práctica.Director: Ing. Agr. Elke Noellemeyer, Cátedra de Edafología y Manejo de Suelos

Spatially Resolved Mapping of Local Polarization Dynamics in an Ergodic Phase of Ferroelectric Relaxor

Spatial variability of polarization relaxation kinetics in relaxor ferroelectric 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 is studied using time-resolved Piezoresponse Force Microscopy. Local relaxation attributed to the reorientation of polar nanoregions is shown to follow stretched exponential dependence, exp(-(t/tau)^beta), with beta~~0.4, much larger than the macroscopic value determined from dielectric spectra (beta~~0.09). The spatial inhomogeneity of relaxation time distributions with the presence of 100-200 nm "fast" and "slow" regions is observed. The results are analyzed to map the Vogel-Fulcher temperatures on the nanoscale.Comment: 23 pages, 4 figures, supplementary materials attached; to be submitted to Phys. Rev. Let

Analytical continuation of imaginary axis data for optical conductivity

We compare different methods for performing analytical continuation of spectral data from the imaginary time or frequency axis to the real frequency axis for the optical conductivity sigma(omega). We compare the maximum entropy (MaxEnt), singular value decomposition (SVD), sampling and Pade methods for analytical continuation. We also study two direct methods for obtaining sigma(0). For the MaxEnt approach we focus on a recent modification. The data are split up in batches, a separate MaxEnt calculation is done for each batch and the results are averaged. For the problems studied here, we find that typically the SVD, sampling and modified MaxEnt methods give comparable accuracy, while the Pade approximation is usually less reliable.Comment: 10 pages, 7 figure

Conductance of the single-electron transistor: A comparison of experimental data with Monte Carlo calculations

We report on experimental results for the conductance of metallic single-electron transistors as a function of temperature, gate voltage and dimensionless conductance. In contrast to previous experiments our transistor layout allows for a direct measurement of the parallel conductance and no ad hoc assumptions on the symmetry of the transistors are necessary. Thus we can make a comparison between our data and theoretical predictions without any adjustable parameter. Even for rather weakly conducting transistors significant deviations from the perturbative results are noted. On the other hand, path integral Monte Carlo calculations show remarkable agreement with experiments for the whole range of temperatures and conductances.Comment: 8 pages, 7 figures, revtex4, corrected typos, submitted to PR