207 research outputs found
Current Profiles of Molecular Nanowires; DFT Green Function Representation
The Liouville-space Green function formalism is used to compute the current
density profile across a single molecule attached to electrodes. Time ordering
is maintained in real, physical, time, avoiding the use of artificial time
loops and backward propagations. Closed expressions for molecular currents,
which only require DFT calculations for the isolated molecule, are derived to
fourth order in the molecule/electrode coupling.Comment: 21 page
Determination of the high-pressure crystal structure of BaWO4 and PbWO4
We report the results of both angle-dispersive x-ray diffraction and x-ray
absorption near-edge structure studies in BaWO4 and PbWO4 at pressures of up to
56 GPa and 24 GPa, respectively. BaWO4 is found to undergo a pressure-driven
phase transition at 7.1 GPa from the tetragonal scheelite structure (which is
stable under normal conditions) to the monoclinic fergusonite structure whereas
the same transition takes place in PbWO4 at 9 GPa. We observe a second
transition to another monoclinic structure which we identify as that of the
isostructural phases BaWO4-II and PbWO4-III (space group P21/n). We have also
performed ab initio total energy calculations which support the stability of
this structure at high pressures in both compounds. The theoretical
calculations further find that upon increase of pressure the scheelite phases
become locally unstable and transform displacively into the fergusonite
structure. The fergusonite structure is however metastable and can only occur
if the transition to the P21/n phases were kinetically inhibited. Our
experiments in BaWO4 indicate that it becomes amorphous beyond 47 GPa.Comment: 46 pages, 11 figures, 3 table
Suppression of decoherence via strong intra-environmental coupling
We examine the effects of intra-environmental coupling on decoherence by
constructing a low temperature spin--spin-bath model of an atomic impurity in a
Debye crystal. The impurity interacts with phonons of the crystal through
anti-ferromagnetic spin-spin interactions. The reduced density matrix of the
central spin representing the impurity is calculated by dynamically integrating
the full Schroedinger equation for the spin--spin-bath model for different
thermally weighted eigenstates of the spin-bath. Exact numerical results show
that increasing the intra-environmental coupling results in suppression of
decoherence. This effect could play an important role in the construction of
solid state quantum devices such as quantum computers.Comment: 4 pages, 3 figures, Revtex fil
State Orthogonalization by Building a Hilbert Space: A New Approach to Electronic Quantum Transport in Molecular Wires
Quantum descriptions of many complex systems are formulated most naturally in
bases of states that are not mutually orthogonal. We introduce a general and
powerful yet simple approach that facilitates solving such models exactly by
embedding the non-orthogonal states in a new Hilbert space in which they are by
definition mutually orthogonal. This novel approach is applied to electronic
transport in molecular quantum wires and is used to predict conductance
antiresonances of a new type that arise solely out of the non-orthogonality of
the local orbitals on different sites of the wire.Comment: 4 pages 1 figur
Electron Standing Wave Formation in Atomic Wires
Using the Landauer formulation of transport theory and tight binding models
of the electronic structure, we study electron transport through atomic wires
that form 1D constrictions between pairs of metallic nano-contacts. Our results
are interpreted in terms of electron standing waves formed in the atomic wires
due to interference of electron waves reflected at the ends of the atomic
constrictions. We explore the influence of the chemistry of the atomic
wire-metal contact interfaces on these standing waves and the associated
transport resonances by considering two types of atomic wires: gold wires
attached to gold contacts and carbon wires attached to gold contacts. We find
that the conductance of the gold wires is roughly for the
wire lengths studied, in agreement with experiments. By contrast, for the
carbon wires the conductance is found to oscillate strongly as the number of
atoms in the wire varies, the odd numbered chains being more conductive than
the even numbered ones, in agreement with previous theoretical work that was
based on a different model of the carbon wire and metal contacts.Comment: 14 pages, includes 6 figure
Water stress and temperature effects on germination and early seedling growth of Digitaria eriantha
This study focused on the two major processes critical for plant establishment: Seed germination and seedling survival. We determined the effects of (1) water stress and temperature on the germination, and (2) water stress on early seedling growth of Digitaria eriantha cv ‘Irene’. Seeds harvested in 2007 were used for temperature studies, and those coming from 2006 and 2007 for water stress studies. In 2009, viability decreased by 65.4% from 2006 to 2007. During the first twenty-four hours, germination was more than 50% at constant (30 or 35°C) than alternating (10/30 or 10/35°C) temperatures, although total germination was about 80% for all temperature treatments. Polyethylene glycol 8000 was used to impose water stress conditions. Germination percentages and coefficients of velocity decreased with decreasing water potentials. Early seedling growth was smaller at lower water potentials. D. eriantha cv ‘Irene’ appeared to germinate within a wide range of temperatures, but it varied greatly in germination response to water potentials. Results suggest that this species could be planted in late spring-early summer, when seedbed temperatures are increasing and soil moisture might still be adequate.Fil: Brevedan, Roberto. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); Argentina. Universidad Nacional del Sur. Departamento de AgronomÃa; ArgentinaFil: Busso, Carlos Alberto. Universidad Nacional del Sur. Departamento de AgronomÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); ArgentinaFil: Fioretti, M. N.. Universidad Nacional del Sur. Departamento de AgronomÃa; ArgentinaFil: Toribio, M. B.. Universidad Nacional del Sur. Departamento de AgronomÃa; ArgentinaFil: Baioni, S. S.. Universidad Nacional del Sur. Departamento de AgronomÃa; ArgentinaFil: Torres, Yanina Alejandra. Universidad Nacional del Sur. Departamento de AgronomÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); ArgentinaFil: Fernandez, Osvaldo Alberto. Universidad Nacional del Sur. Departamento de AgronomÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); ArgentinaFil: Giorgetti, H. D.. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; ArgentinaFil: Bentivegna, Diego Javier. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); ArgentinaFil: EntÃo, J.. Universidad Nacional de La Plata. Facultad de Ciencias Agrarias y Forestales; ArgentinaFil: Ithurrart, Leticia Soledad. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); Argentina. Universidad Nacional del Sur. Departamento de AgronomÃa; ArgentinaFil: Montenegro, O.. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; ArgentinaFil: Mujica, M. de las M.. Universidad Nacional de La Plata. Facultad de Ciencias Agrarias y Forestales; ArgentinaFil: RodrÃguez, G.. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; ArgentinaFil: Tucat, Guillermo. Universidad Nacional del Sur. Departamento de AgronomÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); Argentin
Optical Spectra of Four Objects Identified with Variable Radio Sources
We obtained optical spectra of four objects identified with variable radio
sources. Three objects (0029+0554, 0400+0550, 2245+0500) were found to be
quasars with redshifts of 1.314, 0.761, and 1.091. One object (2349+0534) has a
continuum spectrum characteristic of BL Lac objects. We analyze spectra of the
radio sources in the range 0.97-21.7 GHz for the epoch 1997 and in the range
3.9-11.1 GHz for the epoch 1990, as well as the pattern of variability of their
flux densities on time scales of 1.5 and 7 years.Comment: 4 pages, PD
Water stress and temperature effects on germination and early seedling growth of <i>Digitaria eriantha</i>
This study focused on the two major processes critical for plant establishment: Seed germination and seedling survival. We determined the effects of (1) water stress and temperature on the germination, and (2) water stress on early seedling growth of Digitaria eriantha cv ‘Irene’. Seeds harvested in 2007 were used for temperature studies, and those coming from 2006 and 2007 for water stress studies. In 2009, viability decreased by 65.4% from 2006 to 2007. During the first twenty-four hours, germination was more than 50% at constant (30 or 35°C) than alternating (10/30 or 10/35°C) temperatures, although total germination was about 80% for all temperature treatments. Polyethylene glycol 8000 was used to impose water stress conditions. Germination percentages and coefficients of velocity decreased with decreasing water potentials. Early seedling growth was smaller at lower water potentials. D. eriantha cv ‘Irene’ appeared to germinate within a wide range of temperatures, but it varied greatly in germination response to water potentials. Results suggest that this species could be planted in late spring-early summer, when seedbed temperatures are increasing and soil moisture might still be adequate.Facultad de Ciencias Agrarias y Forestale
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