1,179 research outputs found
The quantum Heisenberg antiferromagnet on the square lattice
The pure-quantum self-consistent harmonic approximation, a semiclassical
method based on the path-integral formulation of quantum statistical mechanics,
is applied to the study of the thermodynamic behaviour of the quantum
Heisenberg antiferromagnet on the square lattice (QHAF). Results for various
properties are obtained for different values of the spin and successfully
compared with experimental data.Comment: Proceedings of the Conference "Path Integrals from peV to TeV - 50
Years from Feynman's paper" (Florence, August 1998) -- 2 pages, ReVTeX, 2
figure
Ising transition in the two-dimensional quantum Heisenberg model
We study the thermodynamics of the spin- two-dimensional quantum
Heisenberg antiferromagnet on the square lattice with nearest () and
next-nearest () neighbor couplings in its collinear phase (),
using the pure-quantum self-consistent harmonic approximation. Our results show
the persistence of a finite-temperature Ising phase transition for every value
of the spin, provided that the ratio is greater than a critical value
corresponding to the onset of collinear long-range order at zero temperature.
We also calculate the spin- and temperature-dependence of the collinear
susceptibility and correlation length, and we discuss our results in light of
the experiments on LiVOSiO and related compounds.Comment: 4 page, 4 figure
Weak localization effects in granular metals
The weak localization correction to the conductivity of a granular metal is
calculated using the diagrammatic technique in the reciprocal grain lattice
representation. The properties of this correction are very similar to that one
in disordered metal, with the replacement of the electron mean free path by the grain diameter and the dimensionless conductance by the
tunnelling dimensionless conductance . In particular, we demonstrate
that at zero temperature no conducting phase can exist for dimensions . We also analyze the WL correction to magnetoconductivity in the weak field
limit.Comment: 4 pages, 3 figures; minor corrections adde
Switch on the engine: how the eukaryotic replicative helicase MCM2–7 becomes activated
© 2014, Springer-Verlag Berlin Heidelberg.A crucial step during eukaryotic initiation of DNA replication is the correct loading and activation of the replicative DNA helicase, which ensures that each replication origin fires only once. Unregulated DNA helicase loading and activation, as it occurs in cancer, can cause severe DNA damage and genomic instability. The essential mini-chromosome maintenance proteins 2–7 (MCM2–7) represent the core of the eukaryotic replicative helicase that is loaded at DNA replication origins during G1-phase of the cell cycle. The MCM2–7 helicase activity, however, is only triggered during S-phase once the holo-helicase Cdc45-MCM2–7-GINS (CMG) has been formed. A large number of factors and several kinases interact and contribute to CMG formation and helicase activation, though the exact mechanisms remain unclear. Crucially, upon DNA damage, this reaction is temporarily halted to ensure genome integrity. Here, we review the current understanding of helicase activation; we focus on protein interactions during CMG formation, discuss structural changes during helicase activation, and outline similarities and differences of the prokaryotic and eukaryotic helicase activation process
Changes in leaf size and in the rate of leaf production contribute to cytokinin-mediated growth promotion in Epipremnum aureum L. cuttings
The growth of ornamental foliage plants is often limited by pot size, which exerts a restriction on root growth and, therefore, on the production of root-synthesised cytokinins which play key regulatory roles in the development and growth of the shoot. We studied the effect of exogenous 6-benzylaminopurine (BAP) on plant growth and on the development of leaf area in Epipremnum aureum L. plants grown in pots. The hypothesis was that increasing the concentration of shoot cytokinins by foliar spraying of BAP would promote plant growth by overcoming the effects of root restriction on whole plant development. Three glasshouse experiments were conducted using (i) different concentrations of BAP, (ii) different numbers of spray applications, and (iii) different light environments. The results showed that a single spray application of BAP at 5 mg l–1 significantly (P ≤ 0.05) increased leaf area (by 20 – 40%) and biomass [fresh weight (FW)] accumulation (by 30 – 35%), while higher BAP concentrations, or repeated spray applications had less effect. The maximum effect of BAP was observed under intermediate levels of irradiance. The increased development of leaf area in BAP-sprayed plants resulted from increases in both individual average leaf areas (by 100 – 150% cf. the controls) and the rate of leaf initiation (by 30 – 120% cf. the controls). The latter could be attributed to a shortening of the phyllochron, since no branching was observed under any BAP spray treatment. Alternative physiological explanations, as well as possible commercial applications of these BAP-elicited responses are discussed.Fil: Di Benedetto, A.. Universidad de Buenos Aires. Facultad de Agronomia; Argentina. Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; ArgentinaFil: Galmarini, Claudio Romulo. Instituto Nacional de TecnologÃa Agropecuaria. Centro Regional Cuyo Mendoza-san Juan. Estación Experimental Agropecuaria la Consulta; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Agrarias; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Mendoza; ArgentinaFil: Tognetti, J.. Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones CientÃficas; Argentin
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