6,592 research outputs found
Orbital disorder induced by charge fluctuations in vanadium spinels
Motivated by recent experiments on vanadium spinels, VO, that show
an increasing degree of electronic delocalization for smaller cation sizes, we
study the evolution of orbital ordering (OO) between the strong and
intermediate-coupling regimes of a multi-orbital Hubbard Hamiltonian. The
underlying magnetic ordering of the Mott insulating state leads to a rapid
suppression of OO due to enhanced charge fluctuations along ferromagnetic
bonds. Orbital double-occupancy is rather low at the transition point
indicating that the system is in the crossover region between strong and
intermediate-coupling regimes when the orbital degrees of freedom become
disordered
Random interactions and spin-glass thermodynamic transition in the hole-doped Haldane system YCaBaNiO
Magnetization, DC and AC bulk susceptibility of the =1 Haldane chain
system doped with electronic holes, YCaBaNiO
(0x0.20), have been measured and analyzed. The most striking
results are (i) a sub-Curie power law behavior of the linear susceptibility,
, for temperature lower than the Haldane gap
of the undoped compound (x=0) (ii) the existence of a spin-glass thermodynamic
transition at = 2-3 K. These findings are consistent with (i) random
couplings within the chains between the spin degrees of freedom induced by hole
doping, (ii) the existence of ferromagnetic bonds that induce magnetic
frustration when interchain interactions come into play at low temperature.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Advanced Modeling of Cold-Formed Steel Walls under Fire
This paper discusses an advanced finite element model able to simulate the structural response of cold-formed steel walls during standard fire tests. The model includes experimental thermo-mechanical properties of materials, geometric imperfections, and temperature distributions on studs and sheathing boards. The model is capable of reasonably predicting the thermal bowing of walls, and estimating the shape, size and amount of joint openings between gypsum boards over time of fire exposure. Numerical results validated with experimental data indicate that the maximum out-of-plane displacements due to thermal gradients occur near the wall mid-height. Early in the heating process, joint openings develop on the exposed side of walls due to thermal bowing and contraction of gypsum boards at elevated temperatures, potentially altering the heat transfer and affecting the fire resistance of the entire system. Future work aims to utilize high fidelity modeling to study the response of load bearing cold-formed steel systems subjected to fire, and optimize their fire resistance
Quantum cosmology of scalar-tensor theories and self-adjointness
In this paper, the problem of the self-adjointness for the case of a quantum
minisuperspace Hamiltonian retrieved from a Brans-Dicke (BD) action is
investigated. Our matter content is presented in terms of a perfect fluid, onto
which the Schutz's formalism will be applied. We use the von Neumann theorem
and the similarity with the Laplacian operator in one of the variables to
determine the cases where the Hamiltonian is self-adjoint and if it admits
self-adjoint extensions. For the latter, we study which extension is physically
more suitable.Comment: Latex file, 12 pages. Small changes made in the paper, and a a new
appendix adde
Magnetic field-induced phase transitions in a weakly coupled s = 1/2 quantum spin dimer system BaCrO
By using bulk magnetization, electron spin resonance (ESR), heat capacity,
and neutron scattering techniques, we characterize the thermodynamic and
quantum phase diagrams of BaCrO. Our ESR measurements indicate that
the low field paramagnetic ground state is a mixed state of the singlet and the
S = 0 triplet for . This suggests the presence of an intra-dimer
Dzyaloshinsky-Moriya (DM) interaction with a DM vector perpendicular to the
c-axis
Electron-Doped Manganese Perovskites: The Polaronic State
Using the Lanczos method in linear chains we study the ground state of the
double exchange model including an antiferromagnetic super-exchange in the low
concentration limit. We find that this ground state is always inhomogeneous,
containig ferromagnetic polarons. The extention of the polaron spin distortion,
the dispersion relation and their trapping by impurities, are studied for
diferent values of the super exchange interaction and magnetic field. We also
find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure
Generalized Jordan-Wigner Transformations
We introduce a new spin-fermion mapping, for arbitrary spin generating
the SU(2) group algebra, that constitutes a natural generalization of the
Jordan-Wigner transformation for . The mapping, valid for regular
lattices in any spatial dimension , serves to unravel hidden symmetries in
one representation that are manifest in the other. We illustrate the power of
the transformation by finding exact solutions to lattice models previously
unsolved by standard techniques. We also present a proof of the existence of
the Haldane gap in 1 bilinear nearest-neighbors Heisenberg spin chains and
discuss the relevance of the mapping to models of strongly correlated
electrons. Moreover, we present a general spin-anyon mapping for the case .Comment: 5 pages, 1 psfigur
Behavior of the diffractive cross section in hadron-nucleus collisions
A phenomenological analysis of diffractive dissociation of nuclei in
proton-nucleus and meson-nucleus collisions is presented. The theoretical
approach employed here is able to take into account at once data of the HELIOS
and EHS/NA22 collaborations that exhibit quite different atomic mass
dependences. Possible extensions of this approach to hard diffraction in
nuclear processes are also discussed.Comment: 5 pages, 2 figure
Signatures of 3?6 day planetary waves in the equatorial mesosphere and ionosphere
International audienceCommon periodic oscillations have been observed in meteor radar measurements of the MLT winds at Cariri (7.4° S, 36.5° W) and Ascension Island (7.9° S, 14.4° W) and in the minimum ionospheric virtual height, h'F, measured at Fortaleza (3.9° S, 38.4° W) in 2004, all located in the near equatorial region. Wavelet analysis of these time series reveals that there are 3?4-day, 6?8-day and 12?16-day oscillations in the zonal winds and h'F. The 3?4 day oscillation appeared as a form of a wave packet from 7?17 August 2004. From the wave characteristics analyzed this might be a 3.5-day Ultra Fast Kelvin wave. The 6-day oscillation in the mesosphere was prominent during the period of August to November. In the ionosphere, however, it was apparent only in November. Spectral analysis suggests that this might be a 6.5-day wave previously identified. The 3.5-day and 6.5-day waves in the ionosphere could have important roles in the initiation of equatorial spread F (plasma bubble). These waves might modulate the post-sunset E×B uplifting of the base of the F-layer via the induced lower thermosphere zonal wind and/or the E-region conductivity
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