831 research outputs found
Coexistence of charge density wave and spin-Peierls orders in quarter-filled quasi-one dimensional correlated electron systems
Charge and spin-Peierls instabilities in quarter-filled (n=1/2) compounds
consisting of coupled ladders and/or zig-zag chains are investigated. Hubbard
and t-J models including local Holstein and/or Peierls couplings to the lattice
are studied by numerical techniques. Next nearest neighbor hopping and magnetic
exchange, and short-range Coulomb interactions are also considered. We show
that, generically, these systems undergo instabilities towards the formation of
Charge Density Waves, Bond Order Waves and (generalized) spin-Peierls modulated
structures. Moderate electron-electron and electron-lattice couplings can lead
to a coexistence of these three types of orders. In the ladder, a zig-zag
pattern is stabilized by the Holstein coupling and the nearest-neighbor Coulomb
repulsion. In the case of an isolated chain, bond-centered and site-centered
2k_F and 4k_F modulations are induced by the local Holstein coupling. In
addition, we show that, in contrast to the ladders, a small charge ordering in
the chains, strongly enhances the spin-Peierls instability. Our results are
applied to the NaV_2O_5 compound (trellis lattice) and various phases with
coexisting charge disproportionation and spin-Peierls order are proposed and
discussed in the context of recent experiments. The role of the long-range
Coulomb potential is also outlined.Comment: 10 pages, Revtex, 10 encapsulated figure
Study of impurities in spin-Peierls systems including lattice relaxation
The effects of magnetic and non-magnetic impurities in spin-Peierls systems
are investigated allowing for lattice relaxation and quantum fluctuations. We
show that, in isolated chains, strong bonds form next to impurities, leading to
the appearance of magneto-elastic solitons. Generically, these solitonic
excitations do not bind to impurities. However, interchain elastic coupling
produces an attractive potential at the impurity site which can lead to the
formation of bound states. In addition, we predict that small enough chain
segments do not carry magnetic moments at the ends
Advancing the field of organizations through the study of military organizations
The article of record as published may be found at http://dx.doi.org/10.1093/icc/dtt059This article argues that the field of organization studies may learn from closer study of decision-making and behaviors in military organizations. It describes some of the intellectual roots of organizational studies within a strategic, military context; discusses some recent characteristics of strategic competition that organ- ization scholars may find fruitful to study; and view some of the key contemporary challenges in military organizations through the lens of strategic organization design, a framework the builds on, and integrates, several streams of research in organizational behavior that have implications for, and influence, how organizations make strategic decisions
Dynamic spin Jahn-Teller effect in small magnetic clusters
We study the effect of spin-phonon coupling in small magnetic clusters,
concentrating on a S=1/2 ring of 4 spins coupled antiferromagnetically. If the
phonons are treated as classical variables, there is a critical value of the
spin-phonon coupling above which a static distortion occurs. This is a good
approximation if the zero point energy is small compared to the energy gain due
to the distortion, which is true for large exchange interactions compared to
the phonons energy (). In the opposite limit, one can
integrate out the phonon degrees of freedom and get an effective spin
hamiltonian. Using exact diagonalizations to include the quantum nature of both
spins and phonons, we obtain the spectrum in the whole range of parameters and
explicit the crossover between the classical and quantum regimes. We then
establish quantitatively the limits of validity of two widely used approaches
(one in the quantum and one in the classical limits) and show that they are
quite poor for small magnetic clusters. We also show that upon reducing
the first excitation of a 4-site cluster becomes a singlet, a
result that could be relevant for CuTeOBr
What do experimental data "say" about growth of hadronic total cross-section?
We reanalyse and high energy data of the elastic scattering
above GeV on the total cross-section and on the
forward -ratio for various models of Pomeron, utilizing two methods. The
first one is based on analytic amplitudes, the other one relies on assumptions
for and on dispersion relation for . We argue that it is
not possible, from fitting only existing data for forward scattering, to select
a definite asymptotic growth with the energy of . We find
equivalent fits to the data together with a logarithmic Pomeron giving a
behavior , and with
a supercritical Pomeron giving a behavior ,
.Comment: LaTeX, 18 pages, 5 eps figures included, to be published in Il Nuovo
Ciment
Experimental study of stratified turbulence forced with columnar dipoles
International audienceWe present a novel experimental setup aimed at producing a forced strongly stratified turbulent flow. The flow is forced by an arena of 12 vortex pair generators in a large tank. The continuous interactions of the randomly produced vortex pairs give rise to a statistically stationary disordered flow in contrast to previous experiments where the stratified turbulence is decaying. The buoyancy frequency N is set to its highest value N = 1.7 rad/s using salt as stratifying agent so that the horizontal Froude number F h = Ω/N is low, while the buoyancy Reynolds number R=ReFh2 , where Re = Ωa 2/Îœ is the classical Reynolds number, is as high as possible given the experimental constraints (Ω is the maximum angular velocity of the vortices, a their radius and Îœ the viscosity). PIV measurements show that the flow is not homogeneous in the horizontal plane and is organised into horizontal layers along the vertical. When R is increased, we observe a progressive evolution from the viscosity dominated regime with smooth layers to a regime with small scales superimposed on the layers and for which the vertical Froude number is of order one. The latter regime resembles the strongly stratified turbulent regime with a downscale cascade that has been predicted for large R . However, horizontal second order structure functions do not exhibit a clear inertial range for the largest R achieved R=310 . In addition, the corresponding turbulent buoyancy Reynolds number Rt=P/(ÎœN2) based on an estimation of the injection rate of energy P is only of order unity Rtâ0.4 indicating that only the edge of the strongly stratified turbulent regime has been reached. However, these results suggest that sufficiently large turbulent buoyancy Reynolds numbers, Rtâ10 , could be achieved experimentally by scaling up five times this novel set-up
High Energy Hadron-Nucleus Cross Sections and Their Extrapolation to Cosmic Ray Energies
Old models of the scattering of composite systems based on the Glauber model
of multiple diffraction are applied to hadron-nucleus scattering. We obtain an
excellent fit with only two free parameters to the highest energy
hadron-nucleus data available. Because of the quality of the fit and the
simplicity of the model it is argued that it should continue to be reliable up
to the highest cosmic ray energies. Logarithmic extrapolations of proton-proton
and proton-antiproton data are used to calculate the proton-air cross sections
at very high energy. Finally, it is observed that if the exponential behavior
of the proton-antiproton diffraction peak continues into the few TeV energy
range it will violate partial wave unitarity. We propose a simple modification
that will guarantee unitarity throughout the cosmic ray energy region.Comment: 8 pages, 9 postscript figures. This manuscript replaces a partial
manuscript incorrectly submitte
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