112 research outputs found
Spin transport in magnetic multilayers
We study by extensive Monte Carlo simulations the transport of itinerant
spins travelling inside a multilayer composed of three ferromagnetic films
antiferromagnetically coupled to each other in a sandwich structure. The two
exterior films interact with the middle one through non magnetic spacers. The
spin model is the Ising one and the in-plane transport is considered. Various
interactions are taken into account. We show that the current of the itinerant
spins going through this system depends strongly on the magnetic ordering of
the multilayer: at temperatures below (above) the transition temperature
, a strong (weak) current is observed. This results in a strong jump of
the resistance across . Moreover, we observe an anomalous variation,
namely a peak, of the spin current in the critical region just above . We
show that this peak is due to the formation of domains in the temperature
region between the low- ordered phase and the true paramagnetic disordered
phase. The existence of such domains is known in the theory of critical
phenomena. The behavior of the resistance obtained here is compared to a recent
experiment. An excellent agreement with our physical interpretation is
observed. We also show and discuss effects of various physical parameters
entering our model such as interaction range, strength of electric and magnetic
fields and magnetic film and non magnetic spacer thicknesses.Comment: 8 pages, 17 figures, submitted to J. Phys.: Cond Matte
First-Order Phase Transition in Potts Models with finite-range interactions
We consider the -state Potts model on , , ,
with Kac ferromagnetic interactions and scaling parameter \ga. We prove the
existence of a first order phase transition for large but finite potential
ranges. More precisely we prove that for \ga small enough there is a value of
the temperature at which coexist Gibbs states. The proof is obtained by a
perturbation around mean-field using Pirogov-Sinai theory. The result is valid
in particular for , Q=3, in contrast with the case of nearest-neighbor
interactions for which available results indicate a second order phase
transition. Putting both results together provides an example of a system which
undergoes a transition from second to first order phase transition by changing
only the finite range of the interaction.Comment: Soumis pour publication a Journal of statistical physics - version
r\'{e}vis\'{e}
Reexamination of the long-range Potts model: a multicanonical approach
We investigate the critical behavior of the one-dimensional q-state Potts
model with long-range (LR) interaction , using a multicanonical
algorithm. The recursion scheme initially proposed by Berg is improved so as to
make it suitable for a large class of LR models with unequally spaced energy
levels. The choice of an efficient predictor and a reliable convergence
criterion is discussed. We obtain transition temperatures in the first-order
regime which are in far better agreement with mean-field predictions than in
previous Monte Carlo studies. By relying on the location of spinodal points and
resorting to scaling arguments, we determine the threshold value
separating the first- and second-order regimes to two-digit precision within
the range . We offer convincing numerical evidence supporting
$\sigma_c(q)Comment: 18 pages, 18 figure
Deindustrialization and the Polarization of Household Incomes: The Example of Urban Agglomerations in Germany
Institutional Conflict Settlement in Divided Societies: The Role of Subgroup Identities in Self-Government Arrangements
From low-conflict polity to democratic civil peace: Explaining Zambian exceptionalism
An absence of civil war and other significant sub-state violence makes Zambia an exceptional although not unique case in central-southern Africa. The literature devoted to explaining civil war has grown dramatically in recent years, but while it pays much attention to sub-Saharan Africa only rarely does it investigate counterfactual cases like Zambia. Similarly the growing field of research into post-conflict reconstruction fails to capture the distinct features of persistently low-conflict situations where many of the predisposing conditions for violent conflict might seem to be present. This paper examines Zambiaâs experience against a background of general theories that try to explain conflict. It is an âinterpretative case studyâ. The paper proceeds by substantiating Zambiaâs claim to a relatively peaceful record and introduces ideas of conflict and conflict theories, before arguing that no single general theory dwelling on just one primary âcauseâ will suffice to explain Zambian exceptionalism. The precise mix of arguments differs for each one Zambiaâs three republican eras, as the potential threats to peace have themselves evolved over the period since independence. The paperâs main theoretical claim is that over time the explanation is both multi-layered and dynamic. That said, certain features do stand out, most notably an inherited political culture that is predisposed against the violent resolution of conflict and continues to insulate the country against social and economic traumas and democratic shortcomings
Different true-protein sources do not modify the metabolism of crossbred Bos taurus Ă Bos indicus growing heifers
Synthesis, Structures, and Optical Properties of Ruthenium(II) Complexes of the Tris(1-pyrazolyl)methane Ligand
Four new complex salts [Ru^(II)Cl(Tpm)(L^A)_2][PF_6]_n [Tpm = tris(1-pyrazolyl)methane; n = 1, L^A = pyridine (py) 1 or ethyl isonicotinate (EIN) 2; n = 3, L^A = N-methyl-4,4â˛-bipyridinium (MeQ^+) 3 or N-phenyl-4,4â˛-bipyridinium (PhQ^+) 4] have been prepared and characterized. Electronic absorption spectra show intense d â Ď^* metal-to-ligand charge-transfer (MLCT) absorption bands, while cyclic voltammetry reveals a reversible Ru^(III/II) wave, accompanied by quasireversible or irreversible L^A-based reductions for all except 1. Single crystal X-ray structures have been obtained for 1â˘Me_2CO, 2, and 3â˘Me_2CO. For 2â4, molecular first hyperpolarizabilities β have been measured in acetonitrile solutions via the hyper-Rayleigh scattering (HRS) technique at 800 nm. Stark (electroabsorption) spectroscopic studies on the MLCT bands in frozen butyronitrile allow the indirect estimation of static first hyperpolarizabilities β_0. The various physical data obtained for 3 and 4 are compared with those reported previously for related cis-{Ru^(II)(NH_3)_4}^(2+) species [Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 4845]. TD-DFT calculations on the complexes in 1â4 confirm that their lowest energy absorption bands are primarily Ru^(II) â L^A MLCT in character, while Ru^(II) â Tpm MLCT transitions are predicted at higher energies. DFT agrees with the Stark, but not the HRS measurements, in showing that β_0 increases with the electron-accepting strength of L^A. The 2D nature of the chromophores is evidenced by dominant β_(xxy) tensor components
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