1,135 research outputs found
Local spin spirals in the Neel phase of La_{2-x}Sr_xCuO_4
Experimental observations of lightly doped La_{2-x}Sr_xCuO_4, x < 0.02,
revealed remarkable magnetic properties such as the incommensurate noncollinear
ordering (additional to the Neel ordering) and a tremendous doping dependence
of the uniform longitudinal susceptibility. We show that the spiral solution of
the t-t'-t''-J model obtained by taking into account the Coulomb trapping of
holes by Sr ions describes these puzzling data perfectly well. Our solution
firstly explains why the incommensurate structure is directed along the
orthorhombic b-axis, and secondly allows a numerical calculation of the
positions and shapes of the incommensurate neutron scattering peaks. Thirdly,
we calculate the doping dependence of the spin-wave gap, and lastly, we study
the longitudinal magnetic susceptibility and show that its doping dependence is
due to the noncollinearity of the spin spiral.Comment: 13 pages, 8 figure
Origin of primitive ocean island basalts by crustal gabbro assimilation and multiple recharge of plume-derived melts
Chemical Geodynamics relies on a paradigm that the isotopic composition of ocean island basalt (OIB) represents equilibrium with its primary mantle sources. However, the discovery of huge isotopic heterogeneity within olivineâhosted melt inclusions in primitive basalts from Kerguelen, Iceland, Hawaii and South Pacific Polynesia islands implies openâsystem behavior of OIBs, where during magma residence and transport, basaltic melts are contaminated by surrounding lithosphere. To constrain the processes of crustal assimilation by OIBs, we employed the Magma Chamber Simulator (MCS), an energyâconstrained thermodynamic model of recharge, assimilation and fractional crystallization. For a case study of the 21â19 Ma basaltic series, the most primitive series ever found among the Kerguelen OIBs, we performed sixtyâseven simulations in the pressure range from 0.2 to 1.0 GPa using compositions of olivineâhosted melt inclusions as parental magmas, and metagabbro xenoliths from the Kerguelen Archipelago as wallrock. MCS modeling requires that the assimilant is anatectic crustal melts (P2O5ââ€â0.4 wt.% contents) derived from the Kerguelen oceanic metagabbro wallrock. To best fit the phenocryst assemblage observed in the investigated basaltic series, recharge of relatively large masses of hydrous primitive basaltic melts (H2Oâ=â2â3 wt%; MgOâ=â7â10 wt.%) into a middle crustal chamber at 0.2 to 0.3 GPa is required. Our results thus highlight the important impact that crustal gabbro assimilation and mantle recharge can have on the geochemistry of mantleâderived olivineâphyric OIBs. The importance of crustal assimilation affecting primitive plumeâderived basaltic melts underscores that isotopic and chemical equilibrium between ocean island basalts and associated deep plume mantle source(s) may be the exception rather than the rule
A well-posedness theory in measures for some kinetic models of collective motion
We present existence, uniqueness and continuous dependence results for some
kinetic equations motivated by models for the collective behavior of large
groups of individuals. Models of this kind have been recently proposed to study
the behavior of large groups of animals, such as flocks of birds, swarms, or
schools of fish. Our aim is to give a well-posedness theory for general models
which possibly include a variety of effects: an interaction through a
potential, such as a short-range repulsion and long-range attraction; a
velocity-averaging effect where individuals try to adapt their own velocity to
that of other individuals in their surroundings; and self-propulsion effects,
which take into account effects on one individual that are independent of the
others. We develop our theory in a space of measures, using mass transportation
distances. As consequences of our theory we show also the convergence of
particle systems to their corresponding kinetic equations, and the
local-in-time convergence to the hydrodynamic limit for one of the models
Phase diagram of TTB ferroelectric compounds Pb1âxK2xNb2O6
Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1âxK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions.Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1âxK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions
Defects as a reason of continuity of normal-incommensurate phase transitions
Almost all normal-incommensurate phase transitions observed experimentally
are continuous. We show that there is not any theoretical reason for this
general behaviour in perfect crystals. A normal-incommensurate phase transition
that is not too far from the mean-field tricritical point should be
discontinuous and it is highly improbable that so far reported
normal-incommensurate phase transitions lie very far from this point. To
understand this behaviour we study influence of defects on a hypothetical
first-order normal-incommensurate phase transition in a pure material. We have
found that this influence is strikingly different from that on other kinds of
first-order phase transitions. The change of the discontinuity of the order
parameter at the transition is negative and formally diverges within our
approximate theory. At the same time the diminishing of the phase transition
temperature remains finite. We interpret these results as an indication that at
least some of the observed seemingly second-order normal-incommensurate
transitions would be first-order transitions in defectless crystals.Comment: 17 pages, 1 figur
Discerning Aggregation in Homogeneous Ensembles: A General Description of Photon Counting Spectroscopy in Diffusing Systems
In order to discern aggregation in solutions, we present a quantum mechanical
analog of the photon statistics from fluorescent molecules diffusing through a
focused beam. A generating functional is developed to fully describe the
experimental physical system as well as the statistics. Histograms of the
measured time delay between photon counts are fit by an analytical solution
describing the static as well as diffusing regimes. To determine empirical
fitting parameters, fluorescence correlation spectroscopy is used in parallel
to the photon counting. For expedient analysis, we find that the distribution's
deviation from a single Poisson shows a difference between two single fluor
moments or a double fluor aggregate of the same total intensities. Initial
studies were performed on fixed-state aggregates limited to dimerization.
However preliminary results on reactive species suggest that the method can be
used to characterize any aggregating system.Comment: 30 pages, 5 figure
Dielectric Properties Of Lead Potassium Lithium Niobate (Pb1,85K1,15Li0,15Nb5O15) With Tetragonal Tungsten Bronze (TTB) Type Structure
A new tungsten bronze ceramic oxide, Pb2-xK1+xLixNb5O15 (PKLN) (x =0.15) was prepared by high temperature solid-state reaction route. Structural and electrical properties are investigated using X-ray diffraction and dielectric measurements. Room temperature XRD pattern confirms the formation of the compound with an orthorhombic crystal system. The dielectric permittivity and the loss tangent of the sample have been measured in a frequency range 1Hzâ1MHz and a temperature range 35â550 °C. Studies of dielectric properties show that the compound exhibits an anomaly at 425°C (usually called transition temperature).The electrical parameters of the material were studied using complex impedance spectroscopy showing that the compound exhibits non-Debye of relaxation process. In the paraelectric phase, activation energy was determined and the value is EÏ = 0.68 eV. The present ceramic is promising candidate for high dielectric constant and low loss dielectric ceramic.A new tungsten bronze ceramic oxide, Pb2-xK1+xLixNb5O15 (PKLN) (x =0.15) was prepared by high temperature solid-state reaction route. Structural and electrical properties are investigated using X-ray diffraction and dielectric measurements. Room temperature XRD pattern confirms the formation of the compound with an orthorhombic crystal system. The dielectric permittivity and the loss tangent of the sample have been measured in a frequency range 1Hzâ1MHz and a temperature range 35â550 °C. Studies of dielectric properties show that the compound exhibits an anomaly at 425°C (usually called transition temperature).The electrical parameters of the material were studied using complex impedance spectroscopy showing that the compound exhibits non-Debye of relaxation process. In the paraelectric phase, activation energy was determined and the value is EÏ = 0.68 eV. The present ceramic is promising candidate for high dielectric constant and low loss dielectric ceramic
On the stability of high-speed milling with spindle speed variation
Spindle speed variation is a well-known technique to suppress regenerative machine tool vibrations, but it is usually considered to be effective only for low spindle speeds. In this paper, the effect of spindle speed variation is analyzed in the high-speed domain for spindle speeds corresponding to the first flip (period doubling) and to the first Hopf lobes. The optimal amplitudes and frequencies of the speed modulations are computed using the semidiscre- tization method. It is shown that period doubling chatter can effectively be suppressed by spindle speed variation, although, the technique is not effective for the quasiperiodic chatter above the Hopf lobe. The results are verified by cutting tests. Some special cases are also discussed where the practical behavior of the system differs from the predicted one in some ways. For these cases, it is pointed out that the concept of stability is understood on the scale of the principal period of the systemâthat is, the speed modulation period for variable spindle speed machining and the tooth passing period for constant spindle speed machining
Effects of anisotropic interactions on the structure of animal groups
This paper proposes an agent-based model which reproduces different
structures of animal groups. The shape and structure of the group is the effect
of simple interaction rules among individuals: each animal deploys itself
depending on the position of a limited number of close group mates. The
proposed model is shown to produce clustered formations, as well as lines and
V-like formations. The key factors which trigger the onset of different
patterns are argued to be the relative strength of attraction and repulsion
forces and, most important, the anisotropy in their application.Comment: 22 pages, 9 figures. Submitted. v1-v4: revised presentation; extended
simulations; included technical results. v5: added a few clarification
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