797,882 research outputs found
Phonon Mechanism of the Ferromagnetic Transition in La1-xSrxMnO3
Temperature dependence of longitudinal optical phonons with oxygen character
was measured in La1-xSrxMnO3 (x=0.2, 0.3) by inelastic neutron scattering in
the (1 0 0) cubic direction. The zone center mode intensity is consistent with
the Debye-Waller factor. However the intensity of the bond-stretching phonons
half way to the zone boundary and near the zone boundary decreases dramatically
as the temperature increases through the ferromagnetic (FM) transition. We
found evidence that the lost phonon spectral weight might shift into polaron
scattering at the same wavevectors. The temperature evolution starts well below
the onset of the FM transition suggesting that the transition is driven by
phonon renormalization rather than by magnetic fluctuations
Current-driven orbital order-disorder transition in LaMnO3
We report significant influence of electric current on the orbital
order-disorder transition in LaMnO3. The transition temperature T_OO, thermal
hysteresis in the resistivity (rho) versus temperature (T) plot around T_OO,
and latent heat L associated with the transition decrease with the increase in
current density. Eventually, at a critical current density, L reaches zero. The
transition zone, on the other hand, broadens with the increase in current
density. The states at ordered, disordered, and transition zone are all found
to be stable within the time window from ~10^-3 to ~10^4 seconds.Comment: 7 pages including 5 figures; resolution of Fig.1 is better here than
the published versio
The Transition Zone in Balmer-Dominated Shocks
We examine the structure of the post-shock region in supernova remnants
(SNRs). The ``shock transition zone'' is set up by charge transfer and
ionization events between atoms and ions, and has a width
cm , where is the total pre-shock density (including
both atoms and ions). For Balmer-dominated SNRs with shock velocity km s, the Rankine-Hugoniot conditions for ion velocity and
temperature are obeyed instantly, leaving the full width at half-maximum (FWHM)
of the broad H line versus relation intact. However, the spatial
variation in the post-shock densities is relevant to the problem of Ly
resonant scattering in young, core-collapse SNRs. Both two- (pre-shock atoms
and ions) and three-component (pre-shock atoms, broad neutrals and ions) models
are considered. We compute the spatial emissivities of the broad () and
narrow () H lines; a calculation of these emissivities in SN
1006 is in general agreement with the computed ones of Raymond et al. (2007).
The (dimensionless) spatial shift, , between the centroids
of and is unique for a given shock velocity and ,
the pre-shock ion fraction. Measurements of can be used
to constrain .Comment: 25 pages, 8 figures. Accepted by Astrophysical Journa
Siberian flood basalt magmatism and Mongolia-Okhotsk slab dehydration
Experimental data combined with numerical calculations suggest that fast subducting slabs are cold enough to carry into the deep mantle a significant portion of the water in antigorite, which transforms with increasing depth to phase A and then to phase E and/or wadsleyite by solid-solid phase transition. Clathrate hydrates and ice VII are also stable at PT conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Some cold slabs are expected to deflect while crossing the 410 km and stagnate in transition zone being unable to penetrate through 660 km discontinuity. In this way slabs can move a long way beneath continents after long-lived subduction. With time, the stagnant slabs are heated to the temperature of the ambient transition zone and release free H~2~O-bearing fluid. Combining with transition zone water filter model this may cause voluminous melting of overlying upper mantle rocks. If such process operates in nature, magmas geochemically similar to island-arc magmas are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the south-eastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with fast subduction of the Mongolia-Okhotsk slab and originated by dehydration of the stagnant slab in the transition zone. We show that influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually reduced with increasing distance from the subduction zone
Modelling the transitional boundary layer
Recent developments in the modelling of the transition zone in the boundary layer are reviewed (the zone being defined as extending from the station where intermittency begins to depart from zero to that where it is nearly unity). The value of using a new non-dimensional spot formation rate parameter, and the importance of allowing for so-called subtransitions within the transition zone, are both stressed. Models do reasonably well in constant pressure 2-dimensional flows, but in the presence of strong pressure gradients further improvements are needed. The linear combination approach works surprisingly well in most cases, but would not be so successful in situations where a purely laminar boundary layer would separate but a transitional one would not. Intermittency-weighted eddy viscosity methods do not predict peak surface parameters well without the introduction of an overshooting transition function whose connection with the spot theory of transition is obscure. Suggestions are made for further work that now appears necessary for developing improved models of the transition zone
Varying rock responses as an indicator of changes in CO2-H2O fluid composition
The formation of the late Archean charnockite zone of southern India was ascribed to dehydration recrystallization due to an influx of CO2. Pressure temperature conditions for the metamorphism were calculated at about 750 C and 7.5 Kbar. The composition of the volatile species presently contained in fluid inclusions in the rocks changes across the transition zone. The transition zone was studied at Kabbaldurga and the paths taken by the fluids were identified
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