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Non-collinear Magnetic Order in the Double Perovskites: Double Exchange on a Geometrically Frustrated Lattice
Double perovskites of the form A_2BB'O_6 usually involve a transition metal
ion, B, with a large magnetic moment, and a non magnetic ion B'. While many
double perovskites are ferromagnetic, studies on the underlying model reveal
the possibility of antiferromagnetic phases as well driven by electron
delocalisation. In this paper we present a comprehensive study of the magnetic
ground state and T_c scales of the minimal double perovskite model in three
dimensions using a combination of spin-fermion Monte Carlo and variational
calculations. In contrast to two dimensions, where the effective magnetic
lattice is bipartite, three dimensions involves a geometrically frustrated face
centered cubic (FCC) lattice. This promotes non-collinear spiral states and
`flux' like phases in addition to collinear anti-ferromagnetic order. We map
out the possible magnetic phases for varying electron density, `level
separation' epsilon_B - epsilon_B', and the crucial B'-B' (next neighbour)
hopping t'.Comment: 15 pages pdflatex + 19 figs, revision: removed redundant comment
The equatorial ionospheric response over Tirunelveli to the 15 January 2010 annular solar eclipse: observations
In this paper we present a case study of the annular solar eclipse effects
on the ionization of E and F regions of equatorial ionosphere over
Tirunelveli [77.8° E, 8.7° N, dip 0.4° N] by means of
digital ionosonde on 15 January 2010. The maximum obscuration of the eclipse
at this station was 84% and it occurred in the afternoon. The E and F1
layers of the ionosphere showed very clear decrease in their electron
concentrations, whereas the F2 layer did not show appreciable changes. A reduction of 30%
was observed in the <I>fo</I>F1 during the maximum phase of the eclipse.
During the beginning phase of the eclipse, an enhancement of 0.97 MHz was
observed in the <I>fo</I>F2 as compared to that of the control days. But the <I>fo</I>F2
decreased gradually as the eclipse progressed and a decrease of 0.59 MHz was
observed towards the end phase of the eclipse. Observed variations in the
<I>h</I>'F2 and <I>hm</I>F2 showed lower values than the control days, although <I>hm</I>F2 was
found to increase a bit during the eclipse. Observed variability in the E,
F1 and F2 layer ionospheric parameters on the eclipse day and their
departure from the control days are discussed as the combined effect of
annular eclipse and presence of counter equatorial electrojet (CEEJ)
Investigation of nose bluntness and angle of attack effects on slender bodies in viscous hypersonic flows
Hypersonic flows over cones and straight biconic configurations are calculated for a wide range of free stream conditions in which the gas behind the shock is treated as perfect. Effect of angle of attack and nose bluntness on these slender cones in air is studied extensively. The numerical procedures are based on the solution of complete Navier-Stokes equations at the nose section and parabolized Navier-Stokes equations further downstream. The flow field variables and surface quantities show significant differences when the angle of attack and nose bluntness are varied. The complete flow field is thoroughly analyzed with respect to velocity, temperature, pressure, and entropy profiles. The post shock flow field is studied in detail from the contour plots of Mach number, density, pressure, and temperature. The effect of nose bluntness for slender cones persists as far as 200 nose radii downstream
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