22,069 research outputs found
The magnetic behavior of Li2MO3 (M=Mn, Ru and Ir) and Li2(Mn1-xRux)O3
The present study summerizes magnetic and Mossbauer measurements on ceramic
Li2MO3 M= Mn, Ru and Ir and the mixed Li2(Mn1-xRux)O3 materials, which show
many of the features reflecting to antiferromagnetic ordering or to existence
of paramagnetic states. Li2IrO3 and Li2RuO3 are paramagnetic down to 5 K.
Li2(Mn1-xRux)O3 compounds are antiferromagnetically ordered at TN = 48 K for
x=0. TN decreases as the Ru content increases and, for x=0.8, TN =34 K.Comment: accepted to Physica
On the geometry of double field theory
Double field theory was developed by theoretical physicists as a way to
encompass -duality. In this paper, we express the basic notions of the
theory in differential-geometric invariant terms, in the framework of
para-Kaehler manifolds. We define metric algebroids, which are vector bundles
with a bracket of cross sections that has the same metric compatibility
property as a Courant bracket. We show that a double field gives rise to two
canonical connections, whose scalar curvatures can be integrated to obtain
actions. Finally, in analogy with Dirac structures, we define and study
para-Dirac structures on double manifolds.Comment: The paper will appear in J. Math. Phys., 201
Orbital Evolution of Moons in Weakly Accreting Circumplanetary Disks
We investigate the formation of hot and massive circumplanetary disks (CPDs)
and the orbital evolution of satellites formed in these disks. Because of the
comparatively small size-scale of the sub-disk, quick magnetic diffusion
prevents the magnetorotational instability (MRI) from being well-developed at
ionization levels that would allow MRI in the parent protoplanetary disk. In
the absence of significant angular momentum transport, continuous mass supply
from the parental protoplanetary disk leads to the formation of a massive CPD.
We have developed an evolutionary model for this scenario and have estimated
the orbital evolution of satellites within the disk. We find, in a certain
temperature range, that inward migration of a satellite can be stopped by a
change in the structure due to the opacity transitions. Moreover, by capturing
second and third migrating satellites in mean motion resonances, a compact
system in Laplace resonance can be formed in our disk models.Comment: 10 pages, 9 figure
New Symbolic Tools for Differential Geometry, Gravitation, and Field Theory
DifferentialGeometry is a Maple software package which symbolically performs
fundamental operations of calculus on manifolds, differential geometry, tensor
calculus, Lie algebras, Lie groups, transformation groups, jet spaces, and the
variational calculus. These capabilities, combined with dramatic recent
improvements in symbolic approaches to solving algebraic and differential
equations, have allowed for development of powerful new tools for solving
research problems in gravitation and field theory. The purpose of this paper is
to describe some of these new tools and present some advanced applications
involving: Killing vector fields and isometry groups, Killing tensors and other
tensorial invariants, algebraic classification of curvature, and symmetry
reduction of field equations.Comment: 42 page
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
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