16,525 research outputs found
Origin of Ferromagnetism and its pressure and doping dependence in TlMnO
Using NMTO-{\it downfolding} technique, we explore and establish the origin
of ferromagnetism in the pyrochlore system, TlMnO. It is
found to be driven by hybridization induced spin-polarization of the
delocalized charge carriers derived from Tl- and O- states. The
mean-field estimate of the ferromagnetic transition temperature, T,
estimated using computed exchange integrals are found to be in good agreement
with the measurements. We find an enhancement of T for moderate doping
with nonmagnetic Sb and a suppression of T upon application of pressure,
both in agreement with experimental findings.Comment: Accepted for publication in PR
Study of Phase Stability in NiPt Systems
We have studied the problem of phase stability in NiPt alloy system. We have
used the augmented space recursion based on the TB-LMTO as the method for
studying the electronic structure of the alloys. In particular, we have used
the relativistic generalization of our earlier technique. We note that, in
order to predict the proper ground state structures and energetics, in addition
to relativistic effects, we have to take into account charge transfer effects
with precision.Comment: 22 pages, 7 figures. Accepted for publication in JPC
String cosmological model in the presence of a magnetic flux
A Bianchi type I string cosmological model in the presence of a magnetic flux
is investigated. A few plausible assumptions regarding the parametrization of
the cosmic string and magneto-fluid are introduced and some exact analytical
solutions are presented.Comment: 9 pages, 4 Figure
Nonlinear spinor field in cosmology
Within the scope of Bianchi type VI (BVI) model the self-consistent system of
nonlinear spinor and gravitational fields is considered. Exact self-consistent
solutions to the spinor and gravitational field equations are obtained for some
special choice of spatial inhomogeneity and nonlinear spinor term. The role of
inhomogeneity in the evolution of spinor and gravitational field is studied.
Oscillatory mode of expansion of the BVI universe is obtained for some special
choice of spinor field nonlinearity.Comment: RevTex4, 19 pages, 4 figure
Ab-initio study of disorder effects on the electronic and magnetic structures of SrFeMoO
We have investigated the electronic structure of ordered and disordered
SrFeMoO using {\it ab-initio} band structure methods. The effect of
disorder was simulated within super-cell calculations to realize several
configurations with mis-site disorders. It is found that such disorder effects
destroy the half-metallic ferro-magnetic state of the ordered compound. It also
leads to a substantial reduction of the magnetic moments at the Fe sites in the
disordered configurations. Most interestingly, it is found for the disordered
configurations, that the magnetic coupling within the Fe sub-lattice as well as
that within the Mo sub-lattice always remain ferro-magnetic, while the two
sub-lattices couple anti-ferromagnetically, in close analogy to the magnetic
structure of the ordered compound, but in contrast to recent suggestions.Comment: 7 pages, 3 figure
Optical properties of perovskite alkaline earth titanates : a formulation
In this communication we suggest a formulation of the optical conductivity as
a convolution of an energy resolved joint density of states and an
energy-frequency labelled transition rate. Our final aim is to develop a scheme
based on the augmented space recursion for random systems. In order to gain
confidence in our formulation, we apply the formulation to three alkaline earth
titanates CaTiO_3, SrTiO_3 and BaTiO_3 and compare our results with available
data on optical properties of these systems.Comment: 19 pages, 9 figures, Submitted to Journal of Physics: Condensed
Matte
Phase-ordering dynamics in itinerant quantum ferromagnets
The phase-ordering dynamics that result from domain coarsening are considered
for itinerant quantum ferromagnets. The fluctuation effects that invalidate the
Hertz theory of the quantum phase transition also affect the phase ordering.
For a quench into the ordered phase a transient regime appears, where the
domain growth follows a different power law than in the classical case, and for
asymptotically long times the prefactor of the t^{1/2} growth law has an
anomalous magnetization dependence. A quench to the quantum critical point
results in a growth law that is not a power-law function of time. Both
phenomenological scaling arguments and renormalization-group arguments are
given to derive these results, and estimates of experimentally relevant length
and time scales are presented.Comment: 6pp., 1 eps fig, slightly expanded versio
Cooperative orbital ordering and Peierls instability in the checkerboard lattice with doubly degenerate orbitals
It has been suggested that the metal-insulator transitions in a number of
spinel materials with partially-filled t_2g d-orbitals can be explained as
orbitally-driven Peierls instabilities. Motivated by these suggestions, we
examine theoretically the possibility of formation of such orbitally-driven
states within a simplified theoretical model, a two-dimensional checkerboard
lattice with two directional metal orbitals per atomic site. We include orbital
ordering and inter-atom electron-phonon interactions self-consistently within a
semi-classical approximation, and onsite intra- and inter-orbital
electron-electron interactions at the Hartree-Fock level. We find a stable,
orbitally-induced Peierls bond-dimerized state for carrier concentration of one
electron per atom. The Peierls bond distortion pattern continues to be period 2
bond-dimerization even when the charge density in the orbitals forming the
one-dimensional band is significantly smaller than 1. In contrast, for carrier
density of half an electron per atom the Peierls instability is absent within
one-electron theory as well as mean-field theory of electron-electron
interactions, even for nearly complete orbital ordering. We discuss the
implications of our results in relation to complex charge, bond, and
orbital-ordering found in spinels.Comment: 8 pages, 5 figures; revised versio
Evaluation Of Support Vector Machine Based Forecasting Tool In Electricity Price Forecasting For Australian National Electricity Market Participants
In this paper, we present an analysis of the results of a study into wholesale (spot) electricity price forecasting utilising Neural Networks (NNs) and Support Vector Machines (SVM). Frequent regulatory changes in electricity markets and the quickly evolving market participant pricing (bidding) strategies cause efficient retraining to be crucial in maintaining the accuracy of electricity price forecasting models. The efficiency of NN and SVM retraining for price forecasting was evaluated using Australian National Electricity Market (NEM), New South Wales regional data over the period from September 1998 to December 1998. The analysis of the results showed that SVMs with one unique solution, produce more consistent forecasting accuracies and so require less time to optimally train than NNs, which can result in a solution at any of a large number of local minima. The SVM and NN forecasting accuracies were found to be very similar
Phase stability analysis in Fe-Pt and Co-Pt alloy systems: An augmented space study
We have studied the problem of phase stability in Fe-Pt and Co-Pt alloy
systems. We have used the orbital peeling technique in the conjunction of
augmented space recursion based on the tight binding linear orbital method as
the method for the calculation of pair interaction energies. In particular, we
have generalized our earlier technique to take into account of magnetic effects
for the cases where the magnetic transition is higher than the order disorder
chemical transition temperature as in the case of CoPt. Our theoretical
results obtained within this framework successfully reproduce the
experimentally observed trends.Comment: 17 pages, 9 Figures. Accepted for publication in Journal of Physics :
Condensed Matte
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