Origin of Ferromagnetism and its pressure and doping dependence in Tl2_{2}Mn2_{2}O7_{7}

Using NMTO-{\it downfolding} technique, we explore and establish the origin of ferromagnetism in the pyrochlore system, Tl$_{2}$Mn$_{2}$O$_{7}$. It is found to be driven by hybridization induced spin-polarization of the delocalized charge carriers derived from Tl-$s$ and O-$p$ states. The mean-field estimate of the ferromagnetic transition temperature, T$_c$, estimated using computed exchange integrals are found to be in good agreement with the measurements. We find an enhancement of T$_{c}$ for moderate doping with nonmagnetic Sb and a suppression of T$_{c}$ 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 Sr2_2FeMoO6_6

We have investigated the electronic structure of ordered and disordered Sr$_2$FeMoO$_6$ 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 Co$_3$Pt. 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