191 research outputs found
Coupling between magnetic ordering and structural instabilities in perovskite biferroics: A first-principles study
We use first-principles density functional theory-based calculations to
investigate structural instabilities in the high symmetry cubic perovskite
structure of rare-earth (R La, Y, Lu) and Bi-based biferroic chromites,
focusing on and point phonons of states with para-, ferro-, and
antiferromagnetic ordering. We find that (a) the structure with G-type
antiferromagnetic ordering is most stable, (b) the most dominant structural
instabilities in these oxides are the ones associated with rotations of oxygen
octahedra, and (c) structural instabilities involving changes in Cr-O-Cr bond
angle depend sensitively on the changes in magnetic ordering. The dependence of
structural instabilities on magnetic ordering can be understood in terms of how
super-exchange interactions depend on the Cr-O-Cr bond angles and Cr-O bond
lengths. We demonstrate how adequate buckling of Cr-O-Cr chains can favour
ferromagnetism. Born effective charges (BEC) calculated using the Berry phase
expression are found to be anomalously large for the A-cations, indicating
their chemical relevance to ferroelectric distortions.Comment: 8 pages, 13 figure
Plankton study of Siddheshwar dam of Hingoli district, (M.S.) India.
The present paper deals with the plankton diversity of siddheshwar dam near Aundha dist Hingoli Maharashtra in the year 2001.The water spread area of the dam 40.58 Sq.Km.at FRL 250.85 Mcum. The phytoplankton represented by chlorophyceae, bascillariophyceae, cyanophyceae, and euglenophyceae and zooplankton represents species of rotifer, copepods, cladocera and ostracoda
Seasonal variations of phsycio-chemical characteristics of Jamgavan dam water of Hingoli District (M.S.) India.
Jamgavan dam is a minor irrigation dam. The water in Jamgavan dam is used for irrigation, fish culture, drinking and domestic purposes. The present investigation deal with the study of seasonal variations of physicochemical characteristics of Jamgavan dam water. The study was carried out during the year 2007 for a period of 12 months. The parameters such as atmospheric temperature, water temperature, pH, free carbon dioxide, Electrical conductivity and dissolved oxygen were studied. 
Geometric phases and Wannier functions of Bloch electrons in 1-dimension
We present a formal expression for Wannier functions of composite bands of
1-D Bloch electrons in terms of parallel-transported Bloch functions and their
non-Abelian geometric phases. Spatial decay properties of these Wannier
functions are studied in the case of simple bands of 1-D model insulator and
metal. Within first-principles density functional theory, we illustrate the
formalism through the construction of Wannier functions of polyethylene and
polyacetylene.Comment: 4 pages, 4 figure
Biferroic YCrO3
YCrO3 which has a monoclinic structure, shows weak ferromagnetism below 140 K
(TN) and a ferroelectric transition at 473 K accompanied by hysteresis. We have
determined the structure and energetics of YCrO3 with ferromagnetic and
antiferromagnetic ordering by means of first-principles density functional
theory calculations, based on pseudopotentials and a plane wave basis. The
non-centrosymmetric monoclinic structure is found to be lower in energy than
the orthorhombic structure, supporting the biferroic nature of YCrO3.Comment: 16 pages including figure
Influence of Nonlinear Fluid Viscous Dampers on Seismic Response of RC Elevated Storage Tanks
The numerical investigation on the seismic response of RC elevated liquid storage tanks installed with viscous dampers is presented. A discrete two-mass model for the liquid and multi-degree of freedom system for staging, installed with the dampers are developed for Reinforced Concrete (RC) elevated liquid storage tanks. The elevated tank is assessed for seismic response reduction when provided with Linear Viscous Damper (LVD) and Nonlinear Viscous Damper (NLVD), installed in the staging. The RC elevated liquid storage tanks are analyzed for two levels of liquid containment in the tank, 100% and 25% of the tank capacity. Three Configurations of placements of dampers viz. dampers at alternate levels (Configuration I and Configuration II) and dampers at all the panels of the staging of the tank (Configuration III) are considered. To study the effect of peak ground acceleration, eight real earthquake time histories with accelerations varying from 0.1 g to 0.93 g are considered. The nonlinearity in the viscous damper is modified by taking force proportional to various velocity exponents. It is found that the nonlinear viscous dampers with lower damping constant result in a comparable reduction in the response of RC elevated liquid storage tank, to that of linear viscous dampers with higher damping constant. A lower damping constant signifies compact the size of the damper. Doi: 10.28991/cej-2020-SP(EMCE)-09 Full Text: PD
First-principles indicators of metallicity and cation off-centricity in the IV-VI rocksalt chalcogenides of divalent Ge, Sn, and Pb
We use first-principles density functional theory to calculate the phonon frequencies, electron localization lengths, Born effective charges, dielectric response, and conventional electronic structures of the IV-VI chalcogenide series. The goals of our work are twofold: first, to determine the detailed chemical composition of lone pairs and, second, to identify the factors that cause lone pairs to favor high- or low-symmetry environments. Our results show that the traditional picture of cation s-p mixing causing localization of the lone pair lobe is incomplete, and instead the p states on the anion also play an important role. In addition these compounds reveal a delicate balance between two competing instabilities-structural distortion and tendency to metallicity-leading, at the same time, to anomalously large Born effective charges as well as large dielectric constants. The magnitude of the LO-TO splitting, which depends on the relative strength of both instabilities, shows a trend consistent with the structural distortions in these compounds
Coupled Phonons, Magnetic Excitations and Ferroelectricity in AlFeO3: Raman and First-principles Studies
We determine the nature of coupled phonons and magnetic excitations in AlFeO3
using inelastic light scattering from 5 K to 315 K covering a spectral range
from 100-2200 cm-1 and complementary first-principles density functional
theory-based calculations. A strong spin-phonon coupling and magnetic ordering
induced phonon renormalization are evident in (a) anomalous temperature
dependence of many modes with frequencies below 850 cm-1, particularly near the
magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band
positions of high frequency Raman bands between 1100-1800 cm-1, in particular a
broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon
Raman scattering. We also observe weak anomalies in the mode frequencies at ~
100 K, due to a magnetically driven ferroelectric phase transition.
Understanding of these experimental observations has been possible on the basis
of first-principles calculations of phonons spectrum and their coupling with
spins
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