106 research outputs found
Realistic Modeling of Complex Oxide Materials
Since electronic and magnetic properties of many transition-metal oxides can
be efficiently controlled by external factors such as the temperature,
pressure, electric or magnetic field, they are regarded as promising materials
for various applications. From the viewpoint of electronic structure, these
phenomena are frequently related to the behavior of a small group of states
close to the Fermi level. The basic idea of this project is to construct a
low-energy model for the states near the Fermi level on the basis of
first-principles density functional theory, and to study this model by modern
many-body techniques. After a brief review of the method, the abilities of this
approach will be illustrated on a number of examples, including multiferroic
manganites and spin-orbital-lattice coupled phenomena in RVO3 (R being the
three-valent element).Comment: 3 pages, 6 figures, Conference on Computational Physics 200
Modeling of complex oxide materials from the first principles: systematic applications to vanadates RVO3 with distorted perovskite structure
"Realistic modeling" is a new direction of electronic structure calculations,
where the main emphasis is made on the construction of some effective
low-energy model entirely within a first-principle framework. Ideally, it is a
model in form, but with all the parameters derived rigorously, on the basis of
first-principles electronic structure calculations. The method is especially
suit for transition-metal oxides and other strongly correlated systems, whose
electronic and magnetic properties are predetermined by the behavior of some
limited number of states located near the Fermi level. After reviewing general
ideas of realistic modeling, we will illustrate abilities of this approach on
the wide series of vanadates RVO3 (R= La, Ce, Pr, Nd, Sm, Gd, Tb, Yb, and Y)
with distorted perovskite structure. Particular attention will be paid to
computational tools, which can be used for microscopic analysis of different
spin and orbital states in the partially filled t2g-band. We will explicitly
show how the lifting of the orbital degeneracy by the monoclinic distortion
stabilizes C-type antiferromagnetic (AFM) state, which can be further
transformed to the G-type AFM state by changing the crystal distortion from
monoclinic to orthorhombic one. Two microscopic mechanisms of such a
stabilization, associated with the one-electron crystal field and electron
correlation interactions, are discussed. The flexibility of the orbital degrees
of freedom is analyzed in terms of the magnetic-state dependence of interatomic
magnetic interactions.Comment: 23 pages, 13 figure
Effect of the orthorhombic distortion on the magneto-optical properties of SrRuO
It is argued that the non-collinear orbital magnetism accompanying the
orthorhombic distortion is an important ingredient which should be taken into
consideration for making promising materials with the best magneto-optical
characteristics on the basis of SrRuO.Comment: 6 pages, 2 figures, Proceedings of ICM '9
Lattice Distortion and Magnetic Ground State of YTiO and LaTiO
Effects of lattice distortion on the magnetic ground state of YTiO and
LaiO are investigated on the basis accurate tight-binding parametrization
of the electronic structure extracted from the local-density
approximation. The complexity of these compounds is related with the fact that
the -level splitting, caused by lattice distortions, is comparable with
the energies of superexchange and spin-orbit interactions. Therefore, all these
interactions are equally important and should be treated on an equal footing.
The Hartree-Fock approximation fails to provide a coherent description
simultaneously for YTiO and LaTiO, and it is essential to go beyond.Comment: 4 pages, 3 figures (good quality figures are available via e-mail
Effect of Thermal Deformation Regimes on the Austenite Growth Kinetics and Recrystallization of Medium Carbon Low-Alloyed Steel for Large-Sized Steam Turbine Rotors
Effect of thermal deformation regimes on the austenite grain growth is studied in the article. The austenite grain growth curve based on the experimental data is constructed. Conclusion about temperature, deformation degree and rate effect is made.
Keywords: deformation, recrystallization, dynamic recrystallization, medium-carbon low-alloyed steel
Noncollinear magnetism in distorted perovskite compounds
Using results of the band structure calculations in the local-spin-density
approximation we demonstrate how the crystal distortions affect the magnetic
structure of orthorhombically distorted perovskites leading to a non-collinear
spin arrangement. Our results suggest that the non-collinearity of the spin
magnetic moments, being generally small in LaO series with =Cr-Fe, is
large in SrRuO.Comment: 6 pagest, 1 figure, Proceedings of ICPTM '9
Coulomb Correlations and Magnetic Anisotropy in ordered CoPt and FePt alloys
We present results of the magneto-crystalline anisotropy energy (MAE)
calculations for chemically ordered CoPt and FePt alloys taking into
account the effects of strong electronic correlations and spin-orbit coupling.
The local spin density + Hubbard U approximation (LSDA+U) is shown to provide a
consistent picture of the magnetic ground state properties when intra-atomic
Coulomb correlations are included for both 3 and 5 elements. Our results
demonstrate significant and complex contribution of correlation effects to
large MAE of these material.Comment: revised version; 4 pages, 2 figure
Exchange coupling in CaMnO and LaMnO: configuration interaction and the coupling mechanism
The equilibrium structure and exchange constants of CaMnO and LaMnO
have been investigated using total energy unrestricted Hartree-Fock (UHF) and
localised orbital configuration interaction (CI) calculations on the bulk
compounds and MnO and MnO clusters. The
predicted structure and exchange constants for CaMnO are in reasonable
agreement with estimates based on its N\'eel temperature. A series of
calculations on LaMnO in the cubic perovskite structure shows that a
Hamiltonian with independent orbital ordering and exchange terms accounts for
the total energies of cubic LaMnO with various spin and orbital orderings.
Computed exchange constants depend on orbital ordering. UHF calculations tend
to underestimate exchange constants in LaMnO, but have the correct sign
when compared with values obtained by neutron scattering; exchange constants
obtained from CI calculations are in good agreement with neutron scattering
data provided the Madelung potential of the cluster is appropriate. Cluster CI
calculations reveal a strong dependence of exchange constants on Mn d e
orbital populations in both compounds. CI wave functions are analysed in order
to determine which exchange processes are important in exchange coupling in
CaMnO and LaMnO.Comment: 25 pages and 9 postscript figure
First principles calculation of uniaxial magnetic anisotropy and magnetostriction in strained CMR films
We performed first - principles relativistic full-potential linearized
augmented plane wave calculations for strained tetragonal ferromagnetic
La(Ba)MnO with an assumed experimental structure of thin strained
tetragonal LaCaMnO (LCMO) films grown on SrTiO[001]
and LaAlO[001] substrates. The calculated uniaxial magnetic anisotropy
energy (MAE) values, are in good quantitative agreement with experiment for
LCMO films on SrTiO substrate. We also analyze the applicability of linear
magnetoelastic theory for describing the stain dependence of MAE, and estimate
magnetostriction coefficient .Comment: Talk given at APS99 Meeting, Atlanta, 199
Structural, electronic, and magneto-optical properties of YVO
Optical and magneto-optical properties of YVO single crystal were studied
in FIR, visible, and UV regions. Two structural phase transitions at 75 K and
200 K were observed and established to be of the first and second order,
respectively. The lattice has an orthorhombic symmetry both above 200 K
as well as below 75 K, and is found to be dimerized monoclinic in
between. We identify YVO as a Mott-Hubbard insulator with the optical gap
of 1.6 eV. The electronic excitations in the visible spectrum are determined by
three -bands at 1.8, 2.4, and 3.3 eV, followed by the charge-transfer
transitions at about 4 eV. The observed structure is in good agreement with
LSDA+ band structure calculations. By using ligand field considerations, we
assigned these bands to the transitions to the , , and states. The strong temperature dependence of these
bands is in agreement with the formation of orbital order. Despite the small
net magnetic moment of 0.01 per vanadium, the Kerr effect of the order
of was observed for all three -bands in the magnetically
ordered phase . A surprisingly strong enhancement of
the Kerr effect was found below 75 K, reaching a maximum of . The
effect is ascribed to the non-vanishing net orbital magnetic moment.Comment: Submitted to Phys. Rev.
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