Firstprinciples Study of Lanthanum Manganite (Lamno3)
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Date
2010-03
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Addis Ababa Universty
Abstract
In this thesis, we have investigated the structural, electronic, and magnetic properties of pure
LaMnO3 from first-principles using the quantum-ESPRESSO open-source code. Unlike previous
theoretical works, most of which implemented experimental structure, we have performed full
structural relaxations for both the cubic and orthorhombic structures within the GGA and LDA+U
approaches. Our simulated structural parameters are in very good agreement with experimental
values. We have used these theoretical geometries in all the calculations. Consistent with the
general theory of the CMR manganites but not reported in earlier theoretical works, the band
structure and DOS calculations for the cubic ferroelectric phase predict half-metallicity. We have
also examined the phonon-dispersion and the corresponding density of states for the theoretically
optimized cubic ferromagnetic LaMnO3 from which we are able to observe two strong crystal
instabilities. The phonon dispersion is calculated along the high-symmetry axes throughout the
Brillouin zone. The densities of states calculated for the orthorhombic structure with different
magnetic orderings strongly suggest the need for structural relaxation in order to get the
experimentally observed insulating ground state for LaMnO3. Both GGA and LDA+U calculations
for the fully relaxed orthorhombic A-type antiferromagnetic structure reproduce the insulating
character. However, the value of the insulating gap obtained within the LDA+U case is much
closer to experiment than that of GGA, which in turn means that a better description of LaMnO3 is
possible with the inclusion of the on-site coulomb interaction.
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Lamno3