1,402 research outputs found
Coulomb interaction and ferroelectric instability of BaTiO3
Using first-principles calculations, the phonon frequencies at the
point and the dielectric tensor are determined and analysed for the cubic and
rhombohedral phases of BaTiO. The dipole-dipole interaction is then
separated \`a la Cochran from the remaining short-range forces, in order to
investigate their respective influence on lattice dynamics. This analysis
highlights the delicate balance of forces leading to an unstable phonon in the
cubic phase and demonstrates the extreme sensitivity of this close compensation
to minute effective charge changes. Within our decomposition, the stabilization
of the unstable mode in the rhombohedral phase or under isotropic pressure has
a different origin.Comment: 9 pages, 4 tables, 1 figur
First-principles study of PbTiO under uniaxial strains and stresses
The behavior of PbTiO under uniaxial strains and stresses is investigated
from first-principles calculations within density functional theory. We show
that irrespectively of the uniaxial mechanical constraint applied, the system
keeps a purely ferroelectric ground-state, with the polarization aligned either
along the constraint direction ( phase) or along one of the pseudo-cubic
axis perpendicular to it ( phase). This contrasts with the cases of
isotropic or biaxial mechanical constraints for which novel phases combining
ferroelectic and antiferrodistortive motions have been previously reported.
Under uniaxial strain, PbTiO switched from a ground state under
compressive strain to ground-state under tensile strain, beyond a
critical strain \%. Under uniaxial stress, PbTiO
exhibits either a ground state under compression () or
a ground state under tension (). Here, however, an
abrupt jump of the structural parameters is also predicted under both
compressive and tensile stresses at critical values
GPa and GPa. This behavior appears similar to that predicted under
negative isotropic pressure and might reveal practically useful to enhance the
piezoelectric response in nanodevices.Comment: Submitted, 9 pages, 9 figure
Electronic and thermoelectric properties of Fe2VAl: The role of defects and disorder
Using first-principles calculations, we show that Fe2VAl is an indirect band
gap semiconductor. Our calculations reveal that its, sometimes assigned,
semimetallic character is not an intrinsic property but originates from the
antisite defects and site disorder, which introduce localized ingap and
resonant states changing the electronic properties close to band gap. These
states negatively affect the thermopower S and power factor PF=S^2\sigma,
decreasing the good thermoelectric performance of intrinsic Fe2VAl.Comment: 4 pages, 6 figures, thermoelectric properties, electronic structure
and transport properties, effect of antisite defects and disorder on
electronic and transport propertie
Engineering multiferroism in CaMnO
From first-principles calculations, we investigate the structural
instabilities of CaMnO. We point out that, on top of a strong
antiferrodistortive instability responsible for its orthorhombic ground-state,
the cubic perovskite structure of CaMnO also exhibit a weak ferroelectric
instability. Although ferroelectricity is suppressed by antiferrodistortive
oxygen motions, we show that it can be favored using strain or chemical
engineering in order to make CaMnO multiferroic. We finally highlight that
the FE instability of CaMnO is Mn-dominated. This illustrates that,
contrary to the common believe, ferroelectricity and magnetism are not
necessarily exclusive but can be driven by the same cation
Structurally Triggered Metal-Insulator Transition in Rare-Earth Nickelates
Rare-earth nickelates form an intriguing series of correlated perovskite
oxides. Apart from LaNiO3, they exhibit on cooling a sharp metal-insulator
electronic phase transition, a concurrent structural phase transition and a
magnetic phase transition toward an unusual antiferromagnetic spin order.
Appealing for various applications, full exploitation of these compounds is
still hampered by the lack of global understanding of the interplay between
their electronic, structural and magnetic properties. Here, we show from
first-principles calculations that the metal-insulator transition of nickelates
arises from the softening of an oxygen breathing distortion, structurally
triggered by oxygen-octahedra rotation motions. The origin of such a rare
triggered mechanism is traced back in their electronic and magnetic properties,
providing a united picture. We further develop a Landau model accounting for
the evolution of the metal-insulator transition in terms of the $R cations and
rationalising how to tune this transition by acting on oxygen rotation motions.Comment: Submitted in Nature Communicatio
First-principles study of lattice instabilities in Ba_xSr_(1-x)TiO_3
Using first-principles calculations based on a variational density functional
perturbation theory, we investigate the lattice dynamics of solid solutions of
barium and strontium titanates. Averaging the information available for the
related pure compounds yields results equivalent to those obtained within the
virtual crystal approximation, providing frequencies which are a good
approximation to those computed for a (111) ordered supercell. Using the same
averaging technique we report the evolution of the ferroelectric and
antiferrodistortive instabilities with composition.Comment: 9 pages, 2 figures, Proceedings for Fundamental Physics of
Ferroelectrics, Aspen (CO), Feb. 13-20, 200
Avoiding the polarization catastrophe in LaAlO3 overlayers on SrTiO3(001) through a polar distortion
A pronounced uniform polar distortion extending over several unit cells
enables thin LaAlO3 overlayers on SrTiO3(001) to counteract the charge dipole
and thereby neutralize the "polarization catastrophe" that is suggested by
simple ion-counting. This unanticipated mechanism, obtained from density
functional theory calculations, allows several unit cells of the LaAlO3
overlayer to remain insulating (hence, fully ionic). The band gap of the
system, defined by occupied O states at the surface and unoccupied Ti 3d
states at the interface in some cases 20 \AA distant, decreases with
increasing thickness of the LaAlO3-film before an insulator-to-metal transition
and a crossover to an electronic reconstruction occurs at around five
monolayers of LaAlO3.Comment: 5 pages, 4 figures, submitted for publicatio
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