595 research outputs found
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 Modeling of Ferroelectric Oxide Nanostructures
The aim of this Chapter is to provide an account of recent advances in the
first-principles modeling of ferroelectric oxide nanostructures. Starting from
a microscopic description of ferroelectricity in bulk materials and considering
then, successively, different kinds of nanostructures (films, multilayers,
wires, and particles), we try to identify the main trends and to provide a
coherent picture of the role of finite size effects in ferroelectric oxides.Comment: 153 pages, 61 figures, 9 tables, 387 reference
Strain-induced ferroelectricity in simple rocksalt binary oxides
The alkaline earth binary oxides adopt a simple rocksalt structure and form
an important family of compounds because of their large presence in the earth's
mantle and their potential use in microelectronic devices. In comparison to the
class of multifunctional ferroelectric perovskite oxides, however, their
practical applications remain limited and the emergence of ferroelectricity and
related functional properties in simple binary oxides seems so unlikely that it
was never previously considered. Here, we show using first-principles density
functional calculations that ferroelectricity can be easily induced in simple
alkaline earth binary oxides such as barium oxide (BaO) using appropriate
epitaxial strains. Going beyond the fundamental discovery, we show that the
functional properties (polarization, dielectric constant and piezoelectric
response) of such strained binary oxides are comparable in magnitude to those
of typical ferroelectric perovskite oxides, so making them of direct interest
for applications. Finally, we show that magnetic binary oxides such as EuO,
with the same rocksalt structure, behave similarly to the alkaline earth
oxides, suggesting a route to new multiferroics combining ferroelectric and
magnetic properties
First-principles study of ferroelectric oxide epitaxial thin films and superlattices: role of the mechanical and electrical boundary conditions
In this review, we propose a summary of the most recent advances in the
first-principles study of ferroelectric oxide epitaxial thin films and
multilayers. We discuss in detail the key roles of mechanical and electrical
boundary conditions, providing to the reader the basic background for a simple
and intuitive understanding of the evolution of the ferroelectric properties in
many nanostructures. Going further we also highlight promising new avenues and
future challenges within this exciting field or researches.Comment: 40 pages, 195 references, 6 eps Figures, submitted to Journal of
Computational and Theoretical Nanoscienc
Orbital-Energy Splitting in Anion Ordered Ruddlesden-Popper Halide Perovskites for Tunable Optoelectronic Applications
The electronic orbital characteristics at the band edges plays an important
role in determining the electrical, optical and defect properties of perovskite
photovoltaic materials. It is highly desirable to establish the relationship
between the underlying atomic orbitals and the optoelectronic properties as a
guide to maximize the photovoltaic performance. Here, using first-principles
calculations and taking anion ordered Ruddlesden-Popper (RP) phase halide
perovskites CsGeICl as an example, we demonstrate
how to rationally optimize the optoelectronic properties (e.g., band gap,
transition dipole matrix elements, carrier effective masses, band width)
through a simple band structure parameter. Our results show that reducing the
splitting energy of p orbitals of B-site atom can effectively
reduce the band gap and carrier effective masses while greatly improving the
optical absorption in the visible region. Thereby, the orbital-property
relationship with is well established through biaxial compressive
strain. Finally, it is shown that this approach can be reasonably extended to
several other non-cubic halide perovskites with similar p orbitals
characteristics at the conduction band edges. Therefore, we believe that our
proposed orbital engineering approach provides atomic-level guidance for
understanding and optimizing the device performance of layered perovskite solar
cells
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