742 research outputs found
Effect of a built-in electric field in asymmetric ferroelectric tunnel junctions
The contribution of a built-in electric field to ferroelectric phase
transition in asymmetric ferroelectric tunnel junctions is studied using a
multiscale thermodynamic model. It is demonstrated in details that there exists
a critical thickness at which an unusual ferroelectric-\'\' polar
non-ferroelectric\rq\rq phase transition occurs in asymmetric ferroelectric
tunnel junctions. In the \'\' polar non-ferroelectric\rq\rq phase, there is
only one non-switchable polarization which is caused by the competition between
the depolarizing field and the built-in field, and closure-like domains are
proposed to form to minimize the system energy. The transition temperature is
found to decrease monotonically as the ferroelectric barrier thickness is
decreased and the reduction becomes more significant for the thinner
ferroelectric layers. As a matter of fact, the built-in electric field does not
only result in smearing of phase transition but also forces the transition to
take place at a reduced temperature. Such findings may impose a fundamental
limit on the work temperature and thus should be further taken into account in
the future ferroelectric tunnel junction-type or ferroelectric capacitor-type
devices.Comment: 9 pages, 8 figures, submitted to PR
Origin of the orbital and spin orderings in rare-earth titanates
Rare-earth titanates RTiO are Mott insulators displaying a rich physical
behavior, featuring most notably orbital and spin orders in their ground state.
The origin of their ferromagnetic to antiferromagnetic transition as a function
of the size of the rare-earth however remains debated. Here we show on the
basis of symmetry analysis and first-principles calculations that although
rare-earth titanates are nominally Jahn-Teller active, the Jahn-Teller
distortion is negligible and irrelevant for the description of the ground state
properties. At the same time, we demonstrate that the combination of two
antipolar motions produces an effective Jahn-Teller-like motion which is the
key of the varying spin-orbital orders appearing in titanates. Thus, titanates
are prototypical examples illustrating how a subtle interplay between several
lattice distortions commonly appearing in perovskites can produce orbital
orderings and insulating phases irrespective of proper Jahn-Teller motions.Comment: Accepted in Physical Review
Cationic ordering control of magnetization in Sr2FeMoO6 double perovskite
The role of the synthesis conditions on the cationic Fe/Mo ordering in
Sr2FeMoO6 double perovskite is addressed. It is shown that this ordering can be
controlled and varied systematically. The Fe/Mo ordering has a profound impact
on the saturation magnetization of the material. Using the appropriate
synthesis protocol a record value of 3.7muB/f.u. has been obtained. Mossbauer
analysis reveals the existence of two distinguishable Fe sites in agreement
with the P4/mmm symmetry and a charge density at the Fe(m+) ions significantly
larger than (+3) suggesting a Fe contribution to the spin-down conduction band.
The implications of these findings for the synthesis of Sr2FeMoO6 having
optimal magnetoresistance response are discussed.Comment: 9 pages, 4 figure
Growth and magnetic properties of multiferroic LaxBi1-xMnO3 thin films
A comparative study of LaxBi1-xMnO3 thin films grown on SrTiO3 substrates is reported. It is shown that these films grow epitaxially in a narrow pressure-temperature range. A detailed structural and compositional characterization of the films is performed within the growth window. The structure and the magnetization of this system are investigated. We find a clear correlation between the magnetization and the unit-cell volume that we ascribe to Bi deficiency and the resultant introduction of a mixed valence on the Mn ions. On these grounds, we show that the reduced magnetization of LaxBi1-xMnO3 thin films compared to the bulk can be explained quantitatively by a simple model, taking into account the deviation from nominal composition and the Goodenough-Kanamori-Anderson rules of magnetic interactions
Origin of band gaps in 3d perovskite oxides
With their broad range of magnetic, electronic and structural properties,
transition metal perovskite oxides ABO3 have long served as a platform for
testing condensed matter theories. In particular, their insulating character -
found in most compounds - is often ascribed to dynamical electronic
correlations through the celebrated Mott-Hubbard mechanism where gaping arises
from a uniform, symmetry-preserving electron repulsion mechanism. However,
structural distortions are ubiquitous in perovskites and their relevance with
respect to dynamical correlations in producing this rich array of properties
remains an open question. Here, we address the origin of band gap opening in
the whole family of 3d perovskite oxides. We show that a single-determinant
mean-field approach such as density functional theory (DFT) successfully
describes the structural, magnetic and electronic properties of the whole
series, at low and high temperatures. We find that insulation occurs via
energy-lowering crystal symmetry reduction (octahedral rotations, Jahn-Teller
and bond disproportionation effects), as well as intrinsic electronic
instabilities, all lifting orbital degeneracies. Our work therefore suggests
that whereas ABO3 oxides may be complicated, they are not necessarily strongly
correlated. It also opens the way towards systematic investigations of doping
and defect physics in perovskites, essential for the full realization of
oxide-based electronics
Mott gapping in 3d ABO3 perovskites without Mott-Hubbard interelectronic U
The existence of band gaps in Mott insulators such as perovskite oxides with
partially filled 3d shells has been traditionally explained in terms of strong,
dynamic inter-electronic repulsion codified by the on-site repulsion energy U
in the Hubbard Hamiltonian. The success of the "DFT+U approach" where an
empirical on-site potential term U is added to the exchange-and correlation
Density Functional Theory (DFT) raised questions on whether U in DFT+U
represents interelectronic correlation in the same way as it does in the
Hubbard Hamiltonian, and if empiricism in selecting U can be avoided. Here we
illustrate that ab-initio DFT without any U is able to predict gapping trends
and structural symmetry breaking (octahedra rotations, Jahn-Teller modes, bond
disproportionation) for all ABO3 3d perovskites from titanates to nickelates in
both spin-ordered and spin disordered paramagnetic phases. We describe the
paramagnetic phases as a supercell where individual sites can have different
local environments thereby allowing DFT to develop finite moments on different
sites as long as the total cell has zero moment. We use a recently developed
exchange and correlation functional ("SCAN") that is sanctioned by the usual
single-determinant, mean-field DFT paradigm with static correlations, but has a
more precise rendering of self-interaction cancelation. Our results suggest
that strong dynamic electronic correlations are not playing a universal role in
gapping of 3d ABO3 Mott insulators, and opens the way for future applications
of DFT for studying a plethora of complexity effects that depend on the
existence of gaps, such as doping, defects, and band alignment in ABO3 oxides
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