821 research outputs found
Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells
We report a model describing the molecular orientation disorder in
CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic
structure calculations, with the nature of the motions informed by ab-initio
molecular dynamics. We investigate the temperature and static electric field
dependence of the equilibrium ferroelectric (molecular) domain structure and
resulting polarisability. A rich domain structure of twinned molecular dipoles
is observed, strongly varying as a function of temperature and applied electric
field. We propose that the internal electrical fields associated with
microscopic polarisation domains contribute to hysteretic anomalies in the
current--voltage response of hybrid organic-inorganic perovskite solar cells
due to variations in electron-hole recombination in the bulk.Comment: 10 pages; 4 figures, 2 SI figure
Hexagonal as semiconducting ferroelectrics
We use a first-principles rational-design approach to identify a
previously-unrecognized class of ferroelectric materials in the LiGaGe
structure type. We calculate structural parameters, polarization and
ferroelectric well depths both for reported and as-yet hypothetical
representatives of this class. Our results provide guidance for the
experimental realization and further investigation of high-performance
materials suitable for practical applications.Comment: 5 pages, 2 figures, 3 table
CaCu_3Ti_4O_12/CaTiO_3 Composite Dielectrics: A Ba/Pb-free Ceramics with High Dielectric Constants
We have measured dielectric properties of CaCuTiO
( = 0, 0.1, 0.5, 1, 1.5, 2, 2.9 and 3), and have found that
CaCuTiO (a composite of CaCuTiO and
CaTiO) exhibits a high dielectric constant of 1800 with a low dissipation
factor of 0.02 below 100 kHz from 220 to 300 K. These are comparable to (or
even better than) those of the Pb/Ba-based ceramics, which could be attributed
to a barrier layer of CaTiO on the surface of the CaCuTiO
grains. The composite dielectric ceramics reported here are environmentally
benign as they do not contain Ba/Pb.Comment: 4 pages, 4 figures, Appl. Phys. Lett. (scheduled on July 25, 2005
Effect of ferroelectric layers on the magnetocapacitance properties of superlattices-based oxide multiferroics
A series of superlattices composed of ferromagnetic
LaCaMnO (LCMO) and ferroelectric/paraelectric
BaSrTiO (0x1) were deposited on SrTiO
substrates using the pulsed laser deposition. Films of epitaxial nature
comprised of spherical mounds having uniform size are obtained.
Magnetotransport properties of the films reveal a ferromagnetic Curie
temperature in the range of 145-158 K and negative magnetoresistance as high as
30%, depending on the type of ferroelectric layers employed for their growth
(\QTR{it}{i.e.} '\QTR{it}{x'} value). Ferroelectricity at temperatures ranging
from 55 K to 105 K is also observed, depending on the barium content. More
importantly, the multiferroic nature of the film is determined by the
appearance of negative magnetocapacitance, which was found to be maximum around
the ferroelectric transition temperature (3% per \QTR{it}{tesla}). These
results are understood based on the role of the ferroelectric/paraelectric
layers and strains in inducing the multiferroism.Comment: Accepted to Applied Physics Letter
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
Conduction of topologically-protected charged ferroelectric domain walls
We report on the observation of nanoscale conduction at ferroelectric domain
walls in hexagonal HoMnO3 protected by the topology of multiferroic vortices
using in situ conductive atomic force microscopy, piezoresponse force
microscopy, and kelvin-probe force microscopy at low temperatures. In addition
to previously observed Schottky-like rectification at low bias [Phys. Rev.
Lett., 104, 217601 (2010)], conductance spectra reveal that negatively charged
tail-to-tail walls exhibit enhanced conduction at high forward bias, while
positively charged head-to-head walls exhibit suppressed conduction at high
reverse bias. Our results pave the way for understanding the semiconducting
properties of the domains and domain walls in small-gap ferroelectrics.Comment: 8 pages, 4 figure
Defects of the Crystal Structure and Jahn-Teller distortion in BiMnO3
Using density-functional theory with the on-site Coulomb correction (the
LDA+U method), we perform the structural optimization of BiMnO3 by starting
from different experimentally reported structures. We confirm that
irrespectively on the starting condition, all calculations converge to the same
centrosymmetric structure, in agreement with the previous finding.
Nevertheless, the structural optimization substantially reduces the Jahn-Teller
(JT) distortion in the system. We attribute this fact to the strong competition
of local distortions around the Mn- and Bi-sites: while the local
Mn-environment experiences the JT instability, the one of the Bi-sites favours
the off-centrosymmetric displacements, which involves the same oxygen atoms.
The existence of the second mechanism explains the difference between BiMnO3
and more canonical JT manganites, such as LaMnO3. Finally, being motivated by
experimental studies, we have investigated the formation of different types of
defects and obtained that BiMnO3 (contrary to other considered systems, such as
LaMnO3 and BiFeO3) can relatively easily form oxygen impurities at interstitial
sites. The impurity oxygen atom tends to form a pair with the host oxygen,
which explains the insulating character of the oxygen-excessive BiMnO3+x.
Moreover, we found that the BiMnO3+x samples experience the "memory effect",
where the optimized crystal structure strongly depends on the starting
configuration. We suggest that such a memory effect may explain stability of
some of the crystal structures of BiMnO3, which have been previously reported
experimentally.Comment: 22 pages, 5 tables, 5 figure
Room-temperature ferromagnetism in Sr_(1-x)Y_xCoO_(3-delta) (0.2 < x < 0.25)
We have measured magnetic susceptibility and resistivity of
SrYCoO ( 0.1, 0.15, 0.2, 0.215, 0.225, 0.25, 0.3,
and 0.4), and have found that SrYCoO is a room
temperature ferromagnet with a Curie temperature of 335 K in a narrow
compositional range of 0.2 0.25. This is the highest transition
temperature among perovskite Co oxides. The saturation magnetization for
0.225 is 0.25 /Co at 10 K, which implies that the observed
ferromagnetism is a bulk effect. We attribute this ferromagnetism to a peculiar
Sr/Y ordering.Comment: 5 pages, 4 figure
Double polarization hysteresis loop induced by the domain pinning by defect dipoles in HoMnO3 epitaxial thin films
We report on antiferroelectriclike double polarization hysteresis loops in
multiferroic HoMnO3 thin films below the ferroelectric Curie temperature. This
intriguing phenomenon is attributed to the domain pinning by defect dipoles
which were introduced unintentionally during film growth process. Electron
paramagnetic resonance suggests the existence of Fe1+ defects in thin films and
first principles calculations reveal that the defect dipoles would be composed
of oxygen vacancy and Fe1+ defect. We discuss migration of charged point
defects during film growth process and formation of defect dipoles along
ferroelectric polarization direction, based on the site preference of point
defects. Due to a high-temperature low-symmetry structure of HoMnO3, aging is
not required to form the defect dipoles in contrast to other ferroelectrics
(e.g., BaTiO3).Comment: 4 figure
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