208 research outputs found
Unusual behaviour of the ferroelectric polarization in PbTiO/SrTiO superlattices
Artificial PbTiO/SrTiO superlattices were constructed using
off-axis RF magnetron sputtering. X-ray diffraction and piezoelectric atomic
force microscopy were used to study the evolution of the ferroelectric
polarization as the ratio of PbTiO to SrTiO was changed. For
PbTiO layer thicknesses larger than the 3-unit cells SrTiO
thickness used in the structure, the polarization is found to be reduced as the
PbTiO thickness is decreased. This observation confirms the primary role
of the depolarization field in the polarization reduction in thin films. For
the samples with ratios of PbTiO to SrTiO of less than one a
surprising recovery of ferroelectricity that cannot be explained by
electrostatic considerations was observed
Ferroelectric Dead Layer Driven by a Polar Interface
Based on first-principles and model calculations we investigate the effect of
polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As
a representative model, we consider a TiO2-terminated BaTiO3 film with LaO
monolayers at the two interfaces that serve as doping layers. We find that the
polar interfaces create an intrinsic electric field that is screened by the
electron charge leaking into the BaTiO3 layer. The amount of the leaking charge
is controlled by the boundary conditions which are different for three
heterostructures considered, namely Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and
SrRuO3/LaO/BaTiO3/LaO. The intrinsic electric field forces ionic displacements
in BaTiO3 to produce the electric polarization directed into the interior of
the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces
that is non-switchable and thus detrimental to ferroelectricity. Our
first-principles and model calculations demonstrate that the effect is stronger
for a larger effective ionic charge at the interface and longer screening
length due to a stronger intrinsic electric field that penetrates deeper into
the ferroelectric. The predicted mechanism for a ferroelectric dead layer at
the interface controls the critical thickness for ferroelectricity in systems
with polar interfaces.Comment: 33 Pages, 5 figure
Quantitative determination of zero-gravity effects on electronic materials processing germanium crystal growth with simultaneous interface demarcation experiment MA-060, section 5
The crystal growth and segregation characteristics of a melt in a directional solidification configuration under near zero g conditions were investigated. The germanium (doped with gallium) system was selected because it was extensively studied on Earth and because it lends itself to a very detailed macroscopic and microscopic characterization. An extensive study was performed of the germanium crystals grown during the Apollo-Soyuz Test Project mission. It was found that single crystal growth was achieved and that the interface demarcation functioned successfully. On the basis of the results obtained to date, there is no indication that convection driven by thermal or surface tension gradients was present in the melt. The gallium segregation, in the absence of gravity, was found to be fundamentally different in its initial and its subsequent stages from that of the ground based tests. None of the existing theoretical models for growth and segregation can account for the observed segregation behavior in the absence of gravity
Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films
X-ray photoelectron diffraction is used to directly probe the intra-cell
polar atomic distortion and tetragonality associated with ferroelectricity in
ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab-initio
calculations, unambiguously demonstrate non-centro-symmetry in films a few unit
cells thick, imply that films as thin as 3 unit cells still preserve a
ferroelectric polar distortion, and also show that there is no thick
paraelectric dead layer at the surface
X-ray photoelectron diffraction study of ultrathin PbTiO3 films
Abstract.: Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate at the atomic level ultrathin epitaxial c-axis oriented PbTiO3 (PTO) films grown on Nb-doped SrTiO3 substrates. Comparison between experiment and theory allows us to identify a preferential ferroelectric polarization state in a 60 Å -thick PTO film. Multiple scattering theory based on a cluster-model [ Phys. Rev. B , 075404 (2001)] is used to simulate the experiment
Competition between Polar and Nonpolar Lattice Distortions in Oxide Quantum Wells: New Critical Thickness at Polar Interfaces
Estrogen Sensitivity of Target Genes and Expression of Nuclear Receptor Co-Regulators in Rat Prostate after Pre- and Postnatal Exposure to the Ultraviolet Filter 4-Methylbenzylidene Camphor
Strain-Engineering Mott-Insulating LaCuO
The transition temperature of unconventional superconductivity
is often tunable. For a monolayer of FeSe, for example, the sweet spot is
uniquely bound to titanium-oxide substrates. By contrast for
LaSrCuO thin films, such substrates are
sub-optimal and the highest is instead obtained using
LaSrAlO. An outstanding challenge is thus to understand the optimal
conditions for superconductivity in thin films: which microscopic parameters
drive the change in and how can we tune them? Here we
demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray
scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of
LaCuO thin films can be enhanced by compressive strain. Our experiments
and theoretical calculations establish that the substrate producing the largest
under doping also generates the largest nearest neighbour
hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest
optimising the parent Mott state as a strategy for enhancing the
superconducting transition temperature in cuprates.Comment: 15 pages, 7 figures and 2 tables (including Supplementary
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