42 research outputs found
Lattice Relaxation in Epitaxial BaTiO Thin Films
We have investigated the out-of-plane lattice relaxation related to the
ferroelectric transitions in epitaxial BaTiO (BTO) films using synchrotron
X-ray diffraction. Under either compressive strain or tensile strain, there is
evidence for two structural phase transitions as a function of temperature. The
transition temperature is a strong function of strain, which can be as
much as 100 K above the corresponding in bulk. Under compressive strain,
the tetragonality of BTO unit cell implies that the polarization of the first
ferroelectric phase is out-of-plane, while under tensile strain, the
polarization is in-plane. The transitions at lower temperature may correspond
to the or transitions, following the notations by Pertsev
\textit{et al}. The orientations of the domains are consistent with theoretical
predictions.Comment: 4 pages, 3 figure
Anomalous Phase Transition in Strained SrTiO Thin Films
We have studied the cubic to tetragonal phase transition in epitaxial
SrTiO films under various biaxial strain conditions using synchrotron X-ray
diffraction. Measuring the superlattice peak associated with TiO octahedra
rotation in the low temperature tetragonal phase indicates the presence of a
phase transition whose critical temperature is a strong function of strain,
with T as much as 50K above the corresponding bulk temperature.
Surprisingly, the lattice constants evolve smoothly through the transition with
no indication of a phase change. This signals an important change in the nature
of the phase transition due to the epitaxy strain and substrate clamping
effect. The internal degrees of freedom (TiO rotations) have become
uncoupled from the overall lattice shape.Comment: 4 pages, 3 figures, REVTeX
Structural phase transitions in epitaxial perovskite films
Three different film systems have been systematically investigated to
understand the effects of strain and substrate constraint on the phase
transitions of perovskite films. In SrTiO films, the phase transition
temperature T was determined by monitoring the superlattice peaks
associated with rotations of TiO octahedra. It is found that T depends
on both SrTiO film thickness and SrRuO buffer layer thickness. However,
lattice parameter measurements showed no sign of the phase transitions,
indicating that the tetragonality of the SrTiO unit cells was no longer a
good order parameter. This signals a change in the nature of this phase
transition, the internal degree of freedom is decoupled from the external
degree of freedom. The phase transitions occur even without lattice relaxation
through domain formation. In NdNiO thin films, it is found that the
in-plane lattice parameters were clamped by the substrate, while out-of-plane
lattice constant varied to accommodate the volume change across the phase
transition. This shows that substrate constraint is an important parameter for
epitaxial film systems, and is responsible for the suppression of external
structural change in SrTiO and NdNiO films. However, in SrRuO films
we observed domain formation at elevated temperature through x-ray reciprocal
space mapping. This indicated that internal strain energy within films also
played an important role, and may dominate in some film systems. The final
strain states within epitaxial films were the result of competition between
multiple mechanisms and may not be described by a single parameter.Comment: REVTeX4, 14 figure
Direct evidence for the suppression of charge stripes in epitaxial LaSrNiO thin films
We have successfully grown epitaxial LaSrNiO films with
a small crystalline mosaic using pulsed laser deposition. With synchrotron
radiation, the x-ray diffraction peaks associated with charge stripes have been
successfully observed for relatively thick films. Anomalies due to the
charge-ordering transition have been examined using four-point probe
resistivity measurements. X-ray scattering provides direct evidence for
suppression of the stripe phase in thin samples; the phase disappears for film
thicknesses 2600 ~\AA{}. The suppression appears to be a result of
shrinking the stripe phase domains. This may reflect the stripe phase
progressing from nematic to isotropic.Comment: 5 pages, 4 figure
Tuning the two-dimensional electron liquid at oxide interfaces by buffer-layer-engineered redox reactions
Polar discontinuities and redox reactions provide alternative paths to create
two-dimensional electron liquids (2DELs) at oxide interfaces. Herein, we report
high mobility 2DELs at interfaces involving SrTiO3 (STO) achieved using polar
La7/8Sr1/8MnO3 (LSMO) buffer layers to manipulate both polarities and redox
reactions from disordered overlayers grown at room temperature. Using resonant
x-ray reflectometry experiments, we quantify redox reactions from oxide
overlayers on STO as well as polarity induced electronic reconstruction at
epitaxial LSMO/STO interfaces. The analysis reveals how these effects can be
combined in a STO/LSMO/disordered film trilayer system to yield high mobility
modulation doped 2DELs, where the buffer layer undergoes a partial
transformation from perovskite to brownmillerite structure. This uncovered
interplay between polar discontinuities and redox reactions via buffer layers
provides a new approach for the design of functional oxide interfaces.Comment: Nano Letters, 201