2,486 research outputs found
Ideal barriers to polarization reversal and domain-wall motion in strained ferroelectric thin films
The ideal intrinsic barriers to domain switching in c-phase PbTiO_3 (PTO),
PbZrO_3 (PZO), and PbZr_{1-x}Ti_xO_3 (PZT) are investigated via
first-principles computational methods. The effects of epitaxial strain on the
atomic structure, ferroelectric response, barrier to coherent domain reversal,
domain-wall energy, and barrier to domain-wall translation are studied. It is
found that PTO has a larger polarization, but smaller energy barrier to domain
reversal, than PZO. Consequentially the idealized coercive field is over two
times smaller in PTO than PZO. The Ti--O bond length is more sensitive to
strain than the other bonds in the crystals. This results in the polarization
and domain-wall energy in PTO having greater sensitivity to strain than in PZO.
Two ordered phases of PZT are considered, the rock-salt structure and a (100)
PTO/PZO superlattice. In these simple structures we find that the ferroelectric
properties do not obey Vergard's law, but instead can be approximated as an
average over individual 5-atom unit cells.Comment: 9 pages, 13 figure
Multiferroic BiFeO3-BiMnO3 Nanocheckerboard From First Principles
We present a first principles study of an unusual heterostructure, an
atomic-scale checkerboard of BiFeO3-BiMnO3, and compare its properties to the
two bulk constituent materials, BiFeO3 and BiMnO3. The "nanocheckerboard" is
found to have a multiferroic ground state with the desired properties of each
constituent: polar and ferrimagnetic due to BiFeO3 and BiMnO3, respectively.
The effect of B-site cation ordering on magnetic ordering in the BiFeO3-BiMnO3
system is studied. The checkerboard geometry is seen to give rise to a a novel
magnetostructural effect that is neither present in the bulk constituent
materials, nor in the layered BiFeO3-BiMnO3 superlattice.Comment: 15 pages, 14 figure
Enhancement of piezoelectricity in a mixed ferroelectric
We use first-principles density-functional total energy and polarization
calculations to calculate the piezoelectric tensor at zero temperature for both
cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest
piezoelectric coefficient for the tetragonal configuration is enhanced by a
factor of about three with respect to that of the cubic configuration. This can
be attributed to both the larger strain-induced motion of cations relative to
anions and higher Born effective charges in the tetragonal case. A normal mode
decomposition shows that both cation ordering and local relaxation weaken the
ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure
Piezoelectric control of the magnetic anisotropy via interface strain coupling in a composite multiferroic structure
We investigate theoretically the magnetic dynamics in a
ferroelectric/ferromagnetic heterostructure coupled via strain-mediated
magnetoelectric interaction. We predict an electric field-induced magnetic
switching in the plane perpendicular to the magneto-crystalline easy axis, and
trace this effect back to the piezoelectric control of the magnetoelastic
coupling. We also investigate the magnetic remanence and the electric
coercivity
Predicting polarization enhancement in multicomponent ferroelectric superlattices
Ab initio calculations are utilized as an input to develop a simple model of
polarization in epitaxial short-period CaTiO3/SrTiO3/BaTiO3 superlattices grown
on a SrTiO3 substrate. The model is then combined with a genetic algorithm
technique to optimize the arrangement of individual CaTiO3, SrTiO3 and BaTiO3
layers in a superlattice, predicting structures with the highest possible
polarization and a low in-plane lattice constant mismatch with the substrate.
This modelling procedure can be applied to a wide range of layered
perovskite-oxide nanostructures providing guidance for experimental development
of nanoelectromechanical devices with substantially improved polar properties.Comment: 4 pages, submitted to PR
Dynamic Behavior in Piezoresponse Force Microscopy
Frequency dependent dynamic behavior in Piezoresponse Force Microscopy (PFM)
implemented on a beam-deflection atomic force microscope (AFM) is analyzed
using a combination of modeling and experimental measurements. The PFM signal
comprises contributions from local electrostatic forces acting on the tip,
distributed forces acting on the cantilever, and three components of the
electromechanical response vector. These interactions result in the bending and
torsion of the cantilever, detected as vertical and lateral PFM signals. The
relative magnitudes of these contributions depend on geometric parameters of
the system, the stiffness and frictional forces of tip-surface junction, and
operation frequencies. The dynamic signal formation mechanism in PFM is
analyzed and conditions for optimal PFM imaging are formulated. The
experimental approach for probing cantilever dynamics using frequency-bias
spectroscopy and deconvolution of electromechanical and electrostatic contrast
is implemented.Comment: 65 pages, 15 figures, high quality version available upon reques
Surface Display of Complex Enzymes by in Situ SpyCatcher-SpyTag Interaction
The display of complex proteins on the surface of cells is of great importance for protein engineering and other fields of biotechnology. Herein, we describe a modular approach, in which the membrane anchor protein Lpp-OmpA and a protein of interest (passenger) are expressed independently as genetically fused SpyCatcher and SpyTag units and assembled in situ by post-translational coupling. Using fluorescent proteins, we first demonstrate that this strategy allows the construct to be installed on the surface of E. coli cells. The scope of our approach was then demonstrated by using three different functional enzymes, the stereoselective ketoreductase Gre2p, the homotetrameric glucose 1-dehydrogenase GDH, and the bulky heme- and diflavin-containing cytochrome P450 BM3 (BM3). In all cases, the SpyCatcher-SpyTag method enabled the generation of functional whole-cell biocatalysts, even for the bulky BM3, which could not be displayed by conventional fusion with Lpp-OmpA. Furthermore, by using a GDH variant carrying an internal SpyTag, the system could be used to display an enzyme with unmodified N- and C-termini
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