Polarization Rotation, Switching and E-T phase diagrams of BaTiO3_3: A Molecular Dynamics Study

We use molecular dynamics simulations to understand the mechanisms of polarization switching in ferroelectric BaTiO$_3$ achieved with external electric field. For tetragonal and orthorhombic ferroelectric phases, we determine the switching paths, and show that polarization rotation through intermediate monoclinic phases (a) facilitates switching at low fields (b) is responsible for a sharp anisotropy in polarization switching. We develop understanding of this through determination of detailed electric field-temperature phase diagrams, that are fundamental to technological applications based on electromechanical and switching response of ferroelectrics

Ferroelectric Phase Transitions in Ultra-thin Films of BaTiO3

We present molecular dynamics simulations of a realistic model of an ultrathin film of BaTiO$_3$ sandwiched between short-circuited electrodes to determine and understand effects of film thickness, epitaxial strain and the nature of electrodes on its ferroelectric phase transitions as a function of temperature. We determine a full epitaxial strain-temperature phase diagram in the presence of perfect electrodes. Even with the vanishing depolarization field, we find that ferroelectric phase transitions to states with in-plane and out-of-plane components of polarization exhibit dependence on thickness; it arises from the interactions of local dipoles with their electrostatic images in the presence of electrodes. Secondly, in the presence of relatively bad metal electrodes which only partly compensate the surface charges and depolarization field, a qualitatively different phase with stripe-like domains is stabilized at low temperature

Elastic displacements and step interactions on metallic surfaces: GIXD and ab initio study of Au(332)

International audienceWe have studied the energetics, relaxation and interactions of steps on the Au(332) vicinal surface, using a combination of grazing incidence X-ray diffraction (GIXD), anisotropic linear elasticity (ALE) theory, and ab initio density functional theory (DFT). We find that the initial force distribution on a bulk-truncated surface, as well as the resulting pattern of atomic relaxations, can be reproduced excellently by a buried dipole elastic model. The close agreement obtained between experimental and calculated X-ray diffraction profiles allows us to precisely determine the value of the elastic dipole density at the steps. We also use these results to obtain an experimental estimate of the surface stress on an unreconstructed Au(111) facet, 2.3+/-0.4 Nm-1, and the value of the step-step elastic interaction energy: 950 +/- 150 meV.Å

A first principles density functional investigation of ligand-protected eight atom gold nanoclusters

Based on first principles density functional calculations we have studied the effect of ligand attachment on eight atom gold clusters of two-dimensional (2D) and three-dimensional (3D) geometries. Recent experimental synthesis of this magic numbered cluster using glutathione [Muhammed et al., Nano Res. 1, 333 (2008)] has instigated this investigation. We have chosen ethyl mercaptan (CH3-CH2SH) as the ligand which is the simplified form of glutathione (HO2CCH2NHCOCH(NH2)-CH2CH2CONHCH(CO2H)-CH2SH). We have analyzed the energetics, sd-hybridization, density of states and charge density distributions of the bare and ligand-capped clusters. Our findings indicate that attachment of ethyl mercaptan ligands on eight atom gold clusters enhances the stability of planar 2D geometries over 3D like structure

Polarization rotation, switching, and electric-field-temperature phase diagrams of ferroelectric BaTiO<SUB>3</SUB>: a molecular dynamics study

We use molecular dynamics simulations to understand the mechanisms of polarization switching in ferroelectric BaTiO3 achieved with external electric field. For tetragonal and orthorhombic ferroelectric phases, we determine the switching paths and show that polarization rotation through intermediate monoclinic phases (a) facilitates switching at low fields and (b) is responsible for a sharp anisotropy in polarization switching. We develop an understanding of this through the determination of detailed electric-field-temperature phase diagrams that are fundamental to technological applications based on electromechanical and switching response of ferroelectrics

Influence of estrogens on GH-cell network dynamics in females: a live in situ imaging approach

The secretion of endocrine hormones from pituitary cells finely regulates a multitude of homeostatic processes. To dynamically adapt to changing physiological status and environmental stimuli, the pituitary gland must undergo marked structural and functional plasticity. Endocrine cell plasticity is thought to primarily rely on variations in cell proliferation and size. However, cell motility, a process commonly observed in a variety of tissues during development, may represent an additional mechanism to promote plasticity within the adult pituitary gland. To investigate this, we used multiphoton time-lapse imaging methods, GH-enhanced green fluorescent protein transgenic mice and sexual dimorphism of the GH axis as a model of divergent tissue demand. Using these methods to acutely (12 h) track cell dynamics, we report that ovariectomy induces a dramatic and dynamic increase in cell motility, which is associated with gross GH-cell network remodeling. These changes can be prevented by estradiol supplementation and are associated with enhanced network connectivity as evidenced by increased coordinated GH-cell activity during multicellular calcium recordings. Furthermore, cell motility appears to be sex-specific, because reciprocal alterations are not detected in males after castration. Therefore, GH-cell motility appears to play an important role in the structural and functional pituitary plasticity, which is evoked in response to changing estradiol concentrations in the female