The structure of electronic polarization and its strain dependence

The \phi(\kpp)\sim \kpp relation is called polarization structure. By density functional calculations, we study the polarization structure in ferroelectric perovskite PbTiO$_3$, revealing (1) the \kpp point that contributes most to the electronic polarization, (2) the magnitude of bandwidth, and (3) subtle curvature of polarization dispersion. We also investigate how polarization structure in PbTiO$_3$ is modified by compressive inplane strains. The bandwidth of polarization dispersion in PbTiO$_3$ is shown to exhibit an unusual decline, though the total polarization is enhanced. As another outcome of this study, we formulate an analytical scheme for the purpose of identifying what determine the polarization structure at arbitrary \kpp points by means of Wannier functions. We find that \phi(\kpp) is determined by two competing factors: one is the overlaps between neighboring Wannier functions within the plane {\it perpendicular} to the polarization direction, and the other is the localization length {\it parallel} to the polarization direction. Inplane strain increases the former while decreases the latter, causing interesting non-monotonous effects on polarization structure. Finally, polarization dispersion in another paradigm ferroelectric BaTiO$_3$ is discussed and compared with that of PbTiO$_3$.Comment: 5 Figure

Evolution of the rates of mass wasting and fluvial sediment transfer from the epicentral area of the 1999, Mw 7.6 earthquake

The 1999 Chichi earthquake (Mw=7.6) triggered more than 20,000 landslides in the epicentral area in central west Taiwan, and subsequent typhoons have caused an even larger number of slope failures. As a result, the suspended sediment load of the epi- central Choshui River has increased dramatically. Measurements of suspended sedi- ment at a downstream gauging station indicate that the unit sediment concentration increased about six times due to the earthquake, and decreased exponentially due to flushing by subsequent typhoons. The e-folding time scale of the seismic perturbation of sediment transfer in the Choshui River is 3-5 years. Based on this estimate of the de- cay of the erosional response to the earthquake, a mass balance can be calculated for the earthquake, including co-seismic uplift and subsidence, post-seismic relaxation, and erosion. This mass balance shows that the Chi-Chi earthquake has acted to build ridge topography in the hanging wall of the fault, but in the far field, some destruc- tion of topography has occurred. However, our estimate of seismically-driven erosion may be incomplete. A detailed analysis of landsliding in the Chenyoulan tributary of the Choshui River indicates that most co-and post seismic landslide debris remains on hillslopes within the catchment. Recent typhoons have continued to cause high rates of landsliding high in the landscape, but rates of mass wasting near the stream net- work have decreased. The full geomorphic response to the Chi-Chi earthquake may be much larger, and more protracted than indicated by river gauging data

Continuum Theory for Piezoelectricity in Nanotubes and Nanowires

We develop and solve a continuum theory for the piezoelectric response of one dimensional nanotubes and nanowires, and apply the theory to study electromechanical effects in BN nanotubes. We find that the polarization of a nanotube depends on its aspect ratio, and a dimensionless constant specifying the ratio of the strengths of the elastic and electrostatic interactions. The solutions of the model as these two parameters are varied are discussed. The theory is applied to estimate the electric potential induced along the length of a BN nanotube in response to a uniaxial stress.Comment: 4 pages in RevTex4, 2 epsf figure

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

Effect of ferroelectric layers on the magnetocapacitance properties of superlattices-based oxide multiferroics

A series of superlattices composed of ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO) and ferroelectric/paraelectric Ba$_{1-x}$Sr$_x$TiO$_3$ (0$\leq$x$\leq$1) were deposited on SrTiO$_3$ 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

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

Coulomb interaction and ferroelectric instability of BaTiO3

Using first-principles calculations, the phonon frequencies at the $\Gamma$ point and the dielectric tensor are determined and analysed for the cubic and rhombohedral phases of BaTiO$_{3}$. 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

Atomic-scale compensation phenomena at polar interfaces

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density functional theory how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by interfacial charge generated, for example, by oxygen vacancies.Comment: 3 figure

Phase diagram of Pb(Zr,Ti)O3 solid solutions from first principles

A first-principles-derived scheme, that incorporates ferroelectric and antiferrodistortive degrees of freedom, is developed to study finite-temperature properties of PbZr1-xTixO3 solid solutions near its morphotropic phase boundary. The use of this numerical technique (i) resolves controversies about the monoclinic ground-state for some Ti compositions, (ii) leads to the discovery of an overlooked phase, and (iii) yields three multiphase points, that are each associated with four phases. Additional neutron diffraction measurements strongly support some of these predictions.Comment: 10 pages, 2 figure

Optimal configuration of microstructure in ferroelectric materials by stochastic optimization

An optimization procedure determining the ideal configuration at the microstructural level of ferroelectric (FE) materials is applied to maximize piezoelectricity. Piezoelectricity in ceramic FEs differ significantly from that of single crystals because of the presence of crystallites (grains) possessing crystallographic axes aligned imperfectly. The piezoelectric properties of a polycrystalline (ceramic) FE is inextricably related to the grain orientation distribution (texture). The set of combination of variables, known as solution space, which dictates the texture of a ceramic is unlimited and hence the choice of the optimal solution which maximizes the piezoelectricity is complicated. Thus a stochastic global optimization combined with homogenization is employed for the identification of the optimal granular configuration of the FE ceramic microstructure with optimum piezoelectric properties. The macroscopic equilibrium piezoelectric properties of polycrystalline FE is calculated using mathematical homogenization at each iteration step. The configuration of grains characterised by its orientations at each iteration is generated using a randomly selected set of orientation distribution parameters. Apparent enhancement of piezoelectric coefficient $d_{33}$ is observed in an optimally oriented BaTiO$_3$ single crystal. A configuration of crystallites, simultaneously constraining the orientation distribution of the c-axis (polar axis) while incorporating ab-plane randomness, which would multiply the overall piezoelectricity in ceramic BaTiO$_{3}$ is also identified. The orientation distribution of the c-axes is found to be a narrow Gaussian distribution centred around ${45^\circ}$. The piezoelectric coefficient in such a ceramic is found to be nearly three times as that of the single crystal.Comment: 11 pages, 7 figure