Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Vortices and Novel Mechanisms |
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Sponsoring Units: DMP DCOMP Chair: James Rondinelli, Argonne National Laboratory Room: C143/149 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V33.00001: Effect of $A$-site size difference on polar behavior in $M$BiScNbO$_6$ ($M$=Na, K, and Rb) perovskite: Density functional calculations Shigeyuki Takagi, Alaska Subedi, Valentino Cooper, David Singh We investigated the effect of $A$-site size difference in the double perovskites BiScO$_3$-$M$NbO$_3$ ($M$=Na, K, and Rb) using first-principles calculations. The materials studied have increasing ionic radii at the $A$-site ($r_{\rm{Na}^+}<$ $r_{\rm{K}^+}<$ $r_{\rm{Rb}^+}$) but are otherwise chemically similar. We find that the polarization of these materials is 70-90 $\mu$C/cm$^2$ along the rhombohedral direction, which increases as the $A$-site size difference becomes larger. The main contribution to the high polarization comes from large off-centerings of Bi ions, which are strongly enhanced by the suppression of octahedral tilts as the $M$-ion size increases. A high Born effective charge of Nb also contributes to the polarization and this contribution is also enhanced by increasing the $M$-ion size. This work was supported by ONR and DOE, BES, Materials Sciences and Engineering. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V33.00002: First Principles Study of Flexoelectricity Jiawang Hong, David Vanderbilt Flexoelectricity is the linear response of polarization to a strain gradient. Because strain gradients break inversion symmetry, flexoelectricity allows for charge to be extracted from deformations even in materials that are not piezoelectric. The flexoelectric effect is negligible on conventional length scales, but it becomes very strong at the nanoscale where large strain gradients can significantly affect the functional properties of dielectric thin films and superlattices. We present first-principles calculations of flexoelectric effects in nonpiezoelectric materials by introducing the strain gradient artificially in a slab geometry and obtain the flexoelectric coefficients. Furthermore, we model the results in terms of quantities, such as dynamical charges and higher\footnote{R. Resta, Phys. Rev. Lett. {\bf 105}, 127601 (2010).} multipole moments that can be computed in the bulk, bringing us closer to a full theory of flexoelectricity. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V33.00003: Coexistence of ferroelectricity and octahedral rotations in ABX$_3$ perovskites Nicole Benedek, Craig Fennie Nearly all cubic perovskite materials are unstable to energy-lowering structural distortions. The most intensively studied distortions are those that induce ferroelectricity and tilts or rotations of the anion octahedra. The phonon dispersion curves of many perovskites contain both types of instability, although competition between the different types of distortions usually leads to ground-state structures in which one type of instability has been eliminated. Hence, whereas there are many perovskites that are \emph{either} ferroelectric or have rotated octahedra, there are very few perovskites that are \emph{both} ferroelectric and have rotated octahedra. We use a combination of Density Functional Theory, group theoretical techniques and crystal chemistry arguments to study the competition between ferroelectric and octahedral rotation distortions in a family of ferroelectric perovskite fluorides and oxides. By considering both ``long-range'' distortions (phonons) and the local bonding environment of each ion, we are able to build up a picture of which factors favor one type of distortion over the other. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 9:12AM |
V33.00004: Vortex Domains in Ferroelectric Nano-Structures Invited Speaker: Recently the study of submicron-diameter ferroelectric disks and squares and rectangles fabricated from films of ca. 100-300 nm thick have revealed usual domain patterns, qualitatively different from the stripe domains commonly studied in macroscopic specimens in the past. These include doughnut-shaped domains, four-fold vertex closure domains, and fractal domains. The static configurations offer a variety of puzzles, and the structures differ from those in magnetic vortex domains, presumably due to the much larger anisotropy in ferroelectrics, which generally prohibits true vortex configurations with polarization forced out of plane. The dynamics also differ qualitatively from early studies: For decades ferroelectrics were thought to be highly Ising-like, but recent data and theoretical simulations favor Bloch walls and more Heisenberg-like kinetics. This talk will include data from Alina Schilling and Marty Gregg in Belfast, Marin Alexe in Halle, and modeling from Hlinka and Marton in Prague and Bellaiche and Prosandeev in Arkansas. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V33.00005: Multiferroic Vortices and Graph Theory Sang-Wook Cheong, S.C. Chae, Y. Horibe, D.Y. Jeong, N. Lee, S. Rodan, T. Choi Hexagonal REMnO$_{3}$ (RE= rare earths) with RE=Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in h-REMnO$_{3}$ meet in cloverleaf arrangements that cycle through all six domain configurations, Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects: even in a strong electric field they won't annihilate. Recently we have found intriguing, but seemingly irregular configurations of a zoo of topological vortices and antivortices in h-REMnO$_{3}$. These configurations can be neatly analyzed in terms of graph theory. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V33.00006: Hidden Roto Symmetries in Crystals and Handed Structures Venkatraman Gopalan, Daniel Litvin Symmetry is a powerful framework to perceive and predict the physical world. The structure of materials is described by a combination of rotations, rotation-inversions and translational symmetries. By recognizing the reversal of static structural rotations between clockwise and counterclockwise directions as a distinct symmetry operation, here we show that there are many more structural symmetries than are currently recognized in right- or left-handed handed helices, spirals, and in antidistorted structures composed equally of rotations of both handedness. For example, though a helix or spiral cannot possess conventional mirror or inversion symmetries, they can possess them in combination with the rotation reversal symmetry. Similarly, we show that many antidistorted perovskites possess twice the number of symmetry elements as conventionally identified. These new symmetries, referred to as ``roto'' symmetries, predict new forms for roto properties that relate to static rotations, such as rotoelectricity, piezorotation, and rotomagnetism. They also enable a symmetry-based search for new phenomena, such as multiferroicity involving a coupling of spins, electric polarization and static rotations. This work is relevant to structure-property relationships in all material structures with static rotations. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V33.00007: Spontaneous Vortex Nanodomain Arrays at Ferroelectric Heterointerfaces Xiaoqing Pan, Christopher Nelson, Yi Zhang, Benjamin Winchester, Long-Qing Chen, Colin Heikes, Carolina Adamo, Alexander Melville, Darrell Schlom, Chad Folkman, Chang-Beom Eom The polarization of BiFeO$_{3}$ subjected to different electrical boundary conditions by hetero-interfaces is imaged with atomic resolution using a Cs-corrected transmission electron microscope. Unusual nanodomains are seen and their role in providing polarization closure is understood through phase-field simulations. Hetero-interfaces are key to the performance of ferroelectric devices and this first observation of vortex arrays at ferroelectric hetero-interfaces reveals properties unlike the surrounding film including mixed Ising-N\'{e}el domain walls, which will affect switching behavior, and a drastic increase of in-plane polarization. Imaging this magnetic analogous effect at ferroelectric hetero-interfaces provides the ability to see device-relevant interface issues. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V33.00008: Helical strain patterns and charge ordering in YbFe$_2$O$_4$ Alexander Hearmon, Paolo Radaelli, Prabhakaran Dharmalingam, Matthias Gutmann, Federica Fabrizi, Dave Allan The $R$Fe$_2$O$_4$ compounds exhibit simultaneously charge ordering (CO) of the Fe$^{2+}$ and Fe$^{3+}$ ions,\footnote{Ikeda \textit{et al}, Nature \textbf{436} 1136 (2005)} together with magnetic ordering of the Fe spins\footnote{Christianson \textit{et al}, PRL \textbf{100} 107601 (2008)} and possible multiferroic behavior.\footnote{S-W Cheong \textit{et al}, Nat. Mater. \textbf{6} 13 (2007)} Synchrotron data collected below the 3D CO transition show intensity concentrated around peaks separated by $\sim$1/3 $c$* but slightly displaced in the $(a^*, b^*)$ plane. Calculations modelling an oxygen displacement pattern are in excellent agreement with the data, suggesting an incommensurate charge ordering of the Fe ions close to the commensurate $\sqrt{3} \times \sqrt{3}$ structure, associated with a helical strain pattern. At high temperature, ordering wavevectors corresponding to many different displacement patterns are simultaneously populated by the system, leading to diffuse but highly structured features in reciprocal space. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V33.00009: Three-dimensional distribution of ferroelectric vortices in multiferroic hexagonal YMnO$_3$ Manfred Fiebig, Elisabeth Soergel, Tobias Jungk, Nicola A. Spaldin, Kris Delaney Multiferroics are a rich source for ``unusual'' forms of ferroelectric order. The spontaneous polarizations is induced by magnetism, charge order, geometric effects, etc., and may lead to novel domain states and functionalities. Recently it was shown that ferroelectric domains in hexagonal multiferroic YMnO$_3$ form vortex-like structures around the direction of polarization [1]. It was assumed that the sixfold character of the domain vortices reflects the uniaxial hexagonal structure. Here we show by piezoresponse force microscopy that high densities of sixfold vortices are also present {\it perpendicular} to the direction of the spontaneous polarization in spite of the merely twofold rotation-inversion symmetry in this direction [2]. We present a simple geometric explanation for this unexpected result and discuss the principal difference between the present case and vortex formation in discommensurate systems.\\[4pt] [1] T. Choi et al., Nature Mater. {\bf 9}, 253 (2010)\\[0pt] [2] T. Jungk et al., Appl. Phys. Lett. {\bf 97}, 012904 (2010) [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V33.00010: Cloverleaf domain patterns in multiferroic RMnO$_{3}$ (R = Ho, Er, and Lu) Y. Horibe, S.C. Chae, N. Lee, S-W. Cheong Hexagonal RMnO$_{3}$ (R=rare earths) exhibits a unique improper ferroelectricity induced by structural trimerization. Intriguing domain pattern associated with ferroelectricity and trimerization, so-called ``cloverleaf'' domain pattern, has been reported in YMnO$_{3}$ [1] In this talk, we will report the domain structures in a series of RMnO$_{3}$ with different rare earth elements, obtained from the results of our transmission electron microscopy. Characteristic cloverleaf domain patterns are clearly observed in RMnO$_{3}$ (R = Ho, Er, and Lu). The results imply that the cloverleaf domain pattern is a common domain feature in the hexagonal manganites. \\[4pt] [1] T. Choi et al., Nature Materials 9, 818 (2010) [Preview Abstract] |
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