Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G6: Focus Session: Ferroic Materials by Design |
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Sponsoring Units: DMP DCOMP Chair: Sang-Wook Cheong, Rutgers University Room: 006A |
Tuesday, March 3, 2015 11:15AM - 11:51AM |
G6.00001: Revealing and understanding the behavior of structural domain walls from first principles Invited Speaker: Jorge Iniguez Ferroelectric and ferroelastic domain walls (DWs) are becoming the focus of renewed excitement. Modern experimental techniques permit an unprecedented control on domain structures, and it is now possible to produce materials with a large volume fraction occupied by the DWs themselves. Also, recent experiments show that DWs can display distinct properties not present in the domains, which suggests the possibility of using the walls themselves as the functional {\em material} in nano-devices. In this talk I will review recent projects in which we have used theory and first-principles simulation to reveal and explain a variety of DW-related effects. The presentation will include the formation of novel two-dimensional crystals at the DWs of a ferroelastic material, the occurrence of ferroic orders (ferroelectric, ferromagnetic) confined at the DWs of various compounds, and cases in which peculiar (and useful) response and switching properties relie on existence of a multi-domain state. I will also summarize experimental evidence for most of these incredible findings, which clearly ratify domain and domain-wall engineering as a powerful strategy to obtain novel functional nano-materials. // Work done in collaboration with many researchers, the main ones being: J.C. Wojde\l\ (ICMAB-CSIC), C. Mag\'en (INA at U. Zaragoza), M. Mostovoy (U. Groningen), P. Zubko (U. College London), as well as the groups of Beatriz Noheda (U. Groningen), R. Ramesh (UC Berkeley) and J.-M. Triscone (U. Geneva). [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G6.00002: Magnetic charge and magnetoelectricity in hexagonal manganites and ferrites Meng Ye, David Vanderbilt Magnetoelectric (ME) materials are of fundamental interest and are investigated for their broad potential for technological applications. Commonly the dominant contribution to the ME response is the lattice-mediated one, which is proportional to both the Born electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$.\footnote{J. \'{I}\~{n}iguez, Phys. Rev. Lett. {\bf 101},117201 (2008).} A previous study has shown that exchange striction acting on noncollinear spins induces much larger magnetic charges than when $Z^{\rm m}$ is driven by spin-orbit coupling.\footnote{M. Ye and D. Vanderbilt, Phys. Rev. B {\bf 89}, 064301 (2014).} The hexagonal manganites $R$MnO$_3$ and ferrites $R$FeO$_3$ (R$=$Sc, Y, In, Ho-Lu) exhibit strong couplings between electric, magnetic and structural degrees of freedom, with the transition-metal ions in the basal plane are antiferromagnetically coupled through super-exchange so as to form a 120$^\circ$ spin arrangement. Here we present a theoretical study of the magnetic charges, and of the spin-lattice and spin-electronic ME constants, in these hexagonal manganites and ferrites, clarifying the conditions under which exchange striction leads to enhanced $Z^{\rm m}$s and anomalously large in-plane spin-lattice ME effects. [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G6.00003: Ferroelectrics with a trilinear coupling of three lattice modes: Response to electrical boundary conditions from first principles Andrew T. Mulder, Craig J. Fennie Recent progress in the design of functional materials has led to the discovery of several ferroelectrics with the polarization coupled trilinearly to two nonpolar lattice modes. Because nonpolar lattice modes in perovskites play a key role in determining electronic and magnetic properties, these materials have a polarization that is strongly coupled to other functionality by design. But many properties of trilinear ferroelectrics remain unknown, and general design rules have been difficult to uncover. For example, the electrical properties are unclear because trilinear ferroelectrics may have a conventional ferroelectric mechanism, or may not (like an improper ferroelectric). Understanding the polarization response to electrical boundary conditions is therefore an open question that will impact the application of trilinear ferroelectrics to practical devices. In this talk, we use first principles methods to study the single domain polarization response to changing electrical boundary conditions in a variety of trilinear ferroelectrics. We clarify the design rules, the connection to phenomenological models (proper, improper, and weak ferroelectricity), and we show how epitaxial strain can tune a single trilinear ferroelectric between proper and improper ferroelectricity. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G6.00004: Epitaxial-strain-induced insulating phase in 1:1 SrCrO$_3$/SrTiO$_3$ superlattices: A first-principles study Yuanjun Zhou, Karin Rabe Using first principles calculations, we studied the structure and electronic properties of the 1:1 superlattice combining the magnetic metallic oxide SrCrO$_3$ and the band insulator SrTiO$_3$. We determined the epitaxial-strain dependent ground-state structures of the superlattice using the ``stacking'' method. An insulating polar ground state is found when the tensile strain is greater than 2.2\%. The insulating character of this phase is related to the $d_{xy}^1d_{yz}^1d_{xz}^0$ orbital ordering of the Cr $t_{2g}$ electrons. Specifically, the 1:1 periodic superlattice structure eliminates the Cr-O-Cr bonds in the direction normal to the interface, which reduces the hopping of 3d electrons in this direction. The band widths of $d_{yz}$ and $d_{xz}$ for Cr are thus significantly reduced, enhancing the effect of the in-plane polar distortions in the SrCrO$_3$ layer responsible for opening the the band gap. The elucidation of this mechanism opens an new approach to band engineering, encouraging further investigations on related metallic/dielectric superlattice systems. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G6.00005: Trilinear couplings and the multi-mode anti-ferroelectric transition of PbZrO$_{3}$: a first-principles investigation Jorge Iniguez, Massimiliano Stengel, Sergey Prosandeev, Laurent Bellaiche We have studied ab initio the phase transition in PbZrO$_{3}$, a perovskite oxide usually presented as the prototypic anti-ferroelectric material. Our work reveals the crucial role that anti-ferrodistortive modes--involving concerted rotations of the oxygen octahedra in the structure--play in the transformation, as they select the observed anti-ferroelectric phase, among competing structural variants, via a cooperative trilinear coupling. The resulting picture is that of a complex transition whose multi-mode character is essential to its very occurrence, and poses the provocative question of whether such an intricate behavior can be taken as representative of anti-ferroelectricity in perovskite oxides. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G6.00006: A new class of in-plane Ferroelectric Mott insulators via oxide hetorostructuring Chanul Kim, Hyowon Park, Chris Marianetti We propose simple design rules based on charge transfer, cation ordering, and size mismatch to design a new class of in-plane ferroelectric Mott insulators in perovskite-based transition metal oxides. Ab Initio DFT+U calculations are then used to selectively scan phase space based on these rules. We begin by exploring pairs of A-type ions (A, A') and pairs of B-type ions (B, B') in $AA^{\prime}BB^{\prime}O_{6}$ which will have nominal charge transfer consistent with valencies that are conducive to a low Mott gap insulator. Additionally, the A-type ions are chosen to have a large size mismatch and stereochemical effect. The ordering of A/A' and B/B' still retains C$_{4v}$ symmetry which may be spontaneously broken to yield an in-plane ferroelectric. We uncover a number of materials which are strong candidates to be in-plane ferroelectric Mott insulators in experiment, including BaBiVCuO$_{6}$, BaBiVNiO$_{6}$, PbLaVCuO$_{6}$. Finally, we will discuss potential applications of in-plane ferroelectric Mott insulators such as ferroelectric photovoltaics, Mott FET, and optoelectronic devices. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G6.00007: Why is the electrocaloric effect so small in ferroelectrics? Gian G. Guzman-Verri, Peter B. Littlewood Ferroelectrics are attractive candidate materials for environmentally friendly solid state refrigeration free of greenhouse gases. Their thermal response upon variations of external electric fields is largest in the vicinity of their phase transitions, which may occur near room temperature. The magnitude of the effect, however, is too small for useful cooling applications even when they are driven close to dielectric breakdown. Insight from microscopic theory is therefore needed to characterize materials and provide guiding principles to search for new ones with enhanced electrocaloric performance [1]. Here, we present meaningful figures of merit derived from well-known microscopic models of ferroelectricity which provide insight into the relation between the strength of the effect and the characteristic interactions of ferroelectrics such as dipole forces. We find that the long range nature of these interactions results in a small effect. A strategy is proposed to make it larger by shortening the correlation lengths of fluctuations of polarization [2]. \\[4pt] [1] X. Moya, S. Kar-Narayan, and N. D. Mathur, Nat. Mater. 13, 439 (2014).\\[0pt] [2] G. G. Guzm\'an-Verri and P. B. Littlewood, arXiv:1411.1326. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G6.00008: Strain{\textendash}Induced Geometric Ferroelectricity in Perovskite{\textendash}structured Fluoroscandates Nenian Charles, James Rondinelli Using first-principles density functional theory calculations we investigate geometric ferroelectricity in epitaxially strained double-perovskite fluorides, Na$_3$ScF$_6$ and K$_2$NaScF$_6$. The experimental room temperature crystal structures of the fluoroscandates are centrosymmetric, i.e. Na$_3$ScF$_6$ ($P2_1/n$) and K$_2$NaScF$_6$ ($Fm\bar{3}m$). However, in their prototypical cubic geometry, we identify soft infrared active modes that are strongly sensitive to pressure: Ferroelectric instabilities are found for negative hydrostatic pressures $\sim\!$ -6 GPa. For Na$_3$ScF$_6$ we observe octahedral rotations ($a^-a^-c^+$ tilt system) are in strong competition with acentric polar distortions, and as a result exceedingly large tensile strain above 8\% are required to stabilize a Pm polar phase. We demonstrate that the strain mismatch required to stabilize the ferroelectric phase can be reduced to approximately 4\% with chemical substitution in ($a^0a^0a^0$) K$_2$NaScF$_6$ by reducing the tendency to octahedral rotations. Our study provides new insights that may prove useful in guiding experimental efforts towards identifying functional polar fluoroperovskites. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G6.00009: Tuning the structure of oxide thin films with strain Elizabeth Nowadnick, Andrew Mulder, Craig Fennie Octahedral rotations are ubiquitous in perovskite oxides and play an important role in determining the functionality of these materials, for example impacting magnetism and ferroelectricity. Pressure applied to bulk perovskites, as well as biaxial strain in epitaxially grown thin films, couples to the octahedral rotations. By performing first principles density functional theory calculations for a wide variety of $ABO_3$ perovskite oxides, we present a systematic study of the structural response of these systems to pressure and strain. We find that the octahedral rotations respond to pressure and biaxial strain in distinct ways. With pressure, the relative compressibilities of the $AO_{12}$ and $BO_6$ polyhedra govern the response of the octahedral rotations, whereas the response to biaxial strain arises from an interplay of the out of plane axis relaxation and the polyhedral compressibilities. Our findings offer insight into how to optimize the sensitivity of octahedral rotations to strain and pressure. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G6.00010: First-principles studies of lone-pair-induced distortions in epitaxial phases of perovskite SnTiO$_3$ and PbTiO$_3$ Krishna Pitike, Lydie Louis, William Parker, Serge Nakhmanson In this project, a computational investigation utilizing density functional theory methods is carried out to elucidate the differences in stereochemical lone-pair activity of Pb and Sn A-site ions in epitaxial polar ATiO$_3$ perovskites. The contrasting tendencies for the lead- and tin-based compounds to form different phases under biaxial tension are connected to the amount of charge concentrated within the lone pair lobes. Specifically, phases are energetically more preferable when as much charge as possible is dissipated out of the lobe, thus lowering the cost of Coulomb repulsions between the lone pair and the surrounding oxygen cage. The insights gained about the electronic-level underpinnings of transitional behavior in such lone-pair active epitaxial ferroelectrics may be used in the design of a new generation of more efficient electromechanical and electrooptical devices. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G6.00011: Tuning ferroelectric polarization in $AA^{\prime}$MnWO$_6$ double perovskites through $A$ cation substitution Joshua Young, James Rondinelli Magnetic ferroelectric materials, which exhibit simultaneous magnetic and electric polarizations, have generated significant interest for application in novel electronic devices. Recent experimental work has shown that the double perovskite NaLaMnWO$_6$ exhibits anti-ferromagnetic order, while computational studies predict it to also exhibit a spontaneous polarization of 16 $\mu$C/cm$^2$ owing to an octahedral rotation induced improper mechanism. Using first principles density functional theory calculations, we investigate nine iso-structural $AA^{\prime}$MnWO$_6$ compounds through chemical substitution of alkali metal ($A$=Na, K, Rb) and rare earth cations ($A^{\prime}$=La, Nd, Y), and find that the ferroelectric polarization can be enhanced by up to 150\% by maximizing the difference in ionic size of the $A$ and $A^{\prime}$ cations. We then identify the microscopic features responsible for this polarization through an examination of the tolerance factors, bond valences, and atomic displacement patterns. We anticipate that the crystal-chemistry criteria and analysis presented here can be extended to additional members of this family as well as guide the targeted design of novel multiferroics. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G6.00012: First principles study of the band gaps and edges of (111) epitaxially strained SrTiO$_3$ Sebastian E. Reyes-Lillo, Robert F. Berger, Jeffrey B. Neaton SrTiO$_3$ is a tunable material with potential use in energy applications. Previous experimental and first principles results demonstrated controllable manipulation of the electronic band gap in (001) biaxially strain, and predicted a large room-temperature band gap reduction (0.3 eV) for (111) epitaxial growth at small and large strains. In this work, we revisit and examine the effect of (111) epitaxial strain on the structural and electronic properties of SrTiO$_3$ using density functional theory calculations at low and room temperatures. Under (111) epitaxial strain at low temperature, we find that SrTiO$_3$ has an antiferrodistortive ground state at large compressive strains ($<$ -1\%) and a ferroelectric phase transition between -1\% $<$ $\eta$ $<$ 4\% biaxial strains. While structural distortions widen the band gap compared to the paraelectric phase, an effective reduction of band gap is observed for the distorted structure at large compressive strains compared to bulk. This work is supported by DOE, computational resources are provided by NERSC. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G6.00013: First principles investigations of structural, elastic, dielectric and piezoelectric properties of \{Ba,Sr,Pb\}TiO$_3$, \{Ba,Sr,Pb\}ZrO$_3$ and \{Ba,Sr,Pb\}\{Zr,Ti\}O$_3$ ceramics Berna Akgenc, Cetin Tasseven, Tahir Cagin We use first-principle density-functional study of structural, anisotropic mechanical, dielectric and piezoelectric properties of \{Ba,Sr,Pb\}TiO$_3$, \{Ba,Sr,Pb\}ZrO$_3$ and \{Ba,Sr,Pb\}\{Zr,Ti\}O$_3$ alloys in cubic perovskite structures at zero temperature. Because there is significant interest in finding new piezoelectrics that do not contain toxic elements such as lead. In this study, we compare piezoelectric response of those alloys to synthesize outstanding piezoelectric materials. In perovskite structures, the spontaneous polarization is due to enormous values of Born effective charges computed by linear response within density functional perturbation theory, which are much larger than predicted nominal charge. We deeply investigated the effects of composition, order and site defects structure on piezoelectric constants. [Preview Abstract] |
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