Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K09: Ordering in Ferroic Oxides IIFocus
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Sponsoring Units: DMP Chair: Fumitaka Kagawa, University of Tokyo Room: LACC 301A |
Wednesday, March 7, 2018 8:00AM - 8:36AM |
K09.00001: Caloric effects in ferroics with antiferroelectric-ferroelectric phase competition and other materials Invited Speaker: Inna Ponomareva Caloric effects are associated with a reversible change in temperature under application or removal of external fields, such as electric, magnetic, or stress fields. Ferroics often exhibit large caloric responses which typically originate from the existence of structural phase transitions. Here we focus on caloric effects in ferroics with antiferroelectric-ferroelectric phase competition, the role of quantum effects in the caloric responses, and some exotic caloric effects predicted from first-principles-based simulations. Antiferroelectrics are interesting for caloric effects investigation since they develop antipolar ordering and exhibit dielectric constants comparable to those of ferroelectrics. We report several features of caloric effects in a prototypical antiferroelectric PbZrO3: negative sign of electrocaloric effect in the antipolar phase, existence of a scaling law for the electrocaloric change in temperature, and prediction of highly tunable piezocaloric effect. Interestingly, PbZrO3 can develop ferroelectric phases at the nanoscale as the temperature is lowered. Transition into a ferroelectric phase results in a large change in polarization which favors electrocaloric response. A combination of direct and indirect simulations is used to predict electrocaloric effect of up to 15 K in PbZrO3 thin films. To the best of our knowledge, all current computational approaches utilize classical framework. We propose a semiclassical approach to direct simulations of caloric effects that takes into account quantum behavior of the heat capacity. Its application to prototypical ferroelectrics reveals a severe underestimation of electrocaloric change in temperature at low temperatures by classical simulations and offers a way to design caloric composites. Finally, we report our findings on some unusual caloric effects, such as flexocaloric effect and elastocaloric effect in nonferroics. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K09.00002: Diffuse scattering studies on polar-nanoregions in PMN-xPT relaxor ferroelectrics Guangyong Xu, Zhijun Xu, Peter Gehring, Chris Stock, Fei Li, shujun zhang PZN-xPT and PMN-xPT relaxor ferroelectric materials have been studied extensively due to their extraordinary piezoelectric and electromechanical responses. One of the key issues in these materials is the role of polar nano-regions (PNR), or, in other words, how the short-range orders coexist with long-range polar order and influence the bulk property. We have performed neutron scattering measurements on single crystal PZN-xPT and PMN-xPT samples. With elastic diffuse scattering measurements carried out under external field along different directions, we were able to monitor how the dominant part of the diffuse scattering - the "butterfly diffuse" that extends along <110> directions - change with field. Our observation was a complete redistribution (of the diffuse scattering intensities) under [111] field, a partial redistribution under [110] field, and no major change under [001] field. Our results suggest that the polar nano-regions are robust local orders that are locked in within the surrounding polar environment below Tc, and also strongly interact with acoustic phonon modes that induces instability in these systems. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K09.00003: Nanoscale order parameter and symmetry fluctuations in ferroelectric BaTiO3 single crystal Yu-Tsun Shao, Jian-Min Zuo Ferroelectric single crystals are widely applied for medical imaging, actuation, and sensor technologies due to their piezoelectric effect. However, the microscopic origin of ferroelectric phase transitions is still debated, despite many past efforts. Here, we report on a study of local crystal symmetry inside the ferroelectric domains of BaTiO3 using convergent beam electron diffraction (SCBED) and nanometer-sized electron probes. The results demonstrate that the crystal symmetry is not homogeneous; regions of few tens nm retaining almost perfect symmetry are interspersed in regions of lower symmetry. The highly symmetric regions have the acentric tetragonal, orthorhombic, and rhombohedral symmetry for the ferroelectric phases of BaTiO3 at different temperatures, which suggest the displacive model of ferroelectric phase transitions in BaTiO3. However, the lowering of symmetry in some regions is consistent with the predictions of order-disorder phase transition mechanism. For future work, the SCBED technique will be further developed for simultaneous identification of polarization domains, determination of the local crystal symmetry, and identification of polarization, structural distortions and chemical bonding. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K09.00004: ``Giant'' magneto-elastic effect, antiferromagnetic domains and topological defects in hexagonal YMnO3 Arpita Paul, Umesh V Waghmare We construct a model Hamiltonian to capture the low energy physics of coupled spins and phonons of hexagonal YMnO3 derived from first-principles density functional theory, and determine its temperature dependent behaviour using Monte Carlo simulations. We demonstrate a first-order Néel transition accompanied by a giant magnetoelastic effect seen experimentally in YMnO3, which is shown here to originate from the coupling between the ordering of spins in the basal plane with Γ3 symmetry and Γ1 phonon. We studied two distinct types of antiferromagnetic domain walls: (a) 180°: antiferromagnetic domains within one structural domain and (b) 60°: structurally locked antiferromagnetic domains. We show that the spins at a 180° antiferromagnetic domain wall within one structural domain order locally with Γ1 symmetry, and consequently give rise to a local linear magneto-electric coupling. Finally, we show that topologically protected magnetic vortices are stabilized along a line of the intersection of six 60° antiferromagnetic domain walls. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K09.00005: Coexistence of Polar Distortion and Metallicity in PbTi1-xNbxO3 Kui-juan Jin, junxing gu, chao ma Ferroelectricity has been believed unable to coexist with metallicity since the free carriers can screen the internal coulomb interactions of dipoles. Very recently, one kind of materials called as ferroelectric metal was reexamined. Here, we report the coexistence of metallicity and polar distortion in a new candidate for ferroelectric metal PbTi1-xNbxO3 via doping engineering. The ferroelectric-like polar distortion in all the doped PbTi1-xNbxO3, with x ranging from 0.04 to 0.12, was observed by the piezoresponse force microscopy and the scanning transmission electron microscopy measurements. PbTi1-xNbxO3 films become more conductive with more doping density, and emerge a metallic behavior when x reaches 0.12. Our first principles calculations further revealed that the doped Nb ions in the films can only provide free electrons, but not be able to damage the dipoles in unite cells even with the heaviest doping density of 0.12. We believe that these results confirm a feasibility of realizing the coexistence of metallicity and polar distortion for other ferroelectrics in a common way, and motivate the synthesis of some new materials with artificially designed properties even incompatible in nature. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K09.00006: Brillouin and angular dependent Raman scattering studies on relaxor ferroelectric 0.83Pb(Mg1/3Nb2/3)O3-0.17PbTiO3 single crystals Helal Md Al, Shinya Tsukada, Md Aftabuzzaman, Sarunas Svirskas, Maksim Ivanov, Juras Banys, Seiji Kojima The relaxor ferroelectrics (RFs) are characterized by their nanometric local region so called polar nanoregions (PNRs) and chemical ordered regions (CORs) [1,2]. In the present study, the temperature and angular dependences of Raman scattering were studied to investigate the local properties in 0.17Pb(Mg1/3Nb2/3)O3-0.83PbTiO3 (PMN-17PT) single crystals with intermediate random fields. The existence of CORs with Fm m and PNRs with R3m symmetry in a cubic phase was shown by the angular dependence of VV and VH Raman scattering spectra. In the temperature dependence of broadband Brillouin scattering, the stretched slowing down mechanism of PNRs towards the Curie temperature was observed in a cubic phase [3]. These results are compared with PMN-56PT with weak random fields. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K09.00007: Electronic signature of the vacancy ordering in NbO (Nb3O3) Anna Efimenko, Nils Hollmann, Katharina Hoefer, Jonas Weinen, Daisuke Takegami, Klaus Wolff, Simone Altendorf, Zhiwei Hu, A Rata, Alexander Komarek, Agustinus Nugroho, Y. Liao, Ku-Ding Tsuei, H. Hsieh, H. Lin, C. Chen, Liu Tjeng, Deepa Kasinathan We investigated the electronic structure of the vacancy-ordered 4d-transition metal monoxide NbO (Nb3O3) using angle-integrated soft- and hard-x-ray photoelectron spectroscopies as well as ultra-violet angle-resolved photoelectron spectroscopy. We found that density-functional-based band structure calculations can describe the spectral features accurately provided that self-interaction effects are taken into account. In the angle-resolved spectra we were able to identify the so-called vacancy band that characterizes the ordering of the vacancies. This together with the band structure results indicates the important role of the very large inter-Nb-4d hybridization for the formation of the ordered vacancies and the high thermal stability of the ordered structure of niobium monoxide. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K09.00008: First-principles study of ferroelectric charge-disproportionated phases in the cation-ordered La1/3Sr2/3FeO3 Se Young Park, Karin Rabe, Jeffrey Neaton We investigate the structure and electronic properties of A-site cation ordered La1/3Sr2/3FeO3 using density functional theory plus Hubbard U corrections (DFT+U). A structure with A-site cation ordering, consisting of (111) planes of La and Sr ions with uniform separation, is considered. We find that depending on the charge ordering patterns, both centrosymmetric and ferroelectric phases exhibit charge disproportionation, with energy difference less than 1 meV per formula unit. The polar phase is induced by the symmetry breaking from A-site cation ordering combined with charge ordering, giving rise to sizable polarization of about 19 μC/cm2 mainly originating from electron transfer between Fe ions. We identify a possible insulating switching path, suggesting that the polar state could be induced by applied electric field, and, depending on the switching process, a ferroelectric hysteresis loop could be observed. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K09.00009: High-mobility ferroelectrics from first principles Xiaohui Liu, Karin Rabe Free carriers can be added to ferroelectric materials through chemical substitution, modulation doping in heterostructures, or through excitation across the band gap, while still maintaining a polar structure. The combination of ferroelectricity and high mobility of added free carriers could extend the application of ferroelectrics in novel electronic devices. However, at room temperature, most perovskite ferroelectrics have low mobility, while the perovskites with high mobility, such as doped BaSnO3, are nonpolar. The challenge is to find ways to stabilize the polar ferroelectric phase in high-mobility systems without degrading the mobility. Here, we formulate design principles for experimentally realizable candidate ferroelectric systems in which the high mobility of the added carriers is promoted by the presence of an isolated low-m* band at the conduction band minimum. We report results for crystal structures, polarizations and band structures of selected systems including perovskite oxide compounds, superlattices and heterostructures, obtained using first-principles methods. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K09.00010: Enhancement of Piezoelectricity from Defect Dipoles in Transition Metal Doped Ferroelectrics Ronald Cohen, Shi Liu, Dhiren Pradham, Muhtar Ahart Using a combination of first-principles density functional theory computations and fundamental experiments we are studying transitional metal doped ferroelectrics. We have performed molecular dynamics simulations using first-principles based potentials, and affirm the importance of defect dipoles in the ageing effect. We assume that defect dipoles, consisting of transition metals coupled with oxygen vacancies, align in the poling direction. Greatly enhanced piezoelectric coupling is obtained as the polarization rotates away from the poling direction. The defect dipoles provide a restoring force for the polarization to rotate back on reduction of the applied field. We have studied this as a function of dopant concentration and find that less than 1\% dopants are required to obtain an appreciable enhancement in electromechanical coupling. We are now performing experiments on transition metal doped samples. S. Liu and R. E. Cohen, APL 111, 082903 (2017); |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K09.00011: Designing Lead-Free Antiferroelectrics for Energy Storage Bin Xu, Jorge Íñiguez, Laurent Bellaiche Dielectric capacitors, although presenting faster charging/discharging rates and better stability compared with supercapacitors or batteries, are limited in applications due to their low energy density. Antiferroelectric compounds, however, show great promise due to their atypical P-E curves. Here we report our first-principles-based theoretical predictions that Bi1-xRxFeO3 systems (R=Nd in this work) can potentially allow high energy densities (100-150 J cm-3) and efficiencies (80–88%) for electric fields that may be within the range of feasibility upon experimental advances (2-3 MV cm-1). Additionally, a simple model is derived to describe the energy density and efficiency of a general antiferroelectric material, providing a framework to assess the effect on the storage properties of variations in doping, electric field magnitude and direction, epitaxial strain, temperature, etc., which can facilitate future search of antiferroelectric materials for energy storage [1]. [1] Xu, Íñiguez, Bellaiche, Nat. Commun. 8, 15682 (2017) |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K09.00012: Quantum Spin Chain Dynamics in YbAlO3 Perovskite Andrei Podlesnyak, L.S. Wu, Z. Wang, W. Zhu, S.E. Nikitin,, Cristian Batista, G. Ehlers, Mark Lumsden, L. Vasylechko Here, we demonstrate experimentally and confirm by theoretical calculations that rare-earth perovskite YbAlO3 is an excellent experimental realization of an effective spin-1/2 quantum XXZ chain. Inelastic neutron scattering spectra taken on YbAlO3 provide evidence for unconventional low-energy magnetic excitations. The spectrum at temperatures above antiferromagnetic ordering TN=0.8K is characterized by a gapless continuum with dispersive boundaries in one direction and bandwidth of ΔE~0.6meV at the zone center. Cooling down through the TN, a different spectrum emerges. A gap is opened abruptly and the excitation spectrum is dominated by a high-intensity sharp mode. The observed excitations have negligible dispersion along the other directions, proving our conclusion that the Yb moments form weakly coupled spin chains despite the three-dimensional perovskite structure. We also argue that the series of rare-earth based perovskites could be an exceptional candidate for low dimensional S=1/2 quantum chain with tunable intrinsic parameters to explore quantum magnetism. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K09.00013: Near Room-Temperature Magnetic Ordering in Doped Multiferroic Hexagonal YbFeO3 Lin Lin, Shumin Wang, Shuhan Zheng, Kunlun Yang, Shuai Dong, Yongqiang Li, Zhi-Bo Yan, Junming Liu Hexagonal rare-earth ferrite RFeO3 family is a unique multiferroic material holding weak ferromagnetic moment, which is predicted to be coupled with trimerization. Until now, only the low temperature magnetic transitions at 120~170 K have been verified. We stabilize this structure in YbFeO3 with finely tuned Sc substitution level, and demonstrate a new near room-temperature magnetic ordering at Tc ~ 225 K in Yb0.42Sc0.58FeO3, accompanying the ferroelectric transition. The weak ferromagnetism is also unambiguously evidenced below Tc, which is originated from the magnetoelectric spin-lattice coupling. More importantly, three successive magnetic transitions at TR1 ~ 165 K, TR2 ~ 33 K and T* ~ 8.5 K have been observed, which may indicate complex interaction between the rare-earth and Fe ions magnetic moments. The present work pushes forward the progress to hunt for room-temperature multiferroics in hexagonal ferrites and related materials. |
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