Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z49: Focus Session: Ferroelectric, Multiferroic and Polar Oxide Heterostructures |
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Sponsoring Units: DMP Chair: Qi Li, Pennsylvania State University Room: Mile High Ballroom 1C |
Friday, March 7, 2014 11:15AM - 11:51AM |
Z49.00001: Interface enhanced functionalities in BaTiO$_3$/CaTiO$_3$ superlattices Invited Speaker: Xifan Wu Interface engineering of oxide thin films has led to the development of many intriguing physical properties and new functionalities, in which the oxygen rotation and tilting take an crucial role. The oxygen octahedral tilt has been considered to be a coherent motion in the oxide thin-films, based on which the tilt is often neglected in the modeling of ABO$_3$ superlattices. However, combined with state-of-art experimental high-resolution electron microscopic image, our first-principles results clearly show that oxygen octahedral tilt should be more appropriately defined by the tilting angles of two individual pyramids. Each pyramid will tilt rather independently as a function of its local chemical environment. Considering the oxygen octahedral rotation at the same time, the new picture of oxygen octahedral tilting will induce a novel interface effect, in which an unstable structure in bulk CaTiO$_3$ will be stabilized at the interface in BaTiO$_3$/CaTiO$_3$ superlattice. This novel interface effect induces large polarizations both in-plane and out-of-plane with a corresponding enhanced piezoelectricity. The above scenario successfully explains the recent experimental discoveries in BaTiO$_3$/CaTiO$_3$ superlattices by H. Lee's and P. Evan's groups respectively. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z49.00002: First-principles modeling of piezoelectric response of perovskite superlattices: the case of BaTiO$_3$/CaTiO$_3$ Qibin Zhou, Karin Rabe In multicomponent ABO$_3$ superlattices, instabilities belonging to individual bulk constituents strongly interact with each other through the interfaces. Such interactions in superlattices lead to rich behavior beyond that of simple perovskites, and in particular can lead to enhanced piezoelectric response.* In this work, we studied short-period BaTiO$_3$/CaTiO$_3$ superlattices with varying layer thicknesses and overall composition. Our first-principles calculations reveal a phase transition between ferroelectric and dielectric phases at a BaTiO$_3$ fraction close to 50\% and enhanced piezoelectricity in the ferroelectric phase. A first-principles-based model, extending a previous analysis for PbTiO$_3$/BaTiO$_3$ superlattices, is constructed to predict the phase transition, the polarization and tetragonality, and the enhanced piezoelectricity. The further extension of this modeling approach to a wider range of perovskite superlattices will be discussed. \\[0.3in] *V. R. Cooper and K. M. Rabe, Phys. Rev. B 79, 180101 (2009). [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z49.00003: Temperature-composition phase diagram of PbTiO$_{3}$/CaTiO$_{3}$ superlattices Matthew Dawber, Benjamin Bein, John Sinsheimer, Sara J. Callori, Hsiang-Chun Hsing, Mohammed Humed Yusuf, Huma Yusuf An experimental enhancement of the piezoelectric response and dielectric constant can be achieved in artificially layered epitaxial PbTiO$_{3}$/CaTiO$_{3}$ superlattices through an engineered rotation of the polarization direction. As the relative layer thicknesses within the superlattice are changed from sample to sample, evidence for polarization rotation is found in multiple x-ray diffraction measurements and associated measurements of functional properties. Here we report on synchrotron x-ray diffraction measurements performed at X22C at the National Synchrotron Light Source at Brookhaven National Laboratory and the MS SD beam line at the Swiss Light Source at the Paul Scherrer Institute. Through these measurements we studied the rotation of the ferroelectric polarization direction as a function of both composition and temperature. This work provides significant insight into the polarization rotation mechanism in general, and illuminates routes for exploiting it in artificially layered structures to produce enhanced piezoelectric materials. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z49.00004: In-situ x-ray diffraction studies of the epitaxial growth of BaTiO$_{3}$/SrTiO$_{3}$ superlattices Benjamin Bein, John Sinsheimer, Sara Callori, Hsiang-Chun Hsing, Mohammed Humed Yusuf, Priya Chinta, Randall Headrick, Matthew Dawber The growth of BaTiO$_{3}$/SrTiO$_{3}$ superlattices on SrTiO$_{3}$ substrates using off axis RF magnetron sputtering was monitored by in-situ x-ray diffraction at X21 at the National Synchrotron Light Source at Brookhaven National Laboratory. The (00$\frac{1}{2}$) surface reflection was used to measure the growth rates of BaTiO$_{3}$ and SrTiO$_{3}$. By rocking the sample in front of an area detector, reciprocal space maps around the (001) and (101) peaks can be rapidly acquired during the growth of the superlattice. This allows the evolution of the materials lattice parameters and the superlattice structure to be continually monitored during the growth of these structures. An interesting observation is that despite the elevated deposition temperature, ferroelectric stripe domains appear, and the evolution of these with superlattice thickness was also monitored during the growth process. The eventual relaxation of the superlattices above a critical thickness was also monitored and can be compared to post-deposition atomic force microscopy measurements of film morphology. These studies provide insight into the evolution of ferroelectric properties during the growth of highly strained epitaxial ferroelectric heterostructures. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z49.00005: Direct Observation of Film Polarization and Oxygen Vacancies at the BaTiO$_{3}$/SrTiO$_{3}$/GaAs Interface Qiao Qiao, Rocio Contreras-Guerrero, Ravi Droopad, Stephen Pennycook, Sokrates Pantelides, Serdar Ogut, Robert Klie We report successful growth of BaTiO$_{3}$ thin films on GaAs (001) with a SrTiO$_{3}$ buffer layer using oxide molecular beam epitaxy (MBE), and investigate the oxide/semiconductor interface using atomic-resolution imaging, electron energy loss spectroscopy (EELS) and first principles density functional theory (DFT). Atomic-resolution Z-contrast and annular bright field (ABF) images of BaTiO$_{3}$/SrTiO$_{3}$/GaAs reveal atomically sharp interfaces and show no sign of interfacial diffusion or extensive sensitivity to the electron beam. ABF images also show that the first SrO monolayer in contact with the GaAs substrate is highly oxygen deficient, and the SrTiO$_{3}$ buffer layer has an out of plane polarization due to the presence of oxygen vacancies, which can be directly observed by the displacement between the Ti and O columns. The Ti $L_{2,3}$ and O $K$ edge spectra from the SrTiO$_{3}$/GaAs interfacial Ti columns indicate the presence of oxygen vacancies and a distortion of the TiO$_{6}$ octahedra. DFT calculations show that O vacancies form preferentially at the SrTiO$_{3}$/GaAs interface, where they polarize the SrTiO$_{3}$, and, in turn, inhibit the ferroelectric switching in the BaTiO$_{3}$. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z49.00006: Structural characterization of PbTiO$_{3}$/SrTiO$_{3}$ superlattices under an applied external field Stephanie Fernandez-Pena, Pavlo Zubko, Celine Lichtensteiger, Jean-Marc Triscone Understanding ferroelectricity in ultrathin films is important both from the fundamental as well as the technological points of view. At these thicknesses, electrostatics plays a key role and often leads to the formation of 180 degree domains that form in order to minimize the depolarization field and subsequently dominate the functional properties of ultrathin ferroelectrics. Superlattice structures combining ferroelectrics and dielectrics, where such domains form a regular pattern that can be probed using X-ray diffraction, are an ideal system for studying ferroelectric nanodomains and their response to applied electric fields (P.Zubko, et al., PRL104, 2010) . Reciprocal space maps reveal domain satellites up to third order around the main superlattice peak and their evolution under applied fields is followed from RT to 30 K. Our detailed study maps the domain evolution as well as the piezoelectric response of superlattices with different PbTiO$_{3}$/SrTiO$_{3}$ periodicities. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z49.00007: Spatial Variation and Temporal Fluctuation of Domains at Equilibrium in a PbTiO3/SrTiO3 Superlattice Qingteng Zhang, Pice Chen, Margaret Cosgriff, Mohammed Yusuf, Zhonghou Cai, Ross Harder, Sara Callori, Matthew Dawber, Paul Evans The spontaneous polarization of ferroelectric thin films often forms a periodic domain pattern in order to minimize the electrostatic energy. The coherent x-ray scattering pattern of serpentine striped domains in a PbTiO3/SrTiO3 superlattice exhibits a series of intensity speckles arising from the nanoscale disorder of the domain pattern. The detailed variation of the domain pattern in space and time can be measured by observing the decorrelation of the speckles in a series of measurements at varying positions and times. We show here that the serpentine domains do not show repetition of spatial patterns on the order of approximately 1000 domain periods. The temporal fluctuation of the domains is fit by a model that describes similar slow dynamics in jammed soft matter systems. Change of domain structures is observed after repeated excitation by short-duration electric-field pulses. The decorrelation is incomplete following electric pulses that are large enough induce a transition to a uniform polarization state, indicating that the domain pattern regenerated after each pulse is at least partially determined by heterogeneity in the superlattice structure. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z49.00008: Dielectric Properties of Artificially Layered Ferroelectrics Pavlo Zubko, Stephanie Fernandez-Pena, Celine Lichtensteiger, Jean-Marc Triscone Over the past decade, superlattices composed of ferroelectric and paraelectric oxides have received a great deal of attention due to the rich physics arising from the complex electrostatic interactions in these materials, as well as the discovery of novel interface phenomena. The strong depolarizing fields induced by the presence of the paraelectric layers, lead to the formation of a stable structure of regular nanoscale domains, which dominate the functional properties of these artificially layered materials. The dielectric properties of PbTiO3-SrTiO3 superlattices were studied using impedance spectroscopy over a broad range of temperatures. A giant enhancement of the dielectric permittivity due to reversible motion of nanoscale domains was observed and domain-wall relaxation dynamics were studied as a function of ferroelectric and paraelectric layer thickness, temperature, and magnitude and frequency of the applied field. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z49.00009: Studies of local polarization in complex oxide multiferroic interfaces by aberration corrected STEM-EELS Gabriel Sanchez-Santolino, Javier Tornos, Carlos Leon, Mar\'Ia Varela, Stephen J. Pennycook, Jacobo Santamar\'Ia Interfaces in complex oxide heterostructures are responsible for exciting new physics, which is directly related to the chemical, structural and electronic properties at the atomic scale. Here, we study artificial multiferroic heterostructures combining ferromagnetic \textit{La}$_{0.7}$\textit{Sr}$_{0.3}$\textit{MnO}$_{3}$ with ferroelectric \textit{BaTiO}$_{3}$ by atomic resolution aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy. Measurements of the atomic positions in the STEM images permit calculating relative displacements and hence, local polarization. Polarization gradients can be observed in annular bright field images which seem to be correlated to strain gradients associated with the large lattice mismatch between barriers and electrodes. Spectroscopic measurements suggest the presence of O vacancies through the ferroelectric layers. Understanding the effect of the charge carriers associated with the oxygen vacancies may be the key to control the dynamics of domain walls in these heterostructures. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z49.00010: Charge Control of Interface Magnetization at Oxide Heterointerface G. Luepke, X. Ma, H. Zhai, F. Fang, A. Kumar, R.S. Katiyar, S. Dussan, H.B. Zhao, J.F. Scott The complex oxide heterointerface is key to the development of emerging multiferroic and spintronic technologies with new functionality. Even so, direct characterization of the interfacial spin state is missing, which prevents further interpretation of the coupling between spin and other ordering parameters at such oxide heterointerfaces, and impedes the development of future interface-based devices. Here we use the interface-specific Magnetization-induced Second-Harmonic Generation (MSHG) technique to investigate the interfacial magnetic state of the multiferroic (MF) heterostructure PbZr0.52Ti0.48O3 / La0.67Sr0.33MnO3 (PZT/LSMO) and its dependence on the charge state. We observe a gradual transition from ferromagnetic (FM) to canted anti-ferromagnetic (AFM) phase in the first unit cell layer at the heterointerface with increasing hole doping. Moreover, the exchange coupling between interface and bulk is weak, independently of the carrier filling. Our results provide new insight into the interface spin system of MF heterostructures, and have implications for developing electric field control of spin switches and magnetic tunneling junctions. [Preview Abstract] |
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