Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q38: Focus Session: Ferroelectric Films and Finite Size Effects |
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Sponsoring Units: DCMP Chair: Max Stengel, University of California, Santa Barbara Room: Morial Convention Center 230 |
Wednesday, March 12, 2008 11:15AM - 11:51AM |
Q38.00001: Imaging ferroelectric polarization by electron holography Invited Speaker: Understanding solids means analysis of the arrangement of the different atoms, e.g. at interfaces, and the intrinsic electric and magnetic fields, as well as the resulting charge distribution. This is particularly important for functional materials, such as semiconductors, ferroelectrics and ferromagnetics. There is a variety of tools answering these questions partially. In particular since correction of aberrations [1], Transmission Electron Microscopy (TEM) offers a lateral resolution below 0.1nm hence can locally analyze position and species of atoms e.g. at interfaces. The most severe drawback is that the phase of the electron wave is not accessible by conventional imaging methods, and therefore phase-modulating peculiarities of the object such as electric and magnetic fields are invisible. However, these are measurable by TEM-holography rendering both amplitude and phase distributions produced by the object. For an overview see e.g. [2]. The electron phase $\varphi $ is modulated by the electric potential $V(x,y,z)$ as $\varphi (x,y)=\sigma \int\limits_{object} {V(x,y,z)dz} $ with interaction constant $\sigma $. In ferroelectrics, the polarization $\vec {P}$ contributes with a phase shift $\varphi _{pol} (\vec {r})=\frac{\sigma }{\varepsilon _0 }\int\limits_{object} {\left[ {\int\limits_{\vec {r}0}^{\vec {r}} {\vec {P}(x,y,z)} d\vec {r}} \right]dz} $ with respect to a reference point $\vec {r}0$, chosen in field-free space; $\vec {r}=(x,y)$ is the coordinate perpendicular to $z$-direction. Therefore, the projected in-plane polarization $\vec {P}_{proj} (\vec {r})=\int\limits_{object} {\left[ {\int\limits_{\vec {r}0}^{\vec {r}} {\vec {P}(x,y,z)} d\vec {r}} \right]dz} $ would be determined from a phase image. However, the polarization is partly compensated by \textit{compensating charges }at surfaces, interfaces and domain boundaries, which contribute with a corresponding potential distribution. The net effect is found in the phase image reconstructed from an electron hologram. Meanwhile, specific ferroelectric effects are found on micrometer and nanometer, even at atomic dimensions. [1] M. Haider et al., Nature 392 (1998) 768. [2] H. Lichte et al., Ann. Rev. Materials Research 37 (2007), 539 [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:03PM |
Q38.00002: Intrinsic Bipolar Carrier (e$^{-}$/h$^{+}$) Layer on Clean Surface of Insulating BaTiO$_{3}$ Y. Watanabe, D. Matsumoto, Y. Urakami, S. Kaku \textbf{We show experimentally the electron/hole layer on the clean surface of insulating pure BaTiO}$_{3}$\textbf{ single crystals in ultra high vacuum, which is the first report of the }\textbf{\textit{bipolar}}\textbf{ surface carrier layer by the field effect on any oxide to our knowledge. The experiments here demonstrate that the surface carrier layer is not due to impurities or defects but is intrinsic due to the spontaneous polarization }\textbf{\textit{P}}$_{S}$\textbf{.} The surface carrier layer induced by $P_{S}$ was found to exist at least to 80K on atomically ordered surface of insulating BaTiO$_{3}^{\ast }$, which has become the basis of a recent new memory device. However, much skepticism about its intrinsic nature is unresolved. We report here the disappearance of the carrier layer above Curie temperature, its disappearance by the exposure to a low vacuum, which indicates its location as the top surface, and, both electron/hole carrier layers by $P_{S}$. Here, the control of the electron and hole surface layer is only through the direction of poling. The realization of the hole layer is clear evidence that oxygen vacancies are inessential to the formation of the carrier layer, because oxygen vacancies kill the hole carrier layers. The present results have various implications for the basis of ferroelectric electrics such as nano-ferroelectrics, field effects and domains. $^{\ast }$Watanabe et al., \textit{Phys}.\textit{ Rev}. \textit{Lett.} 86, 332 (2001). [Preview Abstract] |
Wednesday, March 12, 2008 12:03PM - 12:15PM |
Q38.00003: Contrast in ultrathin film ferroelectric behavior between air and vacuum environments A. P. Baddorf, P. Maksymovych, S. V. Kalinin, R. Ramesh At nanoscale dimensions, ferroelectric properties of oxide materials are dominated by depolarizing effects which depend strongly on the electrostatic screening at the interfaces and the environment. Oxide surfaces readily react with adsorbed molecules that may chemically or electronically alter the ferroelectric behavior. We have examined ultrathin (5-20 nm) BiFeO$_{3}$ thin films grown on SrTiO$_{3}$(110) using ultra-high vacuum Piezoresponse Force Microscopy. All the films reveal a characteristic behavior trend that in air the films are uniformly polarized, while multiple domains are observed in vacuum. The monodomain to polydomain transition occurs via either annealing in oxygen or simple evacuation, which suggests that molecules may be weakly adsorbed on the surface. Local ferroelectric switching behavior has also been compared for vacuum and ambient environments on the same substrate. Measurements were made at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. DOE. [Preview Abstract] |
Wednesday, March 12, 2008 12:15PM - 12:27PM |
Q38.00004: X-ray Studies of Chemical Switching of PbTiO$_{3}$ on SrRuO$_{3}$ Carol Thompson, R.-V. Wang, D.D. Fong, F. Jiang, S.K. Streiffer, P.H. Fuoss, J.A. Eastman, G.B. Stephenson Recent studies have shown that monodomain polarization can be stabilized in thin films not only through the presence of electrodes that provide electronic compensation at the film interfaces but also through surface-adsorbed ions or charged interfacial defects/impurities. Here, we use real-time synchrotron x-ray scattering to investigate changes in the polarization of PbTiO$_{3}$ films induced by varying the chemistry of the vapor above the film surface. We observe that the sign of the polarization can be reversibly switched by changing the partial pressure of oxygen ($p$O$_{2})$ in equilibrium with the film surface. The dependence of film lattice parameter on $p$O$_{2}$ is bistable, following a butterfly loop analogous to that observed under applied voltage. The large compressive strains observed in the thinnest films prior to switching indicate that the chemical switching process can result in electric fields approaching the intrinsic coercive field for PbTiO$_{3}$. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 12:39PM |
Q38.00005: First-principles prediction of switchable stoichiometry at interfaces Sergey V. Levchenko, Andrew M. Rappe We present a first-principles density functional theory (DFT) study of the relative thermodynamic stability of ferroelectric (FE) lithium niobate (LiNbO$_3$) (0001) surfaces of different stoichiometry. We predict that the equilibrium stoichiometries are different for the positively and negatively polarized LiNbO$_3$ surfaces under the same conditions. A correct way of calculating surface charges for ferroelectric materials with intrinsic polar stacking is developed. It is found that surface charge passivation by ions is thermodynamically favored over passivation by mobile carriers in a wide range of chemical potentials. [Preview Abstract] |
Wednesday, March 12, 2008 12:39PM - 12:51PM |
Q38.00006: Phase Field Modeling of Ferroelectric Thin Films with Space Charge Rajeev Ahluwalia, Nathaniel Ng, Haibin Su, Freddy Boey The time-dependent Ginzburg-Landau (TDGL) equations and phase field modeling have been used to describe various phenomena in ferroelectric materials, such as domain nucleation and evolution, and hysteresis. This work applies the TDGL model to explain the behavior of perovskite ferroelectric thin film with space charge. Results show that the presence of space charge at the surface significantly influences the switching process and domain structures in ferroelectric thin films. The role of space charge on size effects is also studied. [Preview Abstract] |
Wednesday, March 12, 2008 12:51PM - 1:03PM |
Q38.00007: One-dimensional polydomains in Ferroelectric thin films Matias Nunez, Marco Buongiorno Nardelli The local nature of the interface in metal/ferroelectric oxide junctions can drastically affect the polarization in the ferroelectric film. As the thickness of the film is reduced, the intensity of the depolarization field increases and the system will reduce its electrostatic energy in two ways, either by forming lateral 180$^{\circ}$ domains [1], or by reducing the ionic polarization while remaining in a monodomain state. Using DFT, maximally localized Wannier functions [2] and the layer polarization concept [3], we studied the ferroelectricity in thin layers of BaTiO$_{3}$ sandwiched between metal. Our results suggest that the structures associated with this spatial scale are more complex than previously thought, and we show how a pattern of the local polarization provides another way to minimize the internal energy below certain critical thickness. This pattern is characterized by the appearance of \textit{one-dimensional polydomains}, consecutive dipoles with opposed orientations in the direction perpendicular to the ferroelectric thin film/metal interface. \newline \newline [1] V.Nagarajan \textit{et al}, J. Appl. Phys. \textbf{100}, 051609 (2006). \newline [2] N. Marzari, and D. Vanderbilt, Phys. Rev. B \textbf{56}, 12847 (1997); A. Calzolari \textit{et al}., Phys. Rev. B \textbf{69}, 035108 (2004). \newline [3] X. Wu\textit{ et al}, Phys. Rev. Lett. \textbf{97}, 107602 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:15PM |
Q38.00008: Theory of Electric Polarization Induced by Inhomogeneity in Crystals Di Xiao, Junren Shi, Dennis Clougherty, Qian Niu We develop a general theory of electric polarization induced by inhomogeneity in crystals. We show that contributions to polarization can be classified in powers of the gradient of the order parameter. The zeroth order contribution reduces to the well-known result obtained by King-Smith and Vanderbilt for uniform systems. The first order contribution, when expressed in a two- point formula, takes the Chern-Simons 3-form of the vector potentials derived from the Bloch wave functions. Using the relation between polarization and charge density, we demonstrate our formula by studying charge fractionalization in a two-dimensional dimer model recently proposed. [Preview Abstract] |
Wednesday, March 12, 2008 1:15PM - 1:27PM |
Q38.00009: Ferroelectric Thin Films Under Inhomogeneous Electric Fields - Lateral Size and Thickness Dependence Nathaniel Ng, Rajeev Ahluwalia, Haibin Su, Freddy Boey Advances in nanoscale ferroelectric devices have led to interest in studying size effects in ultrathin films whose properties differ substantially from the bulk. In fact, thickness-dependent behavior has been well studied both theoretically and experimentally, but not much attention has been devoted to the role of lateral size. Our investigations indicate that fringing electric fields leads to switching via 90\r{ } domain wedge nucleation for films above a certain critical thickness which stabilize 180\r{ } domain walls, which may be of interest in PFM applications. Results also show a minimum lateral width whereby the film becomes virtually impossible to pole. [Preview Abstract] |
Wednesday, March 12, 2008 1:27PM - 1:39PM |
Q38.00010: Critical thickness for memory in thin ferroelectric films A.M. Bratkovsky, A.P. Levanyuk The finite screening length by real metallic electrodes, albeit very small ($<1\AA$), results in finite depolarizing field that tends to split the film into domains. We prove that, as a result, a critical thickness for ferroelectric memory is a few times larger than the critical thickness for ferroelectricity itself, especially in cubic perovskite films. For instance, the critical thickness of strained BaTiO3 films on SrRuO3/SrTiO3 substrate for onset of (meta)stable memory should be in excess of 4.5 nm compared with critical thickness of 1.6 nm for existence of ferroelectric domains. [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 1:51PM |
Q38.00011: Transport properties of epitaxial ferroelectric trilayer heterostructures with BaTiO$_{3}$ barriers D.A. Felker, H.W. Jang, C.B. Eom, M.S. Rzchowski We studied the influence of the polarization on the transport properties of epitaxial trilayer heterostructures with SrRuO$_{3}$ metallic oxide electrodes and ferroelectric BaTiO$_{3}$ barriers. The heterostructures were grown using atomic-layer controlled pulsed laser deposition, with barrier thickness ranging from 4 nm to 100 nm. We discuss the frequency and thickness dependence of the P-E loops, and both voltage-biased and current-biased current-voltage (I-V) characteristics. We find that the I-V curves of the thinnest barriers show a hysteresis due to the switching of the polarization in the ferroelectric barrier, leading to a change in the conductance of the junction. The reasons for the change in conductance were explored. We considered the shift in the potential barrier height for direct tunneling across the barrier, the role of defects, and possible structural changes in the junction. A ferroelectric junction with clear resistive switching is a candidate for nonvolatile memory applications. [Preview Abstract] |
Wednesday, March 12, 2008 1:51PM - 2:03PM |
Q38.00012: $^{17}$O Nuclear Magnetic Resonance Chemical Shielding Calculations of PZT Solid Solutions Daniel L. Pechkis, Eric J. Walter, Henry Krakauer First principles B3LYP calculations of $^{17}$O NMR in PbTiO$_3$, Pb(Zr$_\frac{1}{2}$Ti$_\frac{1}{2}$)O$_3$ (PZT), and PbZrO$_3$ will be presented. These systems were modeled with finite size quantum clusters embedded in point charge arrays. The embedding reproduces the Ewald Coulomb potential to better simulate the crystal environment.\footnote{M. K. Klintenberg, S. E. Derenzo, and M. J. Weber. Comput. Phys. Commun.{\bf 131}, 120, (2000).} For polar systems, the calculations were performed in the presence of an external electric field to cancel surface depolarization effects.\footnote{S. Li and K. Rabe. APS March Meeting abstract (2007).} PZT was modeled using three chemically ordered structures: P4mm, P2mm, and R3m. Two groupings of $^{17}$O isotropic chemical shifts $\delta_{iso}$ are seen in all of our PZT calculations with [001] ordering. One is at $\delta_{iso}$ $\approx$ 400ppm and the other is at $\delta_{iso} \approx$ 650ppm. We relate these to variations in the Ti-O and Zr-O bond lengths and use this to interpret recent experimental measurements.\footnote{ A. Baldwin, P.A. Thomas and R. Dupree, J. Phys: Cond. Matt. {\bf 17}, 7159 (2005).} [Preview Abstract] |
Wednesday, March 12, 2008 2:03PM - 2:15PM |
Q38.00013: First-principles calculation of phonon dispersions for Ba$_{1-x}$Ca$_{x}$TiO$_{3}$ Taichi Kosugi, Shinji Tsuneyuki Amongst perovskite oxides, BaTiO$_{3}$(BTO) is widely used for its distinct dielectric, piezoelectric and optical properties. Its lattice dynamics, associated with atomic displacements, have been intensively studied both theoretically and experimentally, since is essential for the dielectric instability of this material. Recently Ca-doped BTO(BCTO) single crystal was synthesized by Fu et al. and found to have exotic natures. Using the direct method proposed by Parlinski, we calculated the phonon dispersions of BCTO in a fully \textit{ab initio} manner, in which the force constants, the Born effective charges and the dielectric tensors are determined from first-principles. [Preview Abstract] |
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