Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Interfaces and Optical Properties |
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Sponsoring Units: DMP DCOMP Chair: Venkatraman Gopalan, Pennsylvania State University Room: C143/149 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q33.00001: Engineering exotic phenomena at oxide interfaces Invited Speaker: Complex transition metal oxides form an important class of compounds, exhibiting a wide variety of functional properties exploited in many applications. Thanks to advances in deposition techniques, these oxides can also nowadays be combined in heterostructures, with a structural quality comparable to what is achieved for conventional semiconductors, and the appearance of new phenomena at the interfaces where oxides with different properties meet brought recently the field to an entirely new level. Such phenomena include, for instance, the appearance of a two- dimensional electron gas at the interface between insulator oxides, the possibility of unexpected coupling between structural instabilities at some interfaces yielding unusual functional properties or the tunneling through ferroelectric and multiferroic barriers. Concentrating on few selected examples, I will illustrate how first-principles calculations can efficiently help the experimentalists to characterize the interfaces between complex transition metal oxides and sometimes guide them toward the design of new interesting heterostructures with exotic properties. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q33.00002: Harnessing competition in artificially layered ferroelectric superlattices to engineer enhanced piezoelectrics Matthew Dawber, Benedikt Ziegler, Sara Callori, John Sinsheimer, Valentino Cooper, Tahir Yusufaly, Karin M. Rabe, Premala Chandra First principles calculations by Cooper and Rabe (V. R. Cooper and K.M. Rabe, Phys. Rev. B 79, 180101 (R) (2009)), predicted that in PbTiO$_{3}$/BaTiO$_{3}$ superlattices an enhancement of the d$_{33}$ piezoelectric coefficient could be achieved at a particular ratio of the thickness of the constituent layers. We have fabricated high quality artificially layered PbTiO$_{3}$/BaTiO$_{3}$ superlattices on SrTiO$_{3}$ substrates (with SrRuO$_{3}$ bottom electrodes) using an off-axis RF magnetron sputtering technique, allowing us to perform x-ray diffraction, electrical measurements and atomic force microscopy on this system. The experimental results confirm the prediction from first principles calculations, and we apply a Landau theory model as a useful bridge between the first principles predictions and experimental results at elevated temperature. In this work we have demonstrated that by finely balancing competing material properties in artificial heterostructures, desirable properties that exceed those of the parent compounds can be achieved. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q33.00003: Ferroelectrical and Dielectric Properties of BaTiO$_{3}$/Ba$_{(1-x)}$Sr$_{x}$TiO$_{3}$ Superlattices Nora Ortega, Ashok Kumar, Ram S. Katiyar Artificially designed superlattices (SL) composed of alternate layers of BaTiO$_{3}$ (BT) and SrTiO$_{3}$ (ST) have attracted interests due to the possibility of producing superior and new functional properties, which are attractive for device applications. We have fabricated SL of BT/Ba$_{(1-x)}$Sr$_{x}$TiO$_{3}$ (BST) with x = 0, 0.3, 0.4, 0.5, 0.6, 0.7, 1, utilizing multi-target by pulsed laser deposition technique. The modulation period ($\Lambda )$ in all SL was $\Lambda $= 80 {\AA} and the total thickness of each SL films were 600 nm. The x-ray diffraction revealed well oriented (00l) perovskite structure and the so-called satellite peaks. The polarized Raman spectra showed the substantial transformation of the ferroelectric E(1TO) soft mode, depending of the ratio of Ba/Sr in BST layer. The dielectric constant of SL showed linear frequency dispersion above of 20 kHz, and their values are in the range of 400 to 900 at 1 kHz, while the tangent loss values were below to 0.1 at 1 kHz. Well defined ferroelectric loop was observed in all the SL at different frequencies (1 kHz-10 kHz), with remanent polarization (2P$_{r})$ 10 $\mu $C/cm$^{2}$. Improvement in saturation in the ferroelectric loop was observed with increase of Ba composition in BST layer. All these superlattices show very low leakage current far above its coercive field. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q33.00004: Artificially layered PbTiO$_{3}$/CaTiO$_{3}$ superlattices John Sinsheimer, Youcef Benkara, Jonathan Daley, Sara Callori, Matthew Dawber It has been shown that in artificially layered PbTiO$_{3}$/SrTiO$_{3}$ superlattices, a form of improper ferroelectricity occurs where the rotations of the oxygen octahedra at the interfaces couple with the polar mode and increase the ferroelectric polarization of the material when the layers are very thin. PbTiO$_{3}$/CaTiO$_{3}$ superlattices grown on SrTiO$_{3}$ substrates are also highly likely to display this kind of behavior, as the CaTiO$_{3}$ ground state is dominated by rotational distortions. This system should also play host to a competition between in-plane ferroelectricity (as CaTiO$_{3}$ is subjected to a large tensile strain when grown on SrTiO$_{3}$) and out-of-plane ferroelectricity (the usual result when in PbTiO$_{3}$ is grown on SrTiO$_{3}$). Using off-axis RF magnetron sputtering, we have produced high quality superlattices of PbTiO$_{3}$/CaTiO$_{3}$ with various layer thicknesses on SrTiO$_{3}$ substrates with SrRuO$_{3}$ bottom electrodes. The samples were analyzed using x-ray diffraction, electrical measurements, and atomic force microscopy. Our experimental results reveal a fascinating transition region at certain ratios of the relative layer thicknesses. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q33.00005: Polarization switching and dielectric properties of ferroelectric bilayers Mandana Meisami Azad, Daniel Tinberg, Donald Walko, Susan Trolier-Mckinstry, Alexei Grigoriev In this work, we analyze polarization switching and dielectric properties of ferroelectric multilayer thin films of lead zirconate titanate. The interlayer coupling and polarization dynamics of ferroelectric multilayers are largely unknown. The studies of multilayers present a significant interest due to both fundamental understanding of interlayer interactions and practical applications of ferroelectrics in nanoelectronics and nanoelectromechanical systems. It is predicted that unusual switching characteristics and domain configurations such as the antiparallel alignment of the spontaneous polarization in adjacent layers can be observed in these materials. Using electrical measurements and time-resolved x-ray microdiffraction we analyzed physical properties of $PbZr_{0.8} Ti_{0.2}O_{3}$/$PbZr_{0.6}Ti_{0.4}O_{3}$. Strong nonlinearities in piezoelectric and dielectric responses of the bilayer to applied electric fields, which were observed in our experiments, can be explained by unusual polarization domain dynamics and interface charging effects. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q33.00006: Metallic oxides as dielectrics in artificially layered ferroelectric superlattices Sara Callori, Dong Su, John Sinsheimer, Matthew Dawber Artificially structured oxides provide many opportunities to develop systems with novel and tunable properties. SrRuO$_3$ has a metal-insulator transition as a function of thickness, which suggested to us the idea that we could use extremely thin layers (less than 3 unit cells) of SrRuO$_{3}$ as a novel dielectric component within an artificially layered superlattice system. We have created high quality PbTiO$_3$/SrRuO$_3$ superlattices by using an off-axis RF magnetron sputtering technique. The samples were characterized by x-ray diffraction, atomic force microscopy, transmission electron microscopy, and electrical measurements. When the PbTiO$_3$ layers are above a certain critical thickness, significant out-of-plane ferroelectricity develops in the system and the overall material has a semiconducting character. In this talk we will present a detailed experimental investigation of the behavior of ferroelectric polarization and domain size as the relative thicknesses of the superlattice layers are varied. Our work serves as a demonstration that a new set of materials, metallic oxides, can be considered for inclusion as novel dielectric layers in ferroelectric superlattices. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q33.00007: Stabilizing ferroelectric polarization of ultrathin BaTiO3 films through interface engineering Xiaohui Liu, Yong Wang, Pavel Lukashev, J.D. Burton, Evgeny Tsymbal Ferroelectric tunnel junctions have recently attracted considerable interest due to their potential for device applications [1]. The main challenge for the implementation of these devices is to stabilize ferroelectricity in nanometer- thick films where depolarizing fields and interface effects play an important role. Here, we report results of first- principles calculations of ferroelectric polarization in epitaxial SrRuO3/BaTiO3/SrRuO3 junctions. We show that the ferroelectric polarization is very sensitive to the surface termination of the electrodes and film thickness. In particular, we find that the presence of RuO2/BaO interface is detrimental to ferroelectricity due to the pinning of polar displacements in BaTiO3 in the direction away from the interface making the polarization of ultra-thin films non- switchable. We find that ferroelectricity can be stabilized by adding a thin layer of SrTiO3 at this interface. A phenomenological model is developed to explain the correlation between ferroelectric properties and junction geometry. \\[4pt] [1] E.Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q33.00008: Inverse layer capacitance in perovskite oxide superlattices Xifan Wu, Massimiliano Stengel, David Vanderbilt Ferroelectricity is one of the most important functionalities that can be tuned in perovskite oxide superlattices. At fixed displacement field $D$, the overall polar instability can be accessed by the inverse of the capacitance per basal area as $C^{-1}=\partial V / \partial D$, where $V$ is the potential drop across the supercell.\footnote{M. Stengel, D. Vanderbilt, and N.A. Spaldin, Nature Mater. {\bf 8}, 392 (2009).} Here we propose that $C^{-1}$ can be further rigorously decomposed into contributions from individual AO or BO$_2$ layers, giving an {\it layer inverse capacitance} defined as $c_j^{-1}=\epsilon_0^{-1}(h_j+D\partial h_j/\partial D - \partial p_j/\partial D)$, where $h_j$ and $p_j$ are the layer height and Wannier-based layer polarization\footnote{X. Wu, O. Di\'{e}huez, K.M. Rabe and D. Vanderbilt, Phys. Rev. Lett. {\bf 97}, 107602 (2006).} of layer $j$, respectively. We compute the $c_j^{-1}$ in several typical multicomponent perovskite superlattices such as CaTiO$_3$/BaTiO$_3$ and PbTiO$_3$/SrTiO$_3$, and demonstrate that they satisfy a {\it locality} principle: their behavior depends mainly on the local chemical environment (i.e., the identities of neighboring layers). Thus, we show that the $c_j^{-1}$ can provide an insightful {\it local} analysis of the ferroelectric tendency at interfaces in functional oxide superlattices. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q33.00009: Light-Driven Ferroelectric Polarization Dynamics Probed with Time-Resolved X-ray Scattering D. Daranciang, M. Highland, H. Wen, N. Brandt, H. Hwang, J. Larsson, K. Sokolowski-Tinten, D. Reis, K. Nelson, P. Fuoss, G.B. Stephenson, A.M. Lindenberg We report femtosecond resolution time-resolved x-ray scattering measurements of dynamical changes in the polarization of PbTiO3 (PTO) nanolayers on SrTiO3 (STO) and DyScO3 (DSO) substrates under 400 nm, 40 fs optical excitation. For PTO on STO, an optically-induced polarization enhancement occurs on picosecond timescales that can be associated with a carrier-induced screening of the depolarization field. For PTO on DSO, qualitatively different effects are observed, indicating that the light initially couples to c-domains. We also observe optically-driven ferroelectric to paraelectric phase transitions (and vice versa) near the Curie temperature. The optical response of PTO on STO in the monodomain phase is consistent with a bulk photovoltaic effect. Optical excitation in the stripe phase at 515 C drives strains of order 1 percent, with an associated non-thermal disordering of the stripe domains. For PTO on DSO, temperature-dependent in-plane and out-of-plane structural dynamics are simultaneously captured, allowing the complex coupling between a- and c-domain motions to be mapped out. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q33.00010: Optical Properties of Epitaxial Sr-Ti-O Compounds from First Principles Robert Berger, Jeffrey Neaton SrTiO$_{3}$ is a representative of the property-rich perovskite family, and a material whose ability to convert solar photons to H$_{2}$ fuel would be more efficient if its wide optical bandgap (3.25 eV) better matched the solar spectrum. The Sr- and Ti-based Ruddlesden-Popper (RP) phases, Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$, are structural modifications of SrTiO$_{3}$ with potentially useful electronic properties. While bulk growth is limited to $n<4$ and $n=\infty$ (SrTiO$_{3}$), thin films of larger finite $n$ structures have been grown epitaxially. In optical experiments, bandgaps of these films decrease monotonically with increasing $n$.\footnote{C.-H. Lee et al., to be published.} In density functional theory (DFT), however, the $n=\infty$ gap is larger than those for finite $n>3$. This disagreement could stem from limitations in both experiment and theory. We explore this issue in depth using DFT, many-body perturbation theory, and tight-binding techniques. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q33.00011: Spectroscopic investigation on the electronic structure of a 5$d$ band insulator SrHfO$_{3}$ Yunsang Lee, Y.K. Seo, D.J. Lee, H.J. Noh We investigated the high-energy electronic structure of a 5d perovskite SrHfO$_{3}$. By using optical spectroscopy and O 1$s$ x-ray absorption spectroscopy, the values of electronic structure parameters are estimated properly. In particular, the crystal field splitting energy, which is closely associated with the $p-d$ hybridization strength, is as high as 5 eV, and the Sr 4$d$ bands appear to be strongly mixed with the Hf 5$d$ bands. Moreover, the emission spectra with a 325 nm light excitation exhibit a sizable strength near 500 nm at low temperatures due to oxygen defects. These findings in SrHfO$_{3}$ are compared with electronic properties of similar compounds, 3$d$ SrTiO$_{3}$ and 4$d$ SrZrO$_{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q33.00012: Precise measurements of index of refraction at Brewster angle Wei-Tai Hsu, Christian Bahrim A simple and accurate method is proposed for finding the index of refraction of solid and liquid dielectrics using the polarization of light reflected by a dielectric surface near the Brewster angle. The method allows measuring the Brewster angle with a precision better than 0.01 degrees and the index of refraction with a precision of 0.0001 by running a parabolic fit of the parallel component of the reflectance normalized to the total reflectance in a narrow region of about 15 degrees around the Brewster angle [1]. Our measurement is about 100 times better than other existing methods. The best precision in our measurements is achieved when a computer-based filtering procedure of the experimental reflectance is used during the data acquisition [2]. Our apparatus allows measuring small variation of the index of refraction, such as due to the change in temperature or the interference with another E-field.\\[4pt] [1] Bahrim C and Hsu Wei-Tai, 2009 \textit{Am. J. Phys.} \textbf{77} (4) 337-343; [2] Hsu Wei-Tai and Bahrim C, 2009 \textit{Eur. J. Phys.} \textbf{30} 1325-1336. [Preview Abstract] |
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