Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q34: Focus Session: Interfaces in Complex Oxides - Polar Interfaces |
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Sponsoring Units: DMP Chair: Evgeny Tsybal, University of Nebraska Room: C141 |
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q34.00001: Effect of polar interfaces on thin-film ferroelectricity Yong Wang, Manish Niranjan, Karolina Janicka, Julian Velev, Mikhail Zhuravlev, Sitaram Jaswal, Evgeny Tsymbal Based on first-principles and model calculations we investigate the effect of polar interfaces on the ferroelectric stability of thin-film ferroelectrics [1]. We consider Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and SrRuO3/LaO/BaTiO3/LaO heterostructures as representative systems, where a LaO monolayer at the interface with a TiO2-terminated BaTiO3 produces a polar interface. The polar interfaces create an intrinsic electric field which produces electric polarization in BaTiO3 directed into the interior of the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces that is non-switchable and thus detrimental to ferroelectricity. The effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric field that penetrates deeper into the ferroelectric. The predicted mechanism for a ferroelectric dead layer at the interface controls the critical thickness for ferroelectricity in systems with polar interfaces. [1] Y. Wang et al., Phys. Rev. B. 82, 094114 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q34.00002: Lattice screening of the polar catastrophe in KNbO$_{3}$/BaTiO$_{3}$ interfaces Pablo Garcia-Fernandez, Pablo Aguado-Puente, Javier Junquera The discovery of a bidimensional electron gas (2DEG) in the interface between two insulating lattices like LaAlO$_3$ and SrTiO$_3$ has triggered much interest around bidimensional conductivity in these heterostructures. In this work we study polar interfaces between KNbO$_{3}$ (KNO) and BaTiO$_{3}$ (BTO), equivalent from the layer by layer charge point of view to the LAO/STO. In particular we focus on: (a) the possibility of formation of a 2DEG, (b) its interaction with the ferroelectric distortions of these materials, and (c) the effect of external electric fields. For this, we have performed Density Functional Theory calculations for a KNO($m$)/BTO(2$m$)/KNO($m$) slab ($m$= width in unit cells) with different kinds of interfaces ($n$ or $p$). We find that a 2DEG is formed only in the unrelaxed configuration, where there is no rumpling between the atoms of a given layer. However, when geometry is relaxed, KNbO$_{3}$ polarizes and the 2DEG is effectively screened. This effect is robust even under application of electric fields of moderate size. Finally, we find that an easily-rotated [110] in-plane polarization, driven by electrostatic effects, appears in the vicinity of the KO/TiO$_2$-type interface even thought the system is under in-plane compressive strain. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q34.00003: Evolution of the Band Alignment at Polar Oxide Interfaces J.D. Burton, Evgeny Tsymbal The next generation of electronic devices and systems are envisioned to exploit the multifunctional properties of complex oxide interfaces. Fundamental to this endeavor is an understanding of the electronic band alignment across such interfaces. Engineering this band alignment in all-oxide systems by properly preparing the interfaces is highly desirable. Here we explore an all-oxide metal-insulator interface between, SrTiO$_{3}$ (STO), and La$_{1-x}A_{x}$MnO$_{3}$ (LAMO), where $A$ is a divalent cation [1]. The doping level of the manganite, $x$, offers a parameter which can be varied to engineer the band alignment. We use first-principles density-functional calculations to determine the evolution of the band alignment at La$_{0.7}A_{0.3}$MnO$_{3}\vert $La$_{1-x}A_{x}$O$\vert $TiO$_{2}\vert $SrTiO$_{3}$(001) heterointerfaces as the interfacial composition, La$_{1-x}A_{x}$, is varied. The position of the valence band maximum (VBM) with respect to the Fermi level increases linearly with interfacial composition $x$ due to the linear dependence of the screened electrostatic interface dipole on the interfacial ionic charge. The importance of the polar nature of LAMO and its background dielectric properties will be discussed. Our results are agreement with recent experimental data reported by Hikita \textit{et al} [2].\\[0pt] [1] J. D. Burton and E. Y. Tsymbal, Phys. Rev. B 82, 161407 (2010).\\[0pt] [2] Y. Hikita\textit{ et al.}, Phys. Rev. B 79, 073101 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q34.00004: Polarization discontinuities and compensation mechanisms at oxide-oxide interfaces Massimiliano Stengel, David Vanderbilt Polar interfaces between insulating perovskite materials have been the subject of special attention in the past few years, following the discovery of two-dimensional conductivity in LaAlO$_3$/SrTiO$_3$. In this talk I will introduce the problem by using general concepts of macroscopic electrostatics, in the framework of the modern theory of polarization [1]. Based on these ideas, I will show how we can understand the origin and the spatial distribution of the metallic electron gas in terms of few basic ingredients, which can be readily extracted from bulk calculations of SrTiO$_3$. These results provide an unified view over the factors determining the confinement and decay of the compensating free charge within arbitrary electrical boundary conditions.\\[4pt] [1] M. Stengel and D. Vanderbilt, Phys. Rev. B 80, 241103(R) (2009). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q34.00005: First-principles study of the band offset at the anatase TiO$_{2}$/SrTiO$_{3}$ (001) interface Alexander Demkov, Hosung Seo, Chandrima Mitra Band offsets at the interface of two oxides have recently attracted considerable attention in the context of the high-k dielectric gate stack in field effect transistors. The problem is also important in oxide heteroepitaxy. Recently, Chambers et al. reported that using the x-ray photoelectron spectroscopy no valence band offset is found between anatase TiO$_{2}$ (001) and SrTiO$_{3}$ (001). In this talk we describe the electronic structure at the TiO$_{2}$/SrTiO$_{3}$ (001) interface. We calculate the valence band offset to be 0.94 eV in the Schottky limit and 0.76 eV when the oxides are brought in contact, in apparent contradiction with experiment. A careful analysis of the electronic structure evolution from the bulk region of SrTiO$_{3}$ to the interface and through anatase all the way to the surface allows us to clarify the experimental results. We compare local density approximation results to those obtained with the GW method. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q34.00006: Many-body effects on the capacitance of multilayers made from strongly correlated materials Simon Hale, Jim Freericks Recent work by Kopp and Mannhart on novel electronic systems formed at oxide interfaces has shown interesting effects on the capacitances of these devices. In our work, we identify effects on the capacitance that stem from many-body physics and from other factors. In order to do so, we employ inhomogeneous dynamical mean- field theory to calculate the capacitance for multilayered nanostructures. These multilayered nanostructures are composed of semi-infinite metallic leads coupled via a strongly correlated dielectric barrier region. The barrier region can be adjusted from a metallic regime to a Mott insulator through adjusting the interaction strength. We are able to vary the barrier thickness allowing comparison to the expected geometric capacitance. We also examine the effects of varying the temperature, potential difference, screening length, chemical potential, and electron filling. We set up a system that depletes the charge from the barrier so that the capacitance approaches the geometric capacitance, allowing us to study nonlinear effects. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q34.00007: Electronic Reconstruction at oxide interfaces: from an electron-hole bilayer to a spin-polarized 2DEG Invited Speaker: The origin of conductivity at the interface between the band insulators LaAlO$_{3}$ and SrTiO$_{3}$ has been subject of continued interest and debate. Density functional theory calculations can provide not only insight into the underlying mechanisms but also allow one to identify further parameters to tune the electronic reconstruction in this system in view of device applications. In particular, the potential build up induced by the polarity of the LaAlO$_{3}$ film is found to be counteracted by a strong lattice polarization in the LaAlO$_{3}$ film. The latter allows the system to remain insulating for the first several LaAlO$_{3}$ layers before a crossover to an electronic reconstruction takes place at around 4-5 monolayers (ML). We demonstrate that, owing to a dispersive surface state, an additional SrTiO$_{3}$ capping layer can trigger the insulator-to-metal transition already at two ML of LaAlO$_{3}$. As a result, two spatially separated sheets of carriers emerge: electrons at the interface and holes at the surface that are only 1 nm apart and can be used to study excitonic phenomena. Furthermore, we explore the effect of a metallic contact layer on top of the LaAlO$_{3}$ film which not only eliminates the potential build up but turns out to be a promising way to enhance the carrier concentration and possibly to realize a spin-polarized electron gas at the interface. Work in collaboration with W.E. Pickett, K. Otte, V. Ruiz L\'{o}pez. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q34.00008: Electronic properties of the conducting interface in LaAlO$_3$/SrTiO$_3$ heterostructures - the view from x-ray spectroscopies M. Sing, G. Berner, A. Muller, J. Walde, F. Pfaff, R. Claessen, H. Hollmark, L.-C. Duda, S. Paetel, C. Richter, J. Mannhart, S. Thiess, W. Drube, S.A. Pauli, C.W. Schneider, P.R. Willmott Novel phases with often unexpected electronic and magnetic properties may form at the interfaces of epitaxial heterostructures composed out of complex oxides. A case in point is LaAlO$_3$ (LAO) on TiO$_2$-terminated SrTiO$_3$ (STO), for which a conducting interface has been found if the LAO thickness exceeds 3 unit cells. Although there is growing evidence that the origin is intrinsic and involves a transfer of charge to the interface to compensate the electric potential due to the polar nature of LAO there are a number of open issues. These refer, e.g., to the LAO thickness dependence of the charge carrier concentration, the potential gradient and the role of defects in LAO, the band alignment and bending at the interface, the lateral mobility or confinement of the interface charge carriers etc. We address these questions from the viewpoint of high-energy spectroscopies, i.e. hard x-ray photoelectron spectroscopy and resonant inelastic soft x-ray scattering, both of which we applied for the first time to these materials. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q34.00009: Internal electric field in SrTiO$_3$/LaAlO$_3$ heterostructures probed with hard x-ray photoemission spectroscopy Erik Slooten, Z. Zhong, H. Molegraaf, S. de Jong, F. Massee, E. van Heumen, M. Gorgoi, G. Rijnders, D. Blank, M. Huijben, P. Kelly, M. S. Golden The origin of the conducting layer at the interface between insulating SrTiO$_3$ (STO) and LaAlO$_3$ (LAO) is still widely debated. The alternatingly charged layers within the LAO blocks give rise to an internal electric field, which at some point has to be screened. This built-in potential is predicted to close the LAO bandgap at a critical thickness of 4 layers of LAO. Using hard x-ray photoemission spectroscopy we study the core levels of these systems as a function of the LAO layer thickness. By measuring the La 4d and Al 2s core levels with respect to the Sr 3d core level we carefully determine the core level shifts for samples with 2 to 6 layers of LAO. Although the observed shifts are an order of magnitude smaller than predicted, we do find an interesting increase of the core level shifts for samples with more than 4 layers. We perform DFT slab calculations to show that oxygen vacancies can significantly reduce the potential build-up. Our results suggest that in real materials the electronic reconstruction is pre-empted by other effects of which oxygen vacancies are one possibility. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q34.00010: Interfacial Electronic Properties of LaAlO$_{3}$/SrTiO$_{3}$ superlattice Probed with Hard X-ray Photoemission Di-Jing Huang, Ying-Yi Chu, Jan-Chi Yang, Ku-Ding Tsuei, Wen-Chung Liu, Yen-Fa Liao, Ying-Hao Chu, Jonas Weinen, Stefano Agrestini, Valerio Oliana, Hao Tjeng, C.T. Huang Unexpected electronic and magnetic properties at interfaces between distinct transition-metal oxides have received much attention recently. The electronic phase at the interface of a heterostructure often differs from those of the sandwiching bulks. For instance, the interface between two band insulators LaAlO$_{3}$ and SrTiO$_{3}$ exhibits rich interfacial phases, drastically different from their original bulks. Here we will present measurements of hard X-ray photoemission and x-ray absorption on a LaAlO$_{3}$/SrTiO$_{3}$ superlattice to address the issue of interfacial electron at interfaces. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q34.00011: ARPES studies in LaTiO3/SrTiO3 heterostructures Young Jun Chang, Luca Moreschini, Yong Su Kim, Andrew L. Walter, Davide Innocenti, Aaron Bostwick, Geoffrey A. Gaines, Karsten Horn, Eli Rotenberg Electronic band structure of LaTiO3/SrTiO3 heterostructures was studied using angle resolved photoemission spectroscopy (ARPES). LaTiO3 films, grown by in situ pulsed laser deposition (PLD) on beamline 7.0.1 at Advanced Light Source, exhibit a thickness-dependent phase transition from the correlated metallic interface to Mott insulator. We observed the quasi-particle peak at the Fermi level explaining the metallic interface for ultrathin LaTiO3, and the band gap opened for thick LaTiO3 similar to the bulk. We compare the thickness dependent electronic structure with theoretically calculated phase diagram (S. Okamoto and A. J. Millis, Nature 428, 630 (2004)). [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q34.00012: Direct Spectroscopic Evidence of Charge Reversal at PZT/LSMO Heterointerface Chung-Lin Wu, Pei-Wei Lee, Yi-Chun Chen, Lo Yueh Chang, Chia-Hao Chen, Chen-Wei Liang, Pu Yu, Qing He, Ramamoorthy Ramesh, Ying-Hao Chu At the heterointerface of a top ferroelectric Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ (PZT) ultrathin film and a bottom La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) electrode, we used continuous synchrotron radiation photoelectron spectroscopy (SR-PES) to probe \textit{in situ} and demonstrated that the interfacial charges are reversible and their affected valence-band barrier height becomes modulated on switching the polarization in the top layer. By monitoring the core-level shifting of the buried LSMO layer under continuous illumination of synchrotron radiation, we directly observed a temporal screening of polarization induced by the photon-generated carriers in the top PZT layer. This dynamic characterization of the core-level shifting of the buried layer demonstrates an effective method to probe the electric conduction and ferroelectric polarization of an ultra-thin ferroelectric oxide thin film. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q34.00013: Scanning tunneling microscopy investigation of the electronic structure across LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces Ya-Ping Chiu, Bo-Chao Huang, Jan-Chi Yang, Ying-Hao Chu Atomically controlled polarity discontinuities induced unusual charge states have been found in the model interface between two insulating perovskite oxides, LaAlO$_{3}$ and SrTiO$_{3}$. In this work, by using cross-sectional scanning tunneling microscopy, local and direct evidence of nontrival local structural and electronic information across the heterointerfaces are investigated. A combination of scanning tunneling spectroscopy analysis with atomic resolution across the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface reveals how the oppositely charged atomic planes undergo electronic reconstructions and introduce a built-in electric field across the polar LaAlO$_{3}$ thin films grown on SrTiO$_{3}$ substrates. Further analysis of the related electronic and geometrical properties not only realizes the properties both in bands of electron and hole characters across the LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces but also helps to elucidate the mechanism of the interface conductivity. [Preview Abstract] |
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