Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session X30: Structural and Functional Imaging of Oxide InterfacesFocus
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Sponsoring Units: DMP Chair: Ying-Hao Chu, National Chaio Tung University Room: 329 |
Friday, March 18, 2016 8:00AM - 8:36AM |
X30.00001: A Fresh Twist on The Electron Microscope: Probing Broken Symmetries at a New Level Invited Speaker: Juan Carlos Idrobo The introduction of aberration-correction in scanning transmission electron microscopy (STEM) has allowed the realization of Richard Feynman’s long sought dream, atom-by-atom structural and elemental identification of materials by simply looking “at the thing.” Until now, the goal of aberration-correction in STEM has been to produce the smallest possible electron probes, which essentially corresponds to a near constant phase across the probe. Phases increase the size of electron probes and result in images and spectra with a lower spatial resolution. In this talk, calculations will be presented showing that aberrations in lenses are intrinsic generators of angular momentum, and that phases introduced in atomic-size electron probes can actually be beneficial when studying the symmetry of materials. In particular, examples of mapping magnetic ordering of materials with atomic size electron probes will be shown. Magnetic dichroism is one of the new frontiers where aberration-correction STEM can have a significant impact, and reveal information that is physically out of reach in X-ray and neutron synchrotrons. Current and future limitations in the experiments and requirements to reveal the magnetic moment (orbital and spin), charge ordering, crystal field splitting, spin-orbit-coupling, optical dichroism, and other physical phenomena associated with broken symmetries will be discussed. This research was supported by the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Collaborators: J. Rusz, J. Spiegelberg, M.A. McGuire, C.T. Symons, R.R. Vatsavai, C. Cantoni and A.R. Lupini. [Preview Abstract] |
Friday, March 18, 2016 8:36AM - 8:48AM |
X30.00002: Atomic Structure Refinement of \textit{Pbnm}-type Perovskite Oxide Films Amber Choquette, Cole Smith, Steve May Complex \textit{ABO}$_3$ oxide heterostructures are of interest due to their wide variety of electronic, optical, and magnetic properties. One of the controlling factors to these functionalities is the distortions and rotations of the corner-connected \textit{B}O$_6$ octahedral network. This \textit{B}O$_6$ octahedra network directly couples to the electronic bandwidth of these materials, but the inability to determine the full atomic structure in thin films has inhibited quantitative understanding of how factors such as epitaxial strain alter the octahedral rotations in this broad class of materials. Earlier work of has demonstrate that half-order diffraction peaks can be used to quantify octahedral rotations in thin strained films. Here, we build on this approach to solve for both the oxygen and \textit{A}-site positions in films of the commonly occurring \textit{Pbnm} structure type. We present on epitaxial \textit{R}FeO$_3$ heterostructures, where \textit{R} is a rare earth element, to demonstrate the feasibility of quantifying oxygen and \textit{A}-site displacements in films using synchrotron diffraction. This work is supported by the National Science Foundation (DMR-1151649). [Preview Abstract] |
Friday, March 18, 2016 8:48AM - 9:00AM |
X30.00003: Direct measurement of oxygen octahedral rotations in improper ferroelectric superlattices by STEM Jason Lapano, Ryan Haislmaier, Gregory Stone, Venkat Gopalan, Roman Engel-Herbert Complex ABO$_{3}$ perovskites are an intensely studied class of materials due to their numerous magnetic and electronic functionalities. Using strain and A-site cation, can induce new high temperature functionality known as improper ferroelectricity$^{1}$. Visualizing the interplay between strain, cation ordering and octahedral rotations in improper ferroelectics is crucial to understand how this property manifests itself in thin films$^{2}$. A series of CaTiO$_{3}$n$_{\, }$/SrTiO$_{3\, n\, }$with periodicities n$=$2-10 were grown on (La,Sr)(Al,Ta)O3 by hybrid molecular beam epitaxy. I will discuss how strain and layering affects the cation and oxygen sublattices, and how these distortions propagate through the layers, with direct imaging of the oxygen cations by annular bright field (ABF) STEM. I will then relate these back to understanding how improper ferroelectricity evolves in these films. 1. Rondinelli, J. M. {\&} Fennie, C. J. Octahedral Rotation-Induced Ferroelectricity in Cation Ordered Perovskites. Adv. Mater. 24, 1961--1968 (2012). 2. Biegalski, M. D. et al. Impact of symmetry on the ferroelectric properties of CaTiO3 thin films. Appl. Phys. Lett. 106, 162904 (2015). [Preview Abstract] |
Friday, March 18, 2016 9:00AM - 9:12AM |
X30.00004: Effects of local sample bending on atom positions and polarization mapping in HAADF-STEM images Zhen Wang, Hangwen Guo, Lina Chen, E.W. Plummer, Jiandi Zhang, Jing Tao, Lijun Wu, Yimei Zhu Characterization of the structural distortion/reconstruction in the transition-metal oxide heterostructures play an important role in understanding their novel properties. In recent years, high-angle annular dark field (HAADF) in scanning transmission electron microscopy (STEM) has become a powerful technique to determine local atomic arrangements, particularly near interfaces and boundaries. However, sample bending, especially near the edge of a thin specimen, is often introduced during the TEM sample preparation process. Our recent studies reveal that small sample bending can affect significantly the measurement of atom positions in HAADF-STEM image as a result of channeling effect of the incident electron beam. Here we take SrTiO$_{3}$ (STO) as an example to show how to remove sample bending induced artifact from its intrinsic structural distortions. A polar-related artifact in STO at different bending angles were reveled both in our experiments and imaging simulation. This artifact can be removed successfully by quantitative comparing experimental with simulated HAADF-STEM images under the same imaging condition. The bending angle and thickness of the sample can be determined using convergent beam electron diffraction. Our study provide a useful guidance for removing the sample bending-induced artifact in STEM images for the studies of local lattice structures, polarization and distortion of complex materials. [Preview Abstract] |
Friday, March 18, 2016 9:12AM - 9:24AM |
X30.00005: Quantifying the electronic reconstruction in LaTiO$_3$/LaNiO$_3$/(LaAlO$_3$)$_3$ heterostructures using RIXS Gilberto Fabbris, Ankit S. Disa, Sohab Ismail-Beigi, Frederick J. Walker, Charles H. Ahn, Jonathan Pelliciari, Yaobo Huang, Thorsten Schmitt, Lei Xu, Liviu Hozoi, Jeroen van den Brink, Mark Dean A novel approach for manipulating the 3d state in transition metal oxide heterostructures has emerged with the growth of trilayer nickelate LaTiO$_3$/LaNiO$_3$/(LaAlO$_3$)$_3$ (LTNAO) (Disa et al., PRL 114 026801 (2015)). This heterostructure induces a striking reconstruction of the LaNiO$_3$ electronic structure, which is due to a combination of charge transfer from Ti's 3d state and octahedral elongation along the $c$ axis. We use resonant inelastic x-ray scattering (RIXS) experiments at Ni L$_{2,3}$ and O K edges to spectroscopically resolve the LTNAO electronic structure. Surprisingly, our results show that the octahedral elongation generates minor changes in crystal fields at Ni's 3d state compared to bulk LaNiO$_3$. Instead, heterostructuring creates an anisotropic reconstruction of the Ni 3d - O 2p hybridization. The x$^2$-y$^2$ orbital is significantly more hybridized with O p, leading to a 3z$^2$-r$^2$/x$^2$-y$^2$ hole ratio of $\sim$0.55 and large orbital polarization as measured by x-ray absorption spectroscopy. This work establishes RIXS as an ultra-sensitive probe of complex oxide heterostructures. [Preview Abstract] |
Friday, March 18, 2016 9:24AM - 9:36AM |
X30.00006: Electronic Structure near the Interface of Complex Oxide Heterostructure SmTiO$_{3}$/SrTiO$_{3}$ Ryo Mori, Brandon Isaac, Patrick Marshall, Jonathan Denlinger, Susanne Stemmer, Alessandra Lanzara Quantum wells created from oxide heterostructures induce quantum confinement systems at the heterostructure interface, which show unique properties, such as strong electron correlation, two-dimensional superconductivity, high carrier densities and mobility, and/or magnetism. The rare earth titanate, SmTiO$_{3}$, and the transition metal oxide, SrTiO$_{3}$, create such confined electron systems at their interface, which has a controllable quantum well length by changing the number of SrO layers in SrTiO$_{3}$. By Varying the number of SrO layers, we will present the layer-dependent electronic structure of the SmTiO$_{3}$/SrTiO$_{3}$ interface system from angle-resolved photoemission spectroscopy (ARPES) measurements and discuss these results in terms of strong correlations. [Preview Abstract] |
Friday, March 18, 2016 9:36AM - 9:48AM |
X30.00007: Probing Momentum-Resolved Orbital Polarization at the Oxide Interfaces with SW-ARPES Arian Arab, Slavomir Nemsak, Giuseppina Conti, Vladimir Strocov, Mark Huijben, Jan Minar, Charles Fadley, Alexander Gray Interface electronic structure is critical to the functional properties of strongly-correlated multilayer systems such as the La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ heterostucture, a promising candidate for a magnetic tunnel junction. Recently it was demonstrated that for periodic superlattice samples controllable depth selectivity in angle-resolved photoemission spectroscopy (ARPES) can be accomplished by setting up an x-ray standing-wave (SW) field in the sample and translating it vertically along the surface normal by varying x-ray incidence angle. Here, by varying polarization of the incident x-rays we add orbital sensitivity to SW-ARPES, thus allowing us to distinguish momentum-resolved electronic dispersions for the electronic states of different symmetries (e.g. x$^{\mathrm{2}}$-y$^{\mathrm{2}}$ and 3z$^{\mathrm{2}}$-r$^{\mathrm{2}})$. Distinctly different momentum-resolved orbital polarization maps are obtained for the bulk-like and interface-like Mn 3d electronic states. The results are compared to state-of-the-art first-principles calculations. Future directions and applications are discussed. [Preview Abstract] |
Friday, March 18, 2016 9:48AM - 10:00AM |
X30.00008: Interface properties of LaCrO3/SrTiO3 superlattices studied by standing-wave excited photoemission spectroscopy Cheng-Tai Kuo, Shih Chieh Lin, Ryan Comes, Julien Rault, Peter Sushko, Amina Taleb-Ibrahimi, Scott Chambers, Chuck Fadley The interface between LaCrO3 (LCO) and SrTiO3 (STO) is of interest due to a polar discontinuity, built-in potential [1] and recent evidence of polarization in STO-LCO superlattices (SLs). However, an unambiguous depth profiling of the polarization-induced electronic structure has not been attempted. We here present the quantitative determination of the depth profiles of composition, charge state, potential and momentum-resolved electronic structure for LCO/STO SLs using resonant-excitation x-ray standing wave (SW) photoemission spectroscopy. By varying the incident angle and photon energy around the Bragg condition, the standing wave was moved vertically through the interfaces, giving us the ability to focus on either surface, interface or bulk electronic properties. We are thus able to decompose the valence band spectra into layer-specific contributions for both STO and LCO. We also present momentum-resolved electronic structure using resonant SW angle-resolved photoemission spectroscopy (SW-ARPES) [2] and compare these results to DFT theory for the band dispersions of each layer of the SL. [1] S. Chambers et al. PRL, 107, 206802 (2011) [2] A.X. Gray et al., EPL 104, 17004 (2013) [Preview Abstract] |
Friday, March 18, 2016 10:00AM - 10:12AM |
X30.00009: Built-in electric field and polarization in LaCrO$_{\mathrm{3}}$-SrTiO$_{\mathrm{3}}$ superlattices Peter Sushko, Ryan Comes, Steven Spurgeon, Phuong-Vu Ong, Steve Heald, Shih-Chieh Lin, Cheng-Tai Kuo, Chuck Fadley, Scott Chambers Superlattices combining ferroelectric and non-polar materials exhibit an intriguing induced polarization in the non-ferroelectric phase, such as SrTiO$_{\mathrm{3}}$ (STO). However, there has been no report of a superlattice where two non-ferroelectric materials combine to produce bulk polarization. We present studies of STO-LaCrO$_{\mathrm{3}}$ (LCO) superlattices and show that by controlling interfacial termination between layers we can induce a ferroelectric-type polarization in STO. Density functional theory (DFT) predictions show that by alternating terminations between positively charged TiO$_{\mathrm{2}}$-LaO and negative CrO$_{\mathrm{2}}$-SrO interfaces a polarization is induced in each material. Using molecular beam epitaxy, we have synthesized superlattices with such interfaces and a built-in electric field is observed using x-ray photoelectron spectroscopy. X-ray absorption spectroscopy and electron microscopy confirmed these results and were used to estimate the polarization within the STO layers. Our results agree well with the DFT predictions for the cation displacements and induced polarization. We also present models of the band dispersion to quantify the electronic structure in each of the STO and LCO layers. [Preview Abstract] |
Friday, March 18, 2016 10:12AM - 10:24AM |
X30.00010: XPS characterization scheme for phase-pure epitaxial NbO$_{2}$ Tobias Hadamek, Agham Posadas, Alex Demkov NbO$_{2}$ shows a semiconductor-to-metal transition with an associated structural transition of Peierls type. NbO$_{2}$ and Nb$_{2}$O$_{5}$ or mixtures thereof have also shown electrically induced insulator-to-metal transitions. To shed light on the nature of the electrically induced insulator-to-metal transition it is important to grow high phase purity NbO$_{2}$ and Nb$_{2}$O$_{5}$ and compare electrical measurements with mixed niobium oxides and with different electrode materials. Processing NbO$_{2}$ and avoiding surface oxidation requires ultra-high vacuum (UHV) conditions. Niobium oxide thin films where grown in UHV by molecular beam epitaxy on 111-oriented STO substrates and analyzed by X-ray photoelectron spectroscopy (XPS). It was shown that the NbO$_{2}$ 3d core level spectrum exhibits an asymmetric spin-orbit peak pair with more spectral weight on the high binding energy side. Based on the shape of the Nb 3d core levels, peak positions relative to the oxygen O 1s peak, and the valence band shape and height ratio of the niobium 4d$_{xy}$ split-off band to the oxygen 2p band, an identification scheme for NbO$_{2}$ by XPS was devised. Complementary the NbO$_{2}$ phase was confirmed by reflection high-energy electron and x-ray diffraction analysis. [Preview Abstract] |
Friday, March 18, 2016 10:24AM - 10:36AM |
X30.00011: Thickness-dependent structure variation and novel electronic properties of La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ film on SrTiO$_{\mathrm{3}}$ (001) substrate Lina Chen, Zhen Wang, Jisun Kim, Gaomin Wang, Hangwen Guo, Mohammad Saghayezhiane, Ward Plummer, Jiandi Zhang, Jing Tao, Yimei Zhu In principle, La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO3 (LSMO) is a half metal, which exhibits colossal magnetoresistance. However, it has been observed that the transport properties of LSMO thin films depends on their thickness. By combining \textit{in-situ} scanning tunneling spectroscopy (STS), X-ray photoelectron spectroscopy, and low energy electron diffraction, as well as \textit{ex-situ} scanning transmission electron microscopy, we have studied the structure-property relationship of LSMO on SrTiO$_{\mathrm{3}}$(001) as a function of film thickness and temperature. Studying the electronic properties by STS, we found that LSMO films have the novel zero current bias shifts at low temperature, further enhanced by photons, which can be related to the charging of dielectric layer near the interface and polar surface effect. To figure out it, STS thickness and temperature dependence were systematically studied. Furthermore, film thickness-dependent structure and stoichiometry variation were determined, and their effect to the zero current bias shifts will be discussed. [Preview Abstract] |
Friday, March 18, 2016 10:36AM - 10:48AM |
X30.00012: Aberration Corrected Scanning Transmission Electron Microscopy of $\mathbf{(Ca,Sr)Fe_2O_5}$ Brownmillerite superlattices Debangshu Mukherjee, Greg Stone, Eun Ju Moon, Joshua Young, Venkatraman Gopalan, James Rondinelli, Steven May, Nasim Alem The brownmillerite phase $\mathrm{A_2B_2O_5}$ consists of ordered oxygen vacancies in alternate perovskite layers forming chiral tetrahedral chains. The handedness of these tetrahedral chains control the polarization of the structure. The current study focuses on 1-1 brownmillerite superlattices grown on a $\mathrm{SrTiO_3}$ substrates using molecular beam epitaxy. The B-site in this structure is iron throughout the superlattice film, while the A-site alternates between calcium and strontium in the superlattice layers. In this study, we use atomic resolution aberration corrected scanning transmission electron microscopy (STEM) to investigate the structure and chemistry of the film-substrate interface as well as the chemical structure of the superlattice. Atom positions are determined to measure displacement vectors of A-site cations in the superlattice structure. [Preview Abstract] |
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