Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L46: Complex Oxide Interfaces & Heterostructures -- Ultrafast Dynamics, Defects & General PhenomenaFocus
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Sponsoring Units: DMP Chair: Scott Chambers, Pacific Northwest Natl Lab Room: BCEC 212 |
Wednesday, March 6, 2019 11:15AM - 11:51AM |
L46.00001: Shedding light on materials out of equilibrium: Ultrafast control of magnetism in complex oxide thin-films Invited Speaker: Nicole Benedek The recent development of intense ultrashort mid- and far-infrared laser sources has created an opportunity for functional materials based on the direct excitation of infrared active phonons. Strong excitation of infrared active phonons can produce sizable unidirectional distortions of crystal structure through non-linear couplings between various lattice modes. Complex oxides provide an important test-ground for this experimental approach due to their chemical diversity, strong coupling to optical fields, and demonstrated connection between subtle structural changes and functional properties. Early experiments in complex oxides are intriguing, suggesting that non-linear phonon coupling is responsible for transiently induced insulator-metal phase transitions and enhanced superconductivity in optical experiments. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L46.00002: Ultrafast Carrier Dynamics in La1-xSrxMnO3/SrTiO3 Heterostructures Kun Zhao, Joel E Taylor, Rami A Khoury, Mohammad Saghayezhian, Louis H Haber, Jiandi Zhang, E Ward Plummer Transition metal oxide thin films and heterostructures have attracted interest of community due to manifestation of broken symmetry and dimensional confinement, which create new forms of coupling and consequently new functionality. An important challenge is to probe the excited states of these artificially structured materials. Ultrafast reflectivity measurements can be used to study the highly nonequilibrium exited states and observe subsequent dynamical behavior. We grew La1-xSrxMnO3/SrTiO3(001) heterostructures using different growth conditions, being able to control structural and compositional modulation near the interface, which profoundly changes electronic and magnetic properties of these thin films. Using ultrafast reflectivity measurements, we show novel excited-state relaxation dynamics and phonon oscillations in these heterostructures. The changes in these relaxations will be discussed relevant to the growth conditions. With specific growth condition, we observed the relaxations of electron-phonon coupling and phonon-phonon coupling with larger than expected lifetimes as well as the oscillations of two phonon modes. Furthermore, pump-power dependent reflectivity and lifetimes were also studied. |
Wednesday, March 6, 2019 12:03PM - 12:15PM |
L46.00003: Atomic-Resolution Studies of Radiation-Induced Defects In High Conductivity Delafossite Oxide Metals Celesta Chang, Veronika Sunko, Philippa McGuinness, Andrew Mackenzie, David Anthony Muller Delafossite metals are known for having the highest in-plane conductivity in the oxide family. Room temperature resistivity of PdCoO2 is 2.6μΩ, and at low temperature it drops down to 0.0075μΩ, with a mean free path of 20μm. Such long mean free path raises the question of the nature of the defects – is the density truly that low, or are defects present but somehow hidden from scattering channels? Using scanning transmission electron microscopy (STEM) we deliberately introduced defects at a specific atomic plane of PdCoO2 and PtCoO2 using a high electron dose of 520×10-3C/mm2, then imaging at lower doses. Our results show that damage from an electron beam focused on the Pd (Pt) atomic layers creates local metallic chains and clusters. A beam placed on the O-Co-O layer damages more dramatically by cutting the Co layers off, resulting in Pd (Pt) layers to be pulled towards the neighboring Co layer position. |
Wednesday, March 6, 2019 12:15PM - 12:27PM |
L46.00004: Dynamics of Near-Interface Atomic Plane Rearrangement in WO3/SrTiO3(001) Petr Sushko, Le Wang, Zhenzhong Yang, Steven Spurgeon, Scott Chambers, Yingge Du Interfaces of complex oxides can exhibit properties not realized in the consitituent materials. The chemistry that occurs at such interfaces has received relatively little attention, yet can be very influential and permit control of interfacial properties, if it is well understood. Here we focus on a dramatic interface chemical effect - the apparent dissolution of an entire atomic plane during WO3heteroepitaxy on TiO2-terminated SrTiO3(001). Transmission electron microscopy indicates that the first subsurface SrO monolayer is completely removed, resulting in the formation of a TiO2bi-layer at the interface. In combination with x-ray photoelectron spectroscopy measurements, these data suggest that at least a fraction of the Sr from this layer dissolves into the WO3film. To assist interpretation of the TEM data, we determined the thermodynamic stability of candidate interfacial strucutres using ab initio(density functional theory) simulations and linked these stabilities to the growth conditions. The analysis of near-interface diffusion pathways suggests ways in which SrO dissolution takes place, and how manipulation with the growth conditions can shift the thermodynamic preference from one structure to another. |
Wednesday, March 6, 2019 12:27PM - 1:03PM |
L46.00005: Defects and transport in oxide heterostructures Invited Speaker: Chris Van de Walle Complex oxide heterostructures have been intensively investigated in recent years. However, the widely used transition-metal oxides suffer from low carrier mobility, which limits device applications. For applications in electronics, attention has shifted to materials such as BaSnO3 and Ga2O3. They have large band gaps, rendering them suitable for transparent conductors and for high-frequency and power electronics, but can be highly n-type doped and exhibit good transport properties. Better control of dopants and point defects is still needed in order to improve materials quality and enable further applications. I will show how cutting-edge first-principles modeling, using advanced hybrid functional calculations within density functional theory, can shed light on the multiple aspects of this problem: band alignment and confinement of two-dimensional electron gases [1,2,3,4], carrier scattering and mobility [5,6], doping [7,8], point defects and their impact on carrier concentrations [8], energetics and electronic structure of alloys [3,4], and optical properties [9,10]. |
(Author Not Attending)
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L46.00006: Structural Goldstone modes in 111-strained perovskite SrMnO3 Astrid Marthinsen, Sinead Magella Griffin, Magnus Moreau, Tor Grande, Thomas Tybell, Sverre Selbach Epitaxial strain has been extensively explored to enhance existing and enable new functional properties in perovskite thin films. While most of this work has been done on 001-oriented heterostructures, 111-oriented films can have very different properties. Here we use DFT calculations to predict structural Goldstone modes in the (111)-strained perovskite SrMnO3. Massless Goldstone modes are found in high energy particle physics as well as in low energy systems like superconductors and superfluid helium, while structural Goldstone modes are rare. Here we predict acoustic Goldstone phonon modes under compressive strain resulting from coupling between two in-plane rotational instabilities, giving the characteristic Mexican hat shaped energy surface. Large tensile strain is found to induce in-plane polar instabilities giving rise to a continuous polar ground state. Such phonon modes with U(1) symmetry could emulate structural condensed matter Higgs modes, where the mass of this boson is given by the shape of the Mexican hat energy surface, which is tunable by epitaxial strain. |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L46.00007: Designing Optimal Perovskite Structure for High Ionic Conduction Ran Gao, Abhinav Jain, Shishir Pandya, Yongqi Dong, Yakun Yuan, Hua Zhou, Liv R Dedon, Vincent Thoréton, Sahar Saremi, Ruiijuan Xu, Aileen Luo, Ting Chen, Venkatraman Gopalan, Elif Ertekin, John Kilner, Tatsumi Ishihara, Nicola Perry, Dallas Trinkle, Lane Martin There has been an incomplete understanding of the structure-property relationships that would enable the rational design of better ion-conducting electrolytes for SOFC. Here, using epitaxial thin-film growth, synchrotron X-ray diffraction, impedance spectroscopy, and density-functional theory, we delineate the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity in La0.9Sr0.1Ga0.95Mg0.05O3-δ. As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures we experimentally realize both expanded unit-cell volume and strong octahedral rotations which result in an enhancement of the ionic conductivity by ~250% at around 600°C. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L46.00008: Epitaxial strain effect on site occupancy and conductivity of titanomagnetite Tiffany Kaspar, Steve Michael Heald, mark bowden, Peter Sushko, Scott Chambers Magnetite, Fe3O4, exhibits metallic conductivity via electron hopping between Fe2+ and Fe3+ occupying octahedral sites in the spinel lattice. As Ti4+ is doped into the octahedral sites of magnetite (the titanomagnetite series), an equal fraction of Fe3+ is reduced to Fe2+ to maintain charge neutrality. The site occupancies of Fe2+ and Fe3+ determine the transport properties of the titanomagnetite series; the end-member ulvöspinel, Fe2TiO4, exhibits p-type semiconducting transport properties. The Fe2+/Fe3+ site occupancy remains controversial, but is likely in part a function of the lattice strain induced by doping smaller Ti4+ into the lattice. Here, we have deposited titanomagnetites and ulvöspinel as well-defined epitaxial thin films on both MgO and MgAl2O4 substrates. We have characterized the Fe valence state and site occupancy with XPS, XANES, and EXAFS, and related them to the epitaxial strain induced by the substrate. The impact of these factors on the electrical transport properties of the films will be discussed. |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L46.00009: Electronic and Catalytic Behavior of Mn-based Spinels Grown by Molecular Beam Epitaxy Miles Blanchet, Shalinee Chikara, Alexandria Bredar, Tiffany Kaspar, Byron Farnum, Ryan Comes Previous research on manganese-based spinel structures has shown that some exhibit strong oxygen reduction reaction catalytic behavior for water splitting in hydrogen fuel cells. However, there have been limited efforts to study these materials as epitaxial thin films which can be used for surface catalysis studies. With this in mind, thin films of spinel structures such as MnFe2O4, CoMn2O4, NiMn2O4 and Mn3O4 were synthesized to study the relationship of electronic properties and catalytic behavior. Films were grown using a molecular beam epitaxy system which allowed for precise control of sample stoichiometry. After growth, films were transferred to an in-situ x-ray photoelectron spectroscopy system to be examined without exposure to atmosphere. In addition, measurements characterizing the films’ electronic and catalytic properties such as ellipsometry and cyclic voltammetry were also performed. The combination of these studies allows for a greater understanding of these spinels and how they might be used in future catalytic applications. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L46.00010: Fluorination of Epitaxial Thin Films of Functional Metal Oxides Joseph Cartelli, Francis Walz, Anton Wiggins, David Huston, Sanedya Hernandez, Azriel Weinreb, Rajeswari M Kolagani The goal of this project is to investigate the effects of incorporating fluorine into epitaxial thin films of functional metal oxides including Lanthanum Calcium Manganese Oxide (LCMO), Calcium Manganese Oxide (CMO), and Strontium Titanate (STO), with emphasis on electrical resistivity and structural changes. Electrical resistivity is a property of interest for many technical applications of these advanced and emerging materials. Fluorination of similar perovskite oxide materials including SrFeO3-d in thin films has yielded many interesting results. These results include altering the crystal structures and altering the electrical properties. Fluorine alters the anionic composition of these materials thus providing a way for extending the range of functional properties realizable. Fluorine introduction has the potential for charge doping the materials and altering the cation valence states. This would lead to changing structural order and symmetry through ionic size differences in the cation, and changes in the charge carrier concentration and mobility. |
Wednesday, March 6, 2019 2:03PM - 2:15PM |
L46.00011: Nanoscale Visualization of the Au/HfO2/Si(001) Interface Electrostatics with BEEM Jack Rogers, Daniel Pennock, Hyeonseon Choi, Steven Gassner, Westly Nolting, Vincent LaBella Understanding carrier transport through the electrostatic potential of monolayer-thick oxide layers is important for nanoscale devices. Ballistic electron emission microscopy (BEEM) is an STM-based technique which allows for the visualization of nanoscale fluctuations of metal / insulator / semiconductor interface electrostatics [1]. In this work, the electrostatic barrier of an Au/HfO2/Si(001) interface is examined with BEEM to nanoscale dimensions. Thousands of spectra are taken at equally spaced points in a grid over a square micron. Barrier heights consistent with the band offsets for SiO2, HfO2, and the Schottky barrier for gold on p-type silicon are observed. This suggests that the monolayer-thick HfO2 layer is transparent to the ballistic electrons. X-ray photoemission spectroscopy confirms the presence of native SiO2 formation, which is further supported by computational modeling. |
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