Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S49: Focus Session: Oxide Surfaces, Oxygen Vacancies/Disorder, Doped SrTiO3 |
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Sponsoring Units: DMP Chair: Gervasi Herranz, Institut de Ciencia de Materials de Barcelona Room: Mile High Ballroom 1C |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S49.00001: Electronic structure and thermodynamic stability of the SrTiO$_3$ (111) surface Nikhil Sivadas, Hemant Dixit, Valentino Cooper, Di Xiao We investigate the electronic structure and thermodynamic stability of the SrTiO$_3$ (111) surface using density functional theory. We observe that, for the Ti-terminated SrTiO$_3$ (111) surface, there is indeed some electronic reconstruction, resulting in the emergence of a metallic surface state. Polar distortions play a crucial role in screening the internal electric field; thereby reducing the amount of charge transferred between the surfaces. Our analysis emphasizes the failure of the simple nominal charge counting argument in thin STO (111) slabs, which ignores the effect of polar distortion. As expected, having a surface oxygen atom at the Ti termination can stabilize the system, eliminating any electronic reconstruction, making the system insulating. An analysis of the surface thermodynamic stability suggests that the Ti terminated (111) surface should be experimentally realizable. This surface may be useful for exploring the behavior of electrons in oxide (111) interfaces and may have implications for modern device applications. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S49.00002: Theoretical investigation of oxygen vacancy clustering at SrTiO$_3$ surfaces Juan Shen, Roser Valenti, Harald Jeschke Oxygen vacancies have been found, both in experiment and in theory, to create a twodimensional electron gas at SrTiO$_3$ surfaces. We use density functional theory to search for the most stable structural arrangements of multiple oxygen defects. We find that there is a tendency of oxygen defects to cluster. While individual oxygen defects occupy Ti $t_{2g}$ orbitals and contribute to a metallic state, additional defects closeby form localized states in the former SrTiO$_3$ bulk gap. We discuss the relevance of these findings for photoemission experiments. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S49.00003: Bandgap Controlling of Oxygen-Vacancy-Induced Two-Dimensional Electron Gas in SrTiO3 Zhiqi Liu, Wenlai Lu, Shengwei Zeng, Jiawen Deng, Zhen Huang, Changjian Li, M. Motapothula, Weiming Lu, Jianqiang Zhong, Ping Yang, Nina Bao, Wei Chen, Jingsheng Chen, Yuanping Feng, J.M.D. Coey, T. Venkatesan, Ariando Ariando Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, charge, spin and orbital degrees of freedom. Here we report a large bandgap enhancement in SrTiO3 thin films due to their defective nature, which was found to significantly change the electronic and magnetic phases in the oxygen-vacancy-induced two-dimensional electron gas at the interface between amorphous LaAlO3 and SrTiO3. Density functional theory calculations show the possibility of the role of Sr/Ti antisite defects on the observed properties. This may open an attractive path to tailor electronic, magnetic and optical properties of SrTiO3-based oxide interface systems under intense focus in the oxide electronics community. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S49.00004: Infrared study of metallicity in vacuum annealed strontium titanate Peng Xu, T.J. Huffman, M.M. Qazilbash, In Hae Kwak, Amlan Biswas Widely employed as a substrate and as a dielectric layer in heterostructures, strontium titanate (STO) has been thoroughly studied. Metallicity in Nb-doped SrTiO$_{3}$ and at the interface of SrTiO$_{3}$/LaAlO$_{3}$ superlattices is also well known. In this work, we focus on the charge dynamics of vacuum annealed SrTiO$_{3-\delta}$ crystals which have metallic and atomically smooth surfaces. Far-field and near-field infrared measurements supported by spectroscopic ellipsometry have been carried out to provide insight into the emergence of metallicity due to oxygen deficiency in this insulator. Infrared reflectance and near-field optical microscopy are employed to obtain the dielectric function of SrTiO$_{3-\delta}$. This information is analyzed to extract the characteristics of the electron gas in the metallic layer. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S49.00005: Electron Correlation in Oxygen Vacancy in SrTiO$_3$ Chungwei Lin, Alexander A. Demkov Oxygen vacancies are an important type of defect in transition metal oxides. In SrTiO$_3$ they are believed to be the main donors in an otherwise intrinsic crystal. At the same time, a relatively deep gap state associated with the vacancy is widely reported. To explain this inconsistency we investigate the effect of electron correlation in an oxygen vacancy (OV) in SrTiO$_3$. When taking correlation into account, we find that the OV-induced localized level can at most trap one electron, while the second electron occupies the conduction band. Our results offer a natural explanation of how the OV in SrTiO$_3$ can produce a deep in-gap level (about 1 eV below the conduction band bottom) in photoemission, and at the same time be an electron donor. Our analysis implies an OV in SrTiO$_3$ should be fundamentally regarded as a magnetic impurity, whose deep level is always partially occupied due to the strong Coulomb repulsion. An OV-based Anderson impurity model is derived, and its implications are discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S49.00006: Vacancy Ordering and Emergent Electronic Phenomena in SrTiO$_{3}$ P. Ganesh, Dilpuneet S. Aidhy, Valentino R. Cooper, Paul R.C. Kent Electronic complexity in oxides results from the many different types of atomic defects (substitutional, vacancy or interstitial), local strain effects, electrical boundary conditions and other symmetry lowering effects. Among these, oxygen vacancies are one of the most abundant types of defects and can potentially be controlled during oxide growth. We aim to understand what type of oxygen defect configurations form when they are atomically confined to two dimensions in an oxide material i.e. do they prefer to order or prefer disordered configurations, and what types of electronic reconstruction result? Specifically focusing on SrTiO$_{3}$, one of the constituents of 2DEG forming oxide heterostructures, we have performed extensive density functional theory and cluster expansion based calculations to study vacancy structures and the corresponding changes in the electronic structure. Possibilities for vacancy ordering in the bulk, at surfaces, and at interfaces will be discussed. Acknowledgement: A portion of the research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. A portion of this research was conducted at the Center for Nanophase Material Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S49.00007: Emerging magnetism and electronic phase separation at titanate interfaces Natalia Pavlenko, Thilo Kopp, Jochen Mannhart The emergence of magnetism in otherwise nonmagnetic compounds and its underlying mechanisms have become the subject of intense research. Here we demonstrate that the nonmagnetic oxygen vacancies are responsible for an unconventional magnetic state common for titanate interfaces and surfaces. Using an effective multiorbital modelling, we find that the presence of localized vacancies leads to an interplay of ferromagnetic order in the itinerant t2g band and complex magnetic oscillations in the orbitally-reconstructed eg-band, which can be tuned by gate fields at oxide interfaces. The magnetic phase diagram includes highly fragmented regions of stable and phase-separated magnetic states forming beyond nonzero critical defect concentrations. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S49.00008: Dominant Electron Scattering Mechanisms in SrTiO$_{3}$ Amit Verma, Adam Kajdos, Tyler Cain, Susanne Stemmer, Debdeep Jena Transport studies in the complex oxide, SrTiO$_{3}$ (STO), have been carried out for many decades, but a clear consensus on dominant electron scattering mechanisms in action at different temperature and carrier concentration ranges is lacking. Recent progress in the growth of STO by low energetic deposition techniques like MBE has enabled STO thin films with carefully controlled doping and record high mobilities. Such control in turn enables a careful study of scattering mechanisms. In this work, hall mobility data from La-doped STO thin films grown by hybrid-MBE, have been analyzed and modeled considering various electron scattering mechanisms. By comparing theory to measured mobilities, we find that in addition to longitudinal optical phonon and ionized impurity scattering, a) acoustic phonon, and b) a $\sim$ 6meV transverse optical phonon deformation potential scattering mechanisms are necessary to explain the dependence of transport on temperature (2-300K) as the doping concentration is varied over two orders of magnitude (8x10$^{17}$ -- 2x10$^{20}$ cm$^{-3})$. We hope the understanding of scattering mechanisms acting in STO coming out of this work would seed ideas to improve the mobility in this important material. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S49.00009: Stripes of enhanced transition temperature in superconducting strontium titanate Hilary Noad, Katja Nowack, Eric Spanton, Hisashi Inoue, Minu Kim, Chris Bell, Yasuyuki Hikita, Harold Hwang, Kathryn Moler Strontium titanate (SrTiO$_3$) is used widely in heterostructures that are the subject of intense research, such as the LaAlO$_3$/SrTiO$_3$ interface and FeSe grown on SrTiO$_3$, yet the nature and mechanism of superconductivity in SrTiO$_3$ itself are not fully understood. We used a scanning superconducting quantum interference device susceptometer to map the superfluid density as a function of temperature in a 5.5 nm-thick slab of niobium-doped SrTiO$_3$ embedded in undoped SrTiO$_3$. We find that stripe-like regions of the sample remain superconducting to temperatures typically $\sim$40 mK higher than the transition temperature of featureless regions. We associate the stripes with tetragonal domains in SrTiO$_3$, showing that the orientation of the tetragonal c-axis may be important for tuning the critical temperature. These data may be useful for distinguishing models of superconductivity in SrTiO$_3$. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S49.00010: Transparent conducting oxides: A $\delta$-doped superlattice approach Valentino Cooper, Suyoun Lee, Sung Seok Seo, Jun Sung Kim, Woo Seok Choi, Satoshi Okamoto, Ho Nyung Lee Interfaces between dissimilar insulating oxides have been shown to exhibit intriguing phenomena such as metallic states, superconductivity and magnetism. Despite tremendous progress in understanding their origins, very little is known about how to control the conduction pathways and the distribution of charge carriers. Using first principles simulations we examine the effect of SrTiO$_3$ (STO) spacer layer thickness on the physical and chemical properties of La $\delta$-doped STO superlattices. In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of the quantum mechanical wavefunctions between neighboring $\delta$-doped layers. Experimentally these superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2$p$ and Ti $d$ states. These results highlight the potential for using superlattice thickness as a means for tuning the properties of oxide heterostructures with demonstrated importance for optoelectronic devices; providing a unique route for creating transparent conducting oxides. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S49.00011: Structural stability of highly strained oxide thin films in trilayers Yuyang Zhang, Sokrates T. Pantelides, Timothy J. Pennycook, Stephen J. Pennycook Thin layers of yttria-stabilized zirconia (Y$_{2}$O$_{3}$)$_{x}$(ZrO$_{2}$)$_{1-x}$ (YSZ) sandwiched between SrTiO$_{3}$ (STO) layers with $7\%$ lattice-mismatch strain are known to have colossal ionic conductivity at room temperature [1]. This phenomenon has been attributed to a disordering of the O sublattice [2, 3]. Here we report first-principle calculations, including checking the existence of negative-frequency phonon modes, that probe the stability of such highly-strained films. We find that, when matched to the STO layers, the strained YSZ cation sublattice remains ordered despite the total disorder of the O sublattice. The disordered anion sublattice will lower the energy of the system and lead to a stable configuration without negative-frequency phonon modes. [1] J. G. Barriocanal et al., Science 321, 676 (2008); [2] T. J. Pennycook et al., Phys.Rev.Lett. 104, 115901 (2010); [3] T. J. Pennycook et al., Eur. Phys. J. Appl. Phys. 54, 33507 (2011). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S49.00012: Real-time Observation of Vo ordering dynamics in LaCoO3/STO superlattice Jae Hyuck Jang, Rohan Mishra, Young-Min Kim, Qian He, Liang Qiao, Michael D. Biegalski, Andrew R. Lupini, Sokrates T. Pantelides, Stephen J. Pennycook, Sergei V. Kalinin, Albina Y. Borisevich Properties of solid oxide fuel cell, catalysts etc. is dependent on the distribution and transport behavior of oxygen ions. In this study, we observe the dynamics of vacancy ordering in LaCoO3/SrTiO3 (LCO/STO) superlattice and LCO films using high angle annular dark field and annular bright field (ABF) STEM. Vo ordering was directly observed by tracking interatomic spacings, withs nucleation, propagation and interaction of different Vo nuclei demonstrated. Moreover, ABF images show that on 1-D (110) vacancy channels form in the depleted layers.In the case for superlattice, very small contribution of vacancy injection was observed. When this approach is applied to 15 u.c. LCO film, however, a sequence of different phases is observed, starting from disordered perovskite LaCoO3-x to a brownmillerite La3Co3O8-x to eventually brownmillerite La2Co2O5-x. Kinetics of the ordering and vacancy injection, as well as implications for beam-driven phase-transformation at an atomic scale, will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S49.00013: Oxygen vacancy equilibrium concentrations in strontium-doped lanthanum cobalt iron oxides Heng Luo, Xi Lin Formation of oxygen vacancies by introducing various mixed-valent cation dopants is a common procedure to improve the cathode performance in solid oxide fuel cells. A generic computational procedure is developed in this work to predict the oxygen vacancy equilibrium concentrations at experimentally relevant temperatures and oxygen partial pressures for both bulk and surface oxide phases. The calculations are based on the first-principles density functional theory and a constrained free-energy functional. Quantitative agreements are found by direct comparisons to the thermogravimetry measurements for various strontium-doped lanthanum cobalt iron oxides. Our results indicate that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and all oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S49.00014: Epitaxial SrCoOx oxygen sponge H.N. Lee, H. Jeen, W.S. Choi, M.D. Biegalski, D. Shin, M.F. Chisholm, C.M. Folkman, D.D. Fong, J.W. Freeland, I-C. Tung, H. Ohta Perovskite-based transition metal oxides have been actively developed as the replacements of noble metal-based electrodes in energy and environmental devices due to their high catalytic activity and ionic conductivity. However, the high thermodynamic barrier and the robust cation's oxidation state have limited the realization of fast catalysis and bulk diffusion at low temperature, which can reduce thermomechanical degradation in such devices. Here, we report a low-temperature reversible redox reaction in SrCoO$_{x}$ grown directly by pulsed laser epitaxy as one of two distinct crystalline phases, either the perovskite SrCoO$_{3-\delta}$ or the brownmillerite SrCoO$_{2.5}$.\footnote{H. Jeen \textit{et al.}, Nature Mater. \textbf{12}, 1057 (2013).} Based on real-time temperature dependent x-ray diffraction, we found that the distinct valence state in each phase can be reversibly switched at a remarkably reduced temperature (200 $\sim$ 300 $^{\circ}$C) in a considerably short time (\textless 1 min) without destroying the parent framework. Therefore, our results on low temperature reversible redox reactions provide valuable insight not only in understanding the structure-physical property relationship in multivalent oxides, but also for developing new strategies to avoid thermomechanical degradation in high temperature electrochemical devices, such as solid oxide fuel cells. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S49.00015: Oxygen-vacancy-induced polar behavior in (LaFeO$_{3}$)$_{2}$/(SrFeO$_{3}$) superlattices Rohan Mishra, Sokrates Pantelides, Young-Min Kim, Juan Salafranca, Stephen Pennycook, Albina Borisevich, Seong Keun Kim, Seohyoung Chang, Anand Bhattacharya, Jeffrey Eastman, Dillon Fong Complex oxides displaying ferroelectric and/or multiferroic behavior are of high fundamental and applied interest. In this work, for the first time, we show that it is possible to achieve polar order in a superlattice made up of two non-polar oxides by means of oxygen vacancy ordering. Using scanning transmission electron microscopy imaging, we show polar displacement of magnetic Fe ions in a superlattice of (LaFeO$_{3}$)$_{2}$/(SrFeO$_{3}$) grown on a SrTiO$_{3}$ substrate. Using density functional theory calculations, we systematically study the effect of epitaxial strain. octahedral rotations and surface terminations in the superlattice and find them to have negligible effect on the antiferroelectric displacements of the Fe ions lying in between SrO and LaO layers of the superlattice. Introduction of oxygen vacancies, on the other hand, triggers a polar displacement of the Fe ions. We confirm this important result using electron energy loss spectroscopy, which shows oxygen vacancy ordering in the region where polar displacements are observed and an absence of vacancy ordering outside of that area. Overall our results open up a new pathway to design new ferroelectrics and multiferroics. [Preview Abstract] |
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