Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L30: Functional Defects in Oxide HeterostructuresFocus
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Sponsoring Units: DMP Chair: Hanghui Chen, Columbia University Room: 329 |
Wednesday, March 16, 2016 11:15AM - 11:27AM |
L30.00001: Structural and electronic response via oxygen vacancy control in SrFeO3 heterostructures Alex Krick, Eun Ju Moon, Amanda Huon, Steven May The electronic and structural properties of complex perovskite oxide thin films are often directly influenced by their oxygen vacancy concentration. Here, we investigate epitaxial films of SrFeO$_{\mathrm{3}}$, which exhibits a variety of structural and electronic phases as a function of oxygen content. The ability to control these functional properties via temperature or external fields is not present in conventional semiconductors and is attractive from an application perspective. As-grown films are oxidized using a post-growth anneal in dilute ozone, yielding metallic behavior consistent with bulk SrFeO$_{\mathrm{3}}$. X-ray diffraction and temperature dependent resistivity collected at different stages of oxidation and reduction reveal minute structural transformations that yield large changes in electronic behavior due to oxygen loss. [Preview Abstract] |
Wednesday, March 16, 2016 11:27AM - 11:39AM |
L30.00002: A new kind of line defect in NdTiO3 perovskite Mehmet Topsakal, Jong Seok Jeong, Peng Xu, Bharat Jalan, Renata Wentzcovitch, Andre Mkhoyan We report an observation of a new line defect in strained NdTiO3 perovskite. Aberration-corrected analytical scanning transmission electron microscopy and first-principles calculations are used to characterize its atomic structure and electronic properties. The defect represents a shift and rotation of the core Ti-O-Nd unit accommodating the deficiency of Ti-O units in neighboring columns and strain. The core of the defect has considerably different electronic properties resulting from Ti being in different oxidation states. This observation closes the dimensionality gap between previously observed point and planar defects in complex oxides needed to accommodate the alterations of stoichiometry and strain. This new line defect should also exist in other multi-valent perovskites, and could open a new avenue for tailoring unexpected and highly-desirable electronic properties. [Preview Abstract] |
Wednesday, March 16, 2016 11:39AM - 11:51AM |
L30.00003: Interplay of oxygen vacancies and electronic correlations in SrVO$_3$ Steffen Backes, Aaram J. Kim, Frank Lechermann, Harald O. Jeschke, Marcelo J. Rozenberg, Andres F. Santander Syro, Roser Valenti We investigate the role of oxygen vacancies in SrVO$_3$ within LDA+DMFT (density functional theory combined with dynamical mean-field theory). We show that, in addition to the usual $t_{2g}$ lower Hubbard band, oxygen vacancies are responsible for an additional peak around $-1$~eV of V $3d_{z^2}$ orbital character, which is not present in the bulk system without vacancies. We discuss our results in the light of recent angle-resolved photoemission (ARPES) experiments. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:27PM |
L30.00004: Strain induced defects and charge state transitions in oxides Invited Speaker: Ulrich Aschauer It recently became apparent that bi-axial strain in coherent epitaxial perovskite oxide thin films or heterostructures can not only be accommodated by changes in structural parameters such as bond lengths or octahedral-rotation angles, but also by the formation of point defects. The redox reactions accompanying the formation of anion and cation vacancy defects lead to local volume changes, which facilitate the formation of either defect species under bi-axial strain. In this talk we will, after a general introduction to the phenomenon, use density functional theory (DFT) calculations to explore the generality of this concept for perovskite oxides and binary rock-salt oxides and put an emphasis on defects in different charge states. Moreover we will discuss the interaction of defects with ferroelectric domain walls, leading to novel functionalities in strained thin films. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L30.00005: Antisite defects at oxide interfaces Hanghui Chen, Andrew Millis We use ab initio calculations to estimate formation energies of cation (transition metal) antisite defects at oxide interfaces and to understand the basic physical effects that drive or suppress the formation of these defects. We find [1] that antisite defects are favored in systems with substantial charge transfer across the interface, while Jahn-Teller distortions and itinerant ferromagnetism can prevent antisite defects and help stabilize atomically sharp interfaces. Our results enable identification of classes of systems that are more and less susceptible to the formation of antisite defects and motivate a range of experimental studies and further theoretical calculations to further explicate the oxide interface systems. [1] H. Chen and A. J. Millis, arXiv:1509.06643, (2015). [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L30.00006: Metal-Insulator Transition and Weak Localization in Oxygen Vacancy Doped BaSnO$_{\mathrm{\mathbf{3-\delta }}} Thin Films Koustav Ganguly, Abhinav Prakash, Jong Seok Jeong, K. Andre Mkhoyan, Bharat Jalan, Chris Leighton We present detailed temperature-dependent electronic transport in oxygen vacancy doped BaSnO$_{\mathrm{3}}$ films grown on MgO(001), LaAlO$_{\mathrm{3}}$(001), and GdScO$_{\mathrm{3}}$(110) using high pressure oxygen sputter deposition. Various modes of high-resolution X-ray diffraction and scanning transmission electron microscopy confirm phase-pure, close to stoichiometric, smooth, epitaxial BaSnO$_{\mathrm{3}}$(001). [1] As-grown films are insulating, but can be made conductive with $n$-type carriers \textit{via }vacuum annealing, resulting in 300 K Hall mobilities up to 35 cm$^{\mathrm{2}}$V$^{\mathrm{-1}}$s$^{\mathrm{-1\thinspace }}$at 5\texttimes 10$^{\mathrm{19}}$ carriers per cm$^{\mathrm{3}}$. [1] Film thickness, reduction temperature, and substrate ($i.e$. lattice mismatch) have been systematically varied, enabling study of the insulator-metal transition, and, in particular, 2D weak localization at low temperatures. The results provide significant insight into the active transport mechanisms in BaSnO$_{\mathrm{3}}$ films. [1] Ganguly \textit{et al}. APL Materials 3, 062509 (2015). [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L30.00007: Tuning optical absorption and photoexcited recombination dynamics in La$_{\mathrm{1-x}}$Sr$_{x}$FeO$_{\mathrm{3-\delta }}$ through $A$-site substitution and oxygen vacancies Sergey Smolin, Mark Scafetta, Amber Choquette, Matthew Sfeir, Jason Baxter, Steven May We study optical absorption and recombination dynamics in La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$FeO$_{\mathrm{3-\delta }}$ thin films, uncovering the effects of tuning nominal Fe valence via $A$-site substitution and oxygen stoichiometry. Variable angle spectroscopic ellipsometry was used to measure static optical properties, revealing a linear increase in absorption coefficient at 1.25 eV and a red-shifting of the optical absorption edge with increasing Sr fraction. The absorption spectra can be similarly tuned through the introduction of oxygen vacancies, indicating the critical role that nominal Fe valence plays in optical absorption. Dynamic optoelectronic properties were studied with ultrafast transient reflectance spectroscopy, revealing similar nanosecond photoexcited carrier lifetimes for oxygen deficient and stoichiometric films with the same nominal Fe valence. These results demonstrate that while the static optical absorption is strongly dependent on Fe valence tuned through cation or anion stoichiometry, oxygen vacancies do not appear to play a significantly detrimental role in the recombination kinetics. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L30.00008: Low Dimensional Oxygen Vacancy Ordering and Diffusion in SrCrO$_{\mathrm{3-\delta }}$ Phuong Vu Ong, Peter V. Sushko Oxygen vacancies (V$_{\mathrm{O}})$ are known to strongly affect the structure and electronic properties of complex oxides. An ability to control local concentration and spatial distribution of the vacancies, as well as stability and dimensionality of their aggregates, would enable generating novel materials functionalities, such as fast directional charge and mass transport. We use first-principles simulations to study mechanisms of formation, aggregation and diffusion of oxygen vacancies in SrCrO$_{\mathrm{3-\delta }}$. We found that at low concentrations oxygen vacancies have a tendency to aggregate into one-dimensional (1D) structures oriented along a [110] direction. These V$_{\mathrm{O}}$ clusters induce rearrangements of oxide ions and conversion of Cr-centered perovskite lattice octahedra into tetrahedra. In turn, aggregation of these 1D V$_{\mathrm{O}}$ clusters enables formation of 2D vacancy aggregates parallel to the (111) plane of the cubic perovskite lattice. We provide a simple physical picture for the formation and growth of such low-dimensional V$_{\mathrm{O}}$-structures. Moreover, we found mechanisms of V$_{\mathrm{O}}$ migration which enable a diffusion and expansion of the V$_{\mathrm{O}}$-structures with low activation energies. Our results elucidate the atomic-scale mechanisms of efficient and reversible reduction and oxidation process observed in this material. These mechanisms could be extended to other complex oxides and used in design of high performance electrolytes and cathodes. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L30.00009: Structure and Electronic Transport of Oxygen-Deficient SrTiO$_3$ Thin Films Buffered with DyScO$_3$ Purnima Balakrishnan, Urusa Alaan, Matthew Gray, Yuri Suzuki Oxygen deficiency in SrTiO$_3$ (STO) induces metallic behavior in bulk and thin film form. Thus far, reports of STO thin film metallicity have been limited to homoepitaxial growth on bulk STO substrates. Growth on other substrates has suppressed metallicity, suggesting the important role of lattice distortions. In this presentation, we report on the metallicity and corresponding structure of oxygen-deficient STO films deposited on DyScO$_3$ (DSO) buffered STO substrates and compare to STO films deposited directly on STO substrates. These films are epitaxial, atomically flat, expanded out-of-plane by $\sim$0.6$\%$, and coherently strained to the STO substrate. Oxygen-deficient STO thin films grown on STO and DSO-buffered STO substrates are metallic, while films deposited on LaAlO$_3$, (LaAlO$_3$)(SrTaO$_3$), and DSO substrates are insulating. The resistivities of metallic films follow a T$^3$ dependence near room temperature, transitioning to a T$^2$ dependence below $\sim$100 K, and are increased by the addition of a DSO buffer. Comparison of sheet resistance across films of various thicknesses indicates the presence of an insulating layer around 7 unit cells thick. These properties indicate the importance of both oxygen deficiency and lattice structure in obtaining metallicity. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L30.00010: Non-Fermi liquids in two and three-dimensional doped SrTiO3. Evgeny Mikheev, Santosh Raghavan, Jack Zhang, Patrick Marshall, Adam Kajdos, Leon Balents, Susanne Stemmer A remarkable feature of transport in doped SrTiO$_3$ is the temperature dependence of the electrical resistivity that is proportional to $T^n$ with $n$ $\leq$ 2. This power law suggests electron-electron scattering is the dominant scattering mechanism. It extends to room temperature and above in both three-dimensional, uniformly doped SrTiO$_3$ and in two-dimensional electron liquids (2DELs) at oxide interfaces. In case of $n$ = 2, the behavior is traditionally identified as that of a Landau Fermi liquid. Here we argue that Landau Fermi liquid theory does not apply to the electron liquid in SrTiO$_3$, even when $n$ = 2. Using electrostatic gating and chemical doping, we demonstrate that this regime is associated with a scattering rate and an energy scale that are independent of carrier density. This is in fundamental conflict with the premise of the Fermi liquid theory, where this energy scale is the Fermi energy. This work raises important questions in terms of microscopic scattering mechanism. It appears to be relevant for understanding of transport in many other strongly correlated systems, which also show very robust $T^n$ regimes with carrier density independent scattering rates. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L30.00011: Understanding the Origin of Surface Depletion in $\delta $-doped SrTiO$_{\mathrm{3}}$ Structures Hyeok Yoon, Hisashi Inoue, Adrian G. Swartz, Yasuyuki Hikita, Harold Y. Hwang Unlike most of the conventional semiconductors, the large dielectric constant of SrTiO$_{\mathrm{3}}$ results in a pronounced surface depletion width [1]. In thin films, the effect of surface depletion is even more dramatic: reduction of mobility and two-dimensional carrier density. To avoid this effect, capping and buffering a narrow channel of $n$-type doped SrTiO$_{\mathrm{3}}$, so called $\delta $-doping, is designed to make the channel free from surface scattering, resulting in highly mobile carriers [2-4] We have investigated systematic changes in electronic transport by tuning the thicknesses of the undoped surface buffering cap and the $\delta $-doped layer. This has allowed us to map the phase diagram consisting of a three-dimensional metal, two-dimensional metallic behavior, and an insulating phase. We also show the surface depletion width as a function of doping density in order to study the origin of surface depletion of SrTiO$_{\mathrm{3}}$ [1] A. Ohtomo and H. Y. Hwang, \textit{Appl. Phys. Lett.} \textbf{84}, 1716 (2004). [2] Y. Kozuka \textit{et al}., \textit{Appl. Phys. Lett.} \textbf{97}, 222115 (2010). [3] Y. Kozuka, M. Kim \textit{et al., Nature} \textbf{462}, 487 (2009). [4] M. Kim \textit{et al., Phys. Rev. Lett.} \textbf{107}, 106801 (2011). [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L30.00012: High-mobility SrTiO$_3$ delta-doped field-effect transistors Hisashi Inoue, Adrian Swartz, Yasuyuki Hikita, Harold Hwang Two-dimensional electron systems in SrTiO$_3$ show intriguing properties such as high mobility transport, magnetism, and possible unconventional superconductivity. A delta-doped structure, sandwiching a narrow two-dimensional conducting SrTiO$_3$ channel between two insulating SrTiO$_3$ layers, provides a clean platform to realize such electronic states, with symmetric confining potential in the absence of interface or surface scattering. Electric field gating of the conducting channel in a field-effect transistor (FET) geometry is a powerful method for tuning low-dimensional systems via carrier density modulation. We have synthesized high quality SrTiO$_3$ delta-doped structures using pulsed laser deposition, and optimized the device processing steps to achieve ideal FET characteristics at room temperature. This progress enabled examination of high-mobility transport in the carrier density regime as low as 3 $\times$ 10$^{12}$ cm$^{-2}$ at low-temperatures, opening promising avenues to investigate quantum transport and realization of exotic quantum phases in SrTiO$_3$ two-dimensional electron systems. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L30.00013: A model of dopant diffusion through a strongly correlated p-n junction. Jedrzej Wieteska, Richard Brierley, Gian Guzman-Verri, Gunnar Moller, Peter Littlewood The diffusion of charged ions in a solid depends on an equation of state that balances diffusive and screened electrostatic forces, and is well understood in the case of conventional semiconductors and metals. In the case of a strongly-correlated material, the physics is different, and expected to be relevant, for example, in Li-ion battery cathodes. We propose a model of dopant ion motion through a strongly correlated p-n junction. Our approach is to consider diffusive (Nernst-Planck) dynamics of dopants under screened electrostatic interactions computed [1] within a mean-field (Thomas-Fermi) approximation. Dopant profiles as function of time are calculated for a p-n junction held at constant voltage. In the case where filling levels are near a correlation-induced gap, Mott insulating regions can form at the p-n interface and their dynamics is studied. References: 1. Charlebois et al., Phys. Rev. B, 87 035137 (2013) [Preview Abstract] |
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