Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T49: Focus Session: Oxide Interfaces - Defects & Stoichiometry, Rashba and Spin-Orbit, Competing Phases |
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Sponsoring Units: DMP Chair: Mohit Randeria, Ohio State University Room: Mile High Ballroom 1C |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T49.00001: A unified mechanism for 2DEG at SrTiO3/LaAlO3 interface Liping Yu, Alex Zunger The origin of 2DEG appearing at the TiO$_2$-LaO (n-type) interface between two insulating oxides of polar LaAlO$_3$ (LAO) and nonpolar SrTiO$_3$ (STO) after some critical LAO thickness is still under hot debate. Here applying modern defect theory for bulk, interface and surface, based on DFT and HSE, we investigated the current mechanisms that focus on polar catastrophe scenario, interfacial and surface O vacancies (VO), or interfacial cation defects. We uncovered a unified mechanism that can explain not only the 2DEG at n-type interface, but also the insulating behaviour at SrO/AlO$_2$ (p-type) interface. Specifically, for n-type interface, we found that (i) it is the VO at LAO surface coupled with built-in electric field in LAO film that causes 2DEG and determines the critical thickness. (ii) The interfacial La-on-Sr and Ti-on-Al antisite donor defects cause interfacial mixing, but do not contribute itinerant carriers. (iii) The cation vacancies and acceptor antisite defects can trap partially the 2DEG. For p-type interface, the insulating behaviour is resulted from the spontaneous formation of the defect pair of ``interfacial La-on-Sr defect and surface La vacancy defect'' after a critical thickness smaller than that expected from pure polar catastrophe scenario. [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T49.00002: Electronic structure and stability of LaAlO$_{3}$ surfaces using first-principles calculations Karthik Krishnaswamy, Cyrus E. Dreyer, Anderson Janotti, Chris G. Van de Walle LaAlO$_3$ (LAO) is a wide-band gap complex oxide that is often used as a substrate in the growth of other oxides, and has been considered as a high-$k$ gate dielectric for CMOS devices. In addition, a high-density two-dimensional electron gas (2DEG) at the interface between LAO and SrTiO$_3$ (STO) has been observed, showing strong dependence on surface termination and on the thickness of the LAO top layer. The possibility of tuning the 2DEG has triggered the interest in these heterostructures for electronic devices. In all of these applications, the surface of LAO is expected to play a key role, yet its electronic structure and surface stability are poorly understood. Using first-principles calculations based on hybrid density functional theory, we determine the low-energy (001) terminations of LAO and their reconstructions. We analyze the surface stability as a function of oxygen chemical potential and relate to experimental conditions. We discuss the case of bulk single crystals in which the electric field inside the material can be neglected, and also the case of LAO thin films on STO, in which the surface stability can be affected by the presence of the 2DEG at the STO/LAO interface. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T49.00003: Role of surface defects on the formation of the 2-dimensional electron gas at polar interfaces Emilio Artacho, Pablo Aguado-Puente The discovery of a 2-dimensional electron gas (2DEG) at the interface between two insulators, LaAlO$_3$ and SrTiO$_3$, has fuelled a great research activity on this and similar systems in the last years. The electronic reconstruction model, typically invoked to explain the formation of the 2DEG, while being intuitive and successful on predicting fundamental aspects of this phenomenon like the critical thickness of LaAlO$_3$, fails to explain many other experimental observations. Oxygen vacancies, on the other hand, are known to dramatically affect the physical behaviour of this system, but their role at the atomic level is far from well understood. Here we perform ab initio simulations in order to assess whether the formation of oxygen vacancies at the surface of the polar material can account for various recent experimental results that defy the current theoretical understanding of these interfaces. We simulate SrTiO$_3$/LaAlO$_3$ slabs with various concentrations of surface oxygen vacancies and analyze the role of the defects on the formation of the metallic interface, their electrostatic coupling with the 2DEG and the interplay with the different instabilities of the materials involved. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T49.00004: Effect of LaAlO$_{3}$ Stoichiometry on Magneto- Transport Properties of LaAlO$_{3}$/SrTiO$_{3}$ heterostructure Richard Qui, Nicholas Goble, Richard Akrobetu, Alp Sehirlioglu, Xuan Gao The formation of 2DEG in the interface of two insulators, LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO), has stimulated intense research on its origin and applications. We present our low temperature magneto-transport studies on series of high quality LaAlO$_{3}$/SrTiO$_{3}$ hetero-structure samples with different growth and treatment conditions. Parameters that affect interface properties, such as substrate orientation, deposition oxygen partial pressure and the related oxygen vacancies, LaAlO$_{3}$ layer thickness, and LaAlO$_{3}$~stoichiometry, are controlled through processing modifications and post-processing treatments. By looking at the feature of anisotropic magnetoresistance in different samples, the critical effect of LaAlO$_{3}$ stoichiometry is identified, indicating possible candidates that give raise to the magnetism. The authors thank Air Force Office of Scientific Research (grant {\#}FA9550-12-1-0441) for funding support. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T49.00005: Dependence of interfacial conduction on oxygen annealing in MBE-grown LaAlO3/SrTiO3 heterostructures Hao Zhang, Joseph Ngai, Charles Ahn, Christopher McMahon, David G. Hawthorn, J.Y.T. Wei The observation of interfacial metallicity in thin-film heterostructures of LaAlO3 (LAO) and SrTiO3 (STO) has sparked great interest in recent years. This metallicity has been attributed to electronic reconstruction induced by interfacial polar discontinuity [1]. However, the intrinsic oxygen variability of STO is also believed to influence the conduction of LAO/STO films [2], especially in films grown by pulsed laser deposition which can induce defects in STO [3]. To better understand the role of such defects, we study LAO films of varying thickness grown on STO by molecular beam epitaxy and post-annealed in oxygen. X-ray photoelectron spectroscopy is used to correlate the atomic valences with the conduction properties, in an effort to relate the interfacial electronic structure with the presence of oxygen vacancies. \\[4pt] [1] J. Mannhart \textit{et al.}, MRS Bull. 33, 1027 (2008)\\[0pt] [2] A. Kalabukhov \textit{et al.}, Phys. Rev. B 75, 121404 (2007)\\[0pt] [3] Y. Chen \textit{et al.}, Nano Letters 11, 4 3774 (2011) [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T49.00006: Transport Property Dependence on Surface Preparation Methods of LaAlO$_{3}$/SrTiO$_{3}$ Heterointerfaces J.G. Connell, O.B. Korneta, J. Nichols, S.S.A. Seo LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces have shown metallic, superconducting, insulating, and magnetic properties depending on their growth conditions. Here we show that the choice of substrate preparation method also affects the properties of the interface between LaAlO$_{3}$/SrTiO$_{3}$. Atomically flat SrTiO$_{3}$ (001) substrates have been prepared using the well-known buffered hydrofluoric acid (BHF) etching method and the deionized-water (DI-water) leaching method [1]. Epitaxial LaAlO$_{3}$ thin films then are deposited simultaneously via pulsed laser deposition. Metallic samples with $n_{s}$\textgreater 10$^{14}$ cm$^{-2}$ display little difference in carrier concentrations. However, less metallic samples with $n_{s}$\textless 10$^{13}$ cm$^{-2}$ demonstrate an order of magnitude difference in conducting carriers at low temperatures depending on the method of substrate preparation. This behavior is caused presumably by additional carriers provided by fluorine ions originating from the use of BHF in substrate preparation. These results indicate that the properties of oxide heterointerfaces are not only sensitive to deposition conditions, but also substrate preparation methods. \\[4pt] [1] J. G. Connell, B. J. Isaac, G. B. Ekanayake, D. R. Strachan, and S. S. A. Seo, Appl. Phys. Lett., \textbf{101}, 251607, (2012). [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T49.00007: Origin of Metal-Insulator Transition at the LaAlO$_{3}$/SrTiO$_{3}$ interface induced by ion beam irradiation Alexei Kalaboukhov, Tord Claeson, Pier Paolo Aurino, Nikolina Tuzla, Lunjie Zeng, Dag Winkler, Eva Olsson A quasi-two-dimensional electron gas appears when 4 or more unit cells of the LaAlO$_{3}$ film are deposited on a top of the TiO$_{2}$-terminated STO substrate. We show that it is possible to make the interface insulating using low-energy Ar$+$ ion-beam irradiation. The low energy Ar$+$ ions do neither etch the film below critical thickness nor create oxygen vacancies if the etching is terminated in time. The conductivity can be completely recovered by annealing under low oxygen pressure conditions. The restored interface shows strikingly similar electrical properties to the non-irradiated one and is also resistant to annealing at high oxygen pressure. High resolution transmission electron microscopy revealed that the difference between conducting and non-conducting interfaces is related to a change in stoichiometry of the LAO film where the La/Al ratio is 1.2 in the irradiated non-conducting areas while it is 1.0 in the conducting areas. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T49.00008: Time resolved 2nd harmonic generation at LaAlO$_{3}$/SrTiO$_{3}$ Interfaces Sanjay Adhikari, Chang-Beom Eom, Sangwoo Ryu, Cheng Cen Ultrafast spectroscopy can produce information of carrier/lattice dynamics, which is especially valuable for understanding phase transitions at LaAlO$_{3}$/SrTiO$_{3}$ interfaces. LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) are both associated with wide band gap, which allows deep penetration of commonly used laser wavelengths and therefore usually leads to overwhelming bulk signal background. Here we report a time resolved study of a 2$^{\mathrm{nd}}$ harmonic generation (SHG) signal resulting from impulsive below-the-band-gap optical pumping. The nonlinear nature of the signal enables us to probe the interface directly. Output of a home built Ti:Sapphire laser and BBO crystal were used to generate 30fs pulses of two colors (405nm and 810nm). The 405nm pulse was used to pump the LAO/STO interfaces, while 2$^{\mathrm{nd}}$ harmonics of the 810nm pulse generated at the interfaces was probed as a function of the time delay. Signals from samples with varying LAO thicknesses clearly correlates to the metal-insulator transition. Distinct time dependent signals were observed at LAO/STO interfaces grown on different substrates. Experiments performed at different optical polarization geometries, interface electric fields and temperatures allow us to paint a clearer picture of the novel oxide heterostructures under investigation. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T49.00009: Competing electronic ground states in (LaAlO$_{3})_{M}$/(SrTiO$_{3})_{N}$(111) and (LaAlO$_{3})_{M}$/(LaNiO$_{3})_{N}$(111) quantum wells David Doennig, Warren E. Pickett, Rossitza Pentcheva Complex oxide heterostructures exhibit a broad variability of functional properties and electronic states, not available in the bulk. Beyond the much studied (001)-oriented systems, here we highlight theoretical results on (111) perovskite superlattices with and without a polar discontinuity. Density functional theory calculations including an on-site Coulomb repulsion term (GGA$+U)$ reveal a rich set of competing ground states in (LaAlO$_{3})_{M}$/(SrTiO$_{3})_{N}$(111) [1] and (LaAlO$_{3})_{M}$/(LaNiO$_{3})_{N}$(111) superlattices ranging from spin, orbitally polarized, Dirac point Fermi surface to charge ordered flat band phases. For the bilayer ($N=$2), forming a buckled honeycomb lattice, a Dirac-point Fermi surface is obtained in both cases, while symmetry breaking leads to band gap opening with two inequivalent interfaces. Orbital reconstructions and metal-to-insulator transitions show a pronounced sensitivity on the thickness of the quantum well $N$ and in-plane strain. \\[4pt] [1] D. Doennig, W. E. Pickett, and R. Pentcheva, Phys. Rev. Lett. \textbf{111}, 126804 (2013). [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T49.00010: Electronic Structure, spin-orbit coupling and magnetotransport at the LaAlO$_3$/SrTiO$_3$ interface Daniel Kestner, Sumilan Banerjee, Mohit Randeria We study the LaAlO$_3$/SrTiO$_3$ interface using self-consistent solution of the Poisson and Hartree-Fock equations in a tight-binding framework. We go beyond the analysis of ref.[1] by modeling the non-linear dielectric properties of STO using a Landau-Ginzburg-Devonshire theory with parameters determined from bulk measurements. We show that it is essential to also include the lifting of the Ti t$_{2g}$ orbital degeneracy to match DFT results. This then allows us to investigate the density dependence of the electronic structure. We compare our results with the Lifshitz transition inferred from Hall data [2]. We calculate magneto-transport with an in-plane magnetic field and find a planar Hall effect and a magneto-resistance that oscillates with the magnetic field orientation due to the interplay of Zeeman and spin-orbit couplings. Finally, we comment on the nature of the spin-orbit coupling across the Lifshitz transition. \\[4pt] [1] M. Stengel, PRL 106, 136803 (2011); G. Khalsa, A.H. Macdonald, PRB 86, 125121 (2012); S. Y. Park, A. J. Millis, PRB 87, 205145 (2013).\\[0pt] [2] A. Joshua ${\it et al}$., Nature Comm.~3, 1129 (2012). [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T49.00011: Engineering Rashba interactions at perovskite interfaces Guru Khalsa, Byounghak Lee, Nicole Benedek, Mark Stiles, Allan MacDonald The broken inversion symmetry at surfaces and interfaces allows new spin-orbit interactions (Rashba interactions). Rashba interactions originate from changes in electronic structure due to displacements of the electron density and changes to metal-oxygen-metal bond angles [1]. While bond angle changes are not expected to be important in conventional semiconductor heterostructures, they may dominate in perovskites -- this difference is due to the increased ionic nature of the perovskite crystal. The possibility to control metal-oxygen-metal bond angles at perovskite interfaces or, in a more tunable way, with an electric field or other external perturbation provides a new~strategy for engineering perovskite heterostructures with large Rashba interactions. In this talk, we describe our calculations designed to guide the tailoring of Rashba interactions in perovskite heterostructures. We focus on (001) perovskite interfaces/surfaces, and discuss the role of structural distortions and manipulation of octahedral coordination. These calculations highlight the challenges in creating a large tunable Rashba interaction. \\[4pt] [1] Guru Khalsa, Byounghak Lee, and Allan H. MacDonald, Phys. Rev. B 88, 041302(R) (2013). [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T49.00012: Spin-orbit engineering of LaAlO$_3$/SrTiO$_3$ nanowires Patrick Irvin, Megan Kirkendall, Jeremy Levy, Sangwoo Ryo, Chang-Beom Eom LaAlO$_3$/SrTiO$_3$ heterostructures possess a tunable spin-orbit coupling that strongly influences other properties such as magnetism and superconductivity. Low-temperature transport experiments with nanowires created by conductive AFM show a sizeable non-zero resistance in the superconducting state. Here we present low-temperature magnetotransport of nanowires with 1D corrugations (e.g., triangular and rectangular lattices). We find that these ``zig-zag'' nanostructures possess a robust, fully superconducting state as compared to conventional ``straight'' nanowires. The most likely explanation relates to an effective spin-orbit interaction in which the effective magnetic fields of segments within the zig-zag ``unit cell'' cancel. We discuss implications for engineering spin-orbit couplings in superconducting nanostructures capable of supporting Majorana zero modes. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T49.00013: Construction of a first-principles-based second-principle method containing electronic and lattice degrees of freedom Pablo Garcia-Fernandez, Jacek Wojde\l, Jorge \'{I}\~niguez, Javier Junquera First principles simulations are limited in their application to small physical systems containing, in most realistic cases, a few hundred atoms. While not so restricted in size, second-principles simulations are usually focused on either electronic or lattice properties. However, both degrees of freedom are important in many physical problems and should be treated on the same footing. We present here an approach that accurately reproduces first-principles results for systems including reasonably localized electrons like transition-metal oxides or semiconductors. This scheme combines a reliable model potential for the lattice with a modified tight-binding method including both long-range electron-electron repulsions and short-range strong correlations. The interaction of lattice and electron degrees of freedom includes both electrostatics and short-range terms allowing the description of a wide range of phenomena. We illustrate the applicability of our method by tackling two difficult problems where the interaction between lattice and electrons is fundamental, such as the formation of polarons in bulk SrTiO$_3$ and the metallicity at the LaAlO$_3$/SrTiO$_3$ interface. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T49.00014: Surface Segregation of W doped in ZnO thin films Taku Suzuki, Yutaka Adachi, Noriko Saito, Minako Hashiguchi, Isao Sakaguchi, Naoki Ohashi, Shunichi Hishita We observed surface segregation of W (0.05-4 mol\%) doped in ZnO films by the annealing above 900 K. The segregation coefficient was related with the crystal quality of the film, where slower segregation occurred with the better crystalline film. From the structure analysis using low-energy He$^+$ ion scattering spectroscopy, we found that W occupies the substitutional site of Zn at the outermost surface of O-face ZnO(000$\overline{1}$) as a consequence of the segregation. On the other hand, we observed no sign indicating the occupation of W at a certain site in the ZnO lattice at the subsurface. Ultraviolet photoelectron spectroscopy (He I) on the ZnO surface segregated with W indicates that W is in the valence state of +6, and thus, the segregation of the W atom is most likely accompanied with two Zn vacancies. The ion beam mixing followed by the annealing of ZnO surface deposited with W provided the similar surface electronic structure to that of ZnO segregated with W. [Preview Abstract] |
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