Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q13: Focus Session: Complex Oxide Interfaces - LaAlO3/SrTiO3 II |
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Sponsoring Units: DMP Chair: Sohraib Ismail-Beigi, Yale University Room: 007D |
Wednesday, March 4, 2015 2:30PM - 3:06PM |
Q13.00001: Combining Mott insulators and ferroelectrics Invited Speaker: Agn`es Barthelemy We investigated the properties of high quality heterostructures consisting of a Mott insulator, (Ca,Ce)MnO$_{3}$ (CCMO) and a ferroelectric, BiFeO$_{3}$ in the supertetragonal phase (T-BFO) materials. In particular, we studied the electrical response induced by ferroelectric switching in both planar and vertical devices. In the planar geometry, we used thick T-BFO films to explore the possibility to electrically tune the properties of the CCMO compound in a ferroelectric field-effect device with a CCMO channel and a T-BFO gate. Upon polarization reversal of the T-BFO ferroelectric gate, the CMO channel exhibits a nonvolatile resistance switching by a factor of 4 around room temperature, and up to a factor of 10 at 200 K [1]. We also studied Ferroelectric tunnel junctions (FTJs) composed of an ultrathin ferroelectric tunnel barrier of T-BFO sandwiched between a CCMO electrode and a Co/Pt counter-electrode. In these junctions, the tunneling current significantly depends on the orientation of the ferroelectric polarization, resulting in large electroresistance enabling a simple nondestructive readout of the ferroelectric state [2]. FTJs based on ultrathin T-BFO films show fast, stable multistate switching with very high resistance ratios of up to four orders of magnitude [3]. Combined piezoresponse force microscopy (PFM) and electrical measurements give a clear correlation between ferroelectric domain configurations and multiple resistance states They also provide insights into the switching dynamics in response to trains of nanosecond pulses. Additionnaly, we demonstrated the very good endurance and retention characteristics of these FTJs [4]. \\[4pt] [1] H Yamada et al., Scientific Reports 3, 2834 (214)\\[0pt] [2] V. Garcia et al, Nature 460, 81 (2009)\\[0pt] [3] H Yamada et al., ACS Nano 7, 53855390 (2013)\\[0pt] [4] S. Boyn et al., Appl. Phys Lett. 104, 052909 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q13.00002: First-principles many-body investigation of $\delta$-doped titanates Frank Lechermann, Michael Obermeyer Studying oxide heterostructures provides the possibility for exploring novel composite materials beyond nature's original conception. In this respect, the doping of Mott-insulating distorted-perovskite titanates such as LaTiO$_3$ and GdTiO$_3$ with a single SrO layer gives rise to a very rich correlated electronic structure [1]. A realistic superlattice survey by means of the charge self-consistent combination of density functional theory (DFT) with dynamical mean-field theory (DMFT) reveals layer- and temperature-dependent multi-orbital metal-insulator transitions. In [001] stacking, an orbital-selective metallic layer at the interface dissolves via an orbital-polarized doped-Mott state into an orbital-ordered insulating regime beyond the two conducting TiO$_2$ layers. We find large differences in the scattering behavior within the latter. Breaking the spin symmetry in $\delta$-doped GdTiO$_3$ results in blocks of ferromagnetic itinerant and ferromagnetic Mott-insulating layers which are coupled antiferromagnetically. \\[4pt] [1] F. Lechermann and M. Obermeyer, arXiv:1411.1637 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q13.00003: Towards enhancing two-dimensional electron gas quantum confinement effects in perovskite oxide heterostructures Kesong Yang, Safdar Nazir, Maziar Behtash The two-dimensional electron gas (2DEG) in LaAlO$_{3}$/SrTiO$_{3}$ perovskite-oxide heterostructure has attracted much attention because of its potential applications in nanoelectronic devices. A 2DEG has two landmark characters: strong charge confinement in the third dimension and high electron conductivity in two dimensions. In an ideal 2DEG system, electrons can move freely along the interface but are tightly confined in the $c$-axis that is perpendicular to the interface. Nevertheless, the actual electron gas in the SrTiO$_{3}$-based perovskite heterostructures is extended a few nanometers along the c-axis into the SrTiO$_{3}$ substrate, and thus they are also called as quasi-2DEG. Actually, it is a problem of both fundamental and practical interest to achieve an ideal 2DEG via enhancing the lateral quantum confinement effects. By using first-principles electronic structure calculations, herein we proposed two possible approaches to enhance the quantum charge confinement effects by confining the electron gas within one single atomic layer in the perovskite oxide heterostructure. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q13.00004: Two-dimensional electron gas at the epitaxial alumina/SrTiO$_{3}$ interface: control of oxygen vacancies Kristy Kormondy, Agham Posadas, Thong Ngo, Sirong Lu, Nicholas Goble, Jean Jordan-Sweet, Xuan Gao, Martha McCartney, David Smith, John Ekerdt, Alexander Demkov A highly interesting application for SrTiO$_{3}$ involves the formation of a high mobility two-dimensional electron gas (2DEG) at the oxide/oxide interface. We report on the conducting layer formed at the crystalline $\gamma $-alumina/SrTiO$_{3}$ (STO) interface which is attributed to oxygen vacancies. We describe the structure of thin $\gamma $-alumina layers deposited by molecular beam epitaxy on STO (001), as determined by reflection-high-energy electron diffraction, x-ray diffraction, and high-resolution electron microscopy. \textit{In-situ} x-ray photoelectron spectroscopy was used to confirm the presence of oxygen vacancies at the interface. Electrical characterization indicates a higher sheet resistance for lower deposition temperature. A maximum electron Hall mobility of 3100 cm$^{2}$V$^{-1}$s$^{-1}$ at 3.2 K and room temperature mobility of 22 cm$^{2}$V$^{-1}$s$^{-1}$ are measured. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q13.00005: Optical properties of transition metal oxide quantum wells Alexander Demkov, Miri Choi, Matthew Butcher, Cesar Rodriguez, Qian He, Agham Posadas, Albina Borisevich, Stefan Zollner, Chungwei Lin, Elliott Ortmann We report on the investigation of SrTiO$_{\mathrm{3}}$/LaAlO$_{\mathrm{3}}$ quantum wells (QWs) grown by molecular beam epitaxy (MBE) on LaAlO$_{\mathrm{3}}$ substrate. Structures with different QW thicknesses ranging from two to ten unit cells were grown and characterized using x-ray photoemission spectroscopy, reflection high-energy electron diffraction (RHEED), scanning transmission electron microscopy (STEM). Optical properties (complex dielectric function) were measured by spectroscopic ellipsometry (SE) in the range of 1.0 eV to 6.0 eV at room temperature. We observed that the absorption edge was blue-shifted by approximately 0.39 eV as the STO quantum well thickness was reduced to two unit cells (uc). Density functional theory and tight-binding are used to model the optical response of these heterostructures. Our results demonstrate that the energy level of the first sub-band can be controlled by the QW thickness in a complex oxide material. We acknowledge support from Air Force Office of Scientific Research (FA9550-12-10494). [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q13.00006: Polar-nonpolar Oxide heterostructures for photocatalysis Hongli Guo, Jin Zhao, Wissam Saidi The discovery of two-dimensional electron gas (2DEG) at the interface of polar LaAlO$_{3}$ (LAO) and non-polar SrTiO$_{3}$ (STO) open the research field of layered oxide heterostructures. In this study, we propose new application of oxide heterostructures for photocatalysis. We take a sandwich-like heterostructure STO/LAO/STO as an example and prove it to be a promising photocatalyst which is active for near-infrared light. Because the sandwiched LAO is polarized and generates a build-in electrostatic field, the valance band and conduction band locates on two opposite STO surfaces. First principles calculations prove that the band gap is reduced and the absorption of near-infrared to visible light is improved distinctly. Simultaneously, the build-in electric field in LAO accelerates the electrons and holes into opposite directions, preventing the recombination, and generates an electron doped surface and a hole doped STO surface, which could be used for H$_{2}$O reduction and oxidation separately. Our study gives a new perspective into the applications of oxide heterostructures in solar energy conversion. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q13.00007: Electronic Transport Properties of the PrAlO$_3$/SrTiO$_3$ Interface: Effects of Oxygen Pressure Shirin Mozaffari, Mark C. Monti, Samaresh Guchhait, Jeremy W. Paster, Daniel M. Tennant, John T. Markert We explored the electronic and magnetic behavior of epitaxial PrAlO$_3$ films on TiO$_2$-terminated SrTiO$_3$ (PAO/STO) substrates grown by pulsed laser deposition at various oxygen pressures. We report structural (x-ray and AFM), electronic (van der Pauw resistivity, magnetoresistance (MR), and Hall effect), and magnetic data for PAO films grown in 10$^{-3}$--10$^{-6}$ torr O$_2$. Resistivity data exhibit metallic behavior from 300 K down to 100--150 K (75 K; 40 K) for the interface grown in 10$^{-3}$ (10$^{-4}$; 10$^{-5}$) torr O$_2$, and semiconducting behavior below that. One 10$^{-3}$ torr O$_2$ interface shows typical behavior for current parallel to atomic terraces, and a resistance anomaly in the range 50--100 K for current perpendicular to step edges. MR data for all 10$^{-3}$--10$^{-4}$ torr O$_2$ samples show a small ($\le$0.5\%) positive MR at low fields, and a larger negative MR (2--30\%) at high fields; for 10$^{-5}$ torr O$_2$, the MR is positive up to 9 tesla. Sheet resistivity for the 10$^{-6}$ torr O$_2$ interface is anomalously low, suggesting a thick conducting layer. Hall effect data exhibit several variations in the carrier density. We discuss these data considering intrinsic charge transfer, oxygen vacancies and interstitials, and cation interdiffusion. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q13.00008: Orbital order and effective mass enhancement in $t_{2g}$ two-dimensional electron gases John Tolsma, Alessandro Principi, Marco Polini, Allan MacDonald It is now possible to prepare d-electron two-dimensional electron gas systems that are confined near oxide heterojunctions and contain $t_{2g}$ electrons with a density much smaller than one electron per metal atom. I will discuss a generic model that captures all qualitative features of electron-electron interaction physics in $t_{2g}$ two-dimensional electron gas systems, and the use of a GW approximation to explore $t_{2g}$ quasiparticle properties in this new context. $t_{2g}$ electron gases contain a high density isotropic light mass xy component and low-density xz and yz anisotropic components with light and heavy masses in orthogonal directions. The high density light mass band screens interactions within the heavy bands. As a result the wave vector dependence of the self-energy is reduced and the effective mass is increased. When the density in the heavy bands is low, the difference in anisotropy between the two heavy bands favors orbital order. When orbital order does not occur, interactions still reshape the heavy-band Fermi surfaces. I will discuss these results in the context of recently reported magnetotransport experiments. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q13.00009: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q13.00010: Non-Local Signal in Quasi-2DEG of LAO/STO Mi-JIn Jin, Seon Young Moon, Vijayakumar Modepalli, Junhyeon Jo, Jungmin Park, Seung-Hyub Baek, Jung-Woo Yoo Electron gas arizen at the insulating oxide interfaces exhibits high electron mobility, tunable carrier densities and related unique behaviors such as coexistence of superconductivity and ferromagnetism, Kondo resistance, etc. Itinerant electrons at the oxide hetero-interface are predicted to have long spin diffusion length, while they are under the relatively strong Rashba-type spin orbit coupling due to inversion symmetry breaking. We studied non-local spin signal induced by spin orbit coupling with additional gate-controlled Rashba field in quasi-2DEG of LaAlO$_{3}$/SrTiO$_{}$ (LAO/STO) interface. We fabricated simple hall-bar like geometry to measure non-local signal with the variation of channel length (2 $\sim$ 10$\mu$m). Cleaned sample was patterned using e-beam lithography and reactive ion etching followed by oxygen treatment to anneal out oxygen vacancies. When an electric current flows one line of the hall bar structure, spin orbit coupling will induce the current flow away from the source current channel via spin hall and inverse spin hall effects. The non-local signals were studied under different angles of magnetic field and the variation of applied gate voltage. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q13.00011: Contact resistance tuning at metal / Nb:SrTiO$_3$ interfaces using LaAlO$_3$ interlayers Hisashi Inoue, Adrian Swartz, Takashi Tachikawa, Yasuyuki Hikita, Harold Hwang SrTiO$_3$ (STO) exhibits coexistence of high mobility electrons and possible unconventional superconductivity (SC) [1]. Transition metal (TM) contacts to epitaxial insulator/Nb doped STO (NSTO) structures are prototypically used to probe the electronic structure (ES) of n-type STO in tunneling experiments. However, the field dependent permittivity in STO at low temperatures makes it difficult to probe ES when the barrier height (BH) is large [2]. We show that the contact resistance $R_c$ across TM/NSTO interfaces (IF) can be effectively tuned by inserting a thin LaAlO$_3$ (LAO) interlayer (IL) between the TM and NSTO. Change of IL thickness from 0 to 2 u.c. in Co/LAO/NSTO (100) structures results in systematic reduction of $R_c$ by orders of magnitude, as evidenced by a transition from Schottky-type rectifying to nearly ohmic current-voltage curves. This is because the polar nature of the LAO surface, generating an IF dipole IL, lowers the Schottky BH. This is a useful method to optimize $R_c$ for tunneling experiments in doped STO with possible applications for SC and spintronics. It is also important that this effect fails for $ex$-$situ$ deposited TM and we discuss the distinction. [1]C. Richter et al., Nature 502, 528 (2013) [2]T. Susaki et al., Phys. Rev. B 76, 155110 (2007) [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q13.00012: Spin injection and spin-charge conversion in LaAlO$_3$/SrTiO$_{3}$ Manuel Bibes, Edouard Lesne, Juan Carlos Rojas Sanchez, Simon Oyarzun, Yu Fu, Nicolas Reyren, Mathieu Jamet, Eric Jacquet, Agnes Barthelemy, Jean-Marie George, Albert Fert, Henri Jaffres, Laurent Vila The perovskite oxides family provides materials to efficiently generate and control spin polarized currents using respectively half-metallic ferromagnets such as mixed-valence manganites or ferroelectrics and multiferroics. More recently channel materials to transport spin information have also emerged. These include the LaAlO$_{3}$/SrTiO$_{3}$ two-dimensional electron system (2DES) which, in addition, possesses a gate-tunable spin-orbit coupling. A limitation of this system is however the minimum LaAlO$_{3}$ thickness of 4 uc required for 2DES formation. In this presentation we will show that this thickness can be reduced if the LaAlO$_{3}$ is capped by appropriate metals. We will also present different approaches to inject spins in these engineered LaAlO$_{3}$/SrTiO$_{3}$ 2DES and introduce detection schemes taking advantage of efficient spin-charge conversion via interfacial spin-orbit effects. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q13.00013: Photo-induced Nanopattern Formation on Polarity Patterned Lithium Niobate with ZnO-Modified Surfaces Manpuneet Kaur, Xingye Wang, Brianna Eller, Robert Nemanich This research is focused on modifying the surface of polarity patterned lithium niobate (PPLN) templates with ultra thin layers of ZnO. Photo-induced nanopattern formation is employed to discern the effects of thin ZnO on PPLN. The spontaneous polarization of ZnO is intended to be used to enhance the photo-induced transport of electrons to the surface to reduce Ag$+$ to Ag nanoparticles. The ZnO thin films were deposited by plasma enhanced atomic layer deposition (PEALD) at 150 C with 0.2 nm/cycle. Photo-induced Ag nanopatterns were deposited on bare PPLN and 1, 2, 3 and 10 nm ZnO-PPLN heterostructures, immersed on an aqueous AgNO3 solution and illumination with 254 nm UV light. The photo-induced deposition of 1nm ZnO/PPLN results in enhanced Ag nanoparticle formation at domain boundaries. The positive domain selectivity is not observed on 2nm ZnO/PPLN templates, and the deposition becomes the same on both domains. The nanoparticle patterns were not evident for ZnO films thicker than 3nm. The amorphous structure of thick ZnO on PPLN tends to reduce the effect of the ZnO polarization. The effect of polarity patterned thin PEALD ZnO films is discussed to understand photo-induced electron transfer and AgNO3 reduction on ZnO-PPLN heterostructures. [Preview Abstract] |
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