Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V31: Focus Session: Oxide Interfaces II |
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Sponsoring Units: DMP GMAG Chair: Arunava Gupta, University of Alabama Room: Morial Convention Center 223 |
Thursday, March 13, 2008 11:15AM - 11:51AM |
V31.00001: Field effect tuning of superconductivity at the LaAlO$_{3}$/SrTiO$_{3}$ interface Invited Speaker: At interfaces between complex oxides, electronic systems with unusual properties can be generated [1]. As reported first by Ohtomo and Hwang [2], a highly mobile electron gas is formed at the interface between LaAlO$_{3}$ and SrTiO$_{3}$, two insulating dielectric perovskite oxides. It will be shown that the ground state of this system is superconducting [3]. The superconducting critical temperature is about 200mK. The field effect allows the normal state and superconducting state properties to be spectacularly tuned. The characteristics of the observed superconducting transitions are consistent with a superconducting sheet as thin as a few nm. \newline [1] ``When oxides meet face to face''. E. Dagotto, Science \textbf{318}, 1076 (2007) \newline [2] ``A high mobility electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface''. A. Ohtomo, H. Y. Hwang, Nature \textbf{427}, 423 (2004). \newline [3] ``Superconducting interfaces between insulating oxides''. N. Reyren, \textit{et al.}, Science\textbf{ 317}, 1196 (2007). [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V31.00002: Stabilization mechanisms at LaXO$_3$/SrTiO$_3$ (X=Ti, Al, V) Heterointerfaces R. Pentcheva, W. Pickett Multivalent transition metal ions that are being incorporated into oxide heterointerfaces offer more degrees of freedom to compensate the charge imbalance at the interface than is the case for conventional semiconductor interfaces. Density-functional theory calculations including a Hubbard U (LDA+U) have shown that a charge and orbitally ordered layer of Ti$^{3+}$ and Ti$^{4+}$ is formed at the interface (IF) between the Mott and the band insulator LaTiO$_3$ ($d^1$) and SrTiO$_3$ ($d^0$) [Phys. Rev. Lett. 99, 016802 (2007)] as well as between the two band insulators LaAlO$_3$ and SrTiO$_3$ [Phys. Rev. B. 74, 035112 (2006)]. Additional complexity relating to charge state and orbital occupation arises at the LaVO$_3$/SrTiO$_3$ interface between these compounds that are $d^2$ and $d^0$ respectively in the bulk: the electron-doped ...-TiO$_2$-LaO-... IF may promote a Ti$^{3+}$ charge state, the hole-doped ...-SrO-VO$_2$-... IF may encourage a V$^{4+}$ ion, or a metallic IF may result. We will present LDA+U predictions of charge states, orbital and spin order, and conducting behavior at these heterointerfaces, and contrast the results with the $d^1-d^0$ and $d^0-d^0$ interfaces mentioned above. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V31.00003: Charge accumulation in nonpolar perovskite quantum well sandwiched by polar Mott-insulating perovskites Cheng-Ching Wang, Sahu Bhagawan , Hongki Min, Wei-Cheng Lee, MacDonald Allan. H. We theoretically examine the possibility of having charge accumulation in the (LaTiO3)$_{n}$/Ba2VO4/(LaTiO3)$_{n}$ layered oxide quantum well system with polar barrier material and non-polar quantum well material using a LDA+U approach. The charge accumulation we find reflects electronic reconstruction which tends to occur near polar/nonpolar heterojunctions. We find enormous orbital reconstruction and both antiferromagnetic and ferromagnetic local in different planes. Lattice relaxation in the structure was allowed as a partial test of the robustness of LDA+U predictions for the properties of this system. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V31.00004: A First Principle Study of the LaAlO$_3$/SrTiO$_3$ Heterointerface Hanghui Chen, Sohrab Ismail-Beigi In order to understand the origin of the intriguing high mobility quasi two dimensional electron gas formed at the LaAlO$_3$/SrTiO$_3$(001) heterointerface, we carry out first principle calculations on the electronic structures and properties of complementary interface. The intrinsic polar properties are investigated and the average electronic potential increase by each LaAlO$_3$ layer is calculated, which can account for the recent observed experiment fact that the heterointerface is not metallic until the number of LaAlO$_3$ layers reaches a critical value. We also study the effects of different surface terminations of SrTiO$_3$ which surprisingly turn out to influence the electronic structure of the interface and so far have not been focused on in experiments. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V31.00005: Electrostatic doping in oxide heterostructures Jaekwang Lee, Alexander A. Demkov Recent experiments on perovskite heterostructures grown by molecular beam epitaxy or pulsed laser deposition suggest the possibility of creating high mobility two dimensional electron gas at the oxide/oxide interface. However, the origin of charge in these insulating materials is still not clear and deemed controversial. We report a first-principles study of SrTiO$_{3}$/LaAlO$_{3}$ heterostructures using density functional theory at the LDA+U level. We consider the energetics and electronic structure of the junction, while focusing on the role of electrostatics. Our results suggest that a complex balance of the crystal filed, Jahn-Teller effect, lattice dynamics and internal electric field result in the robust electrostatic doping for carefully chosen thickness of the polar oxide. We explore the possible uses of this effect in other oxide-based heterostructures. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V31.00006: Electronic and magnetic properties of two-dimensional electron gas formed at LaAlO$_{3}$/SrTiO$_{3}$ and LaTiO$_{3}$/SrTiO$_{3}$ interfaces Karolina Janicka, Julian Velev, Evgeny Tsymbal We perform first-principles electronic structure calculations to elucidate the electronic and magnetic properties of LaAlO$_{3}$/SrTiO$_{3}$ and LaTiO$_{3}$/SrTiO$_{3 }$superlattices. We find that TiO$_{2}$-terminated interfaces are $n$-type conducting which is consistent with experimental observations. In both heterostructures we find that charge resides in Ti conduction band and is localized within a few nanometers from the interface. This charge distribution is consistent with metal induced gap states in the gap of SrTiO$_{3}$ produced by the interfacial charge. In order to understand the magnetism of these oxide heterostructures, we performed spin-polarized calculations which reveal that this interface in a (LaAlO$_{3})_{3}$/(SrTiO$_{3})_{3}$ superlattice is magnetic with magnetic moment on the Ti$^{3+}$ atom of 0.2$\mu _{B}$. For thicker SrTiO$_{3}$ layers the magnetism decreases and eventually disappears because the electron gas spreads over more than one unit cell making the electron delocalized and violating the Stoner criterion for magnetism. The inclusion of electron correlations via the LDA+U approximation with U=5eV on Ti atoms makes the two-dimensional electron gas more localized and half-metallic and strengthens the interface magnetization. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V31.00007: Fabrication and characterization of complex oxide \textit{RE}NiO3/LaAlO3 superlattices M. Kareev, J.W. Freeland, J. Liu, B. Kirby, B. Keimer, J. Chakhalian Nowadays there has been growing interest to synthesis of atomically thin complex oxide superlattices which can result in novel electronic and magnetic properties at the interface. Here we report on digital synthesis of single unit cell nickel based heterostructures of \textit{RE}NiO3/LaAlO3 (\textit{RE} = La, Nd and Pr) superlattices on SrTiO3 and LaAlO3 by laser MBE. RHEED analysis, grazing angle XRD and AFM imaging have confirmed the high quality of the epitaxially grown superlattices. The magnetic and electronic properties of the superlattices have been elucidated by polarized X-ray spectroscopies, which show a non-trivial evolution of magnetism and charge of the LNO layer with increasing LNO layer thickness. The work has been supported by U.S. DOD-ARO under Contract No. 0402-17291. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V31.00008: Band gap engineering in digital manganite-titanate superlattices Xiaofang Zhai, Chandra Mohapatra, Amish Shah, Jian-Min Zuo, Zoran Popovic, Sashi Satpathy, Anand Bhattacharya, James Eckstein The electronic structure of short period digital superlattices of two different phases depends strongly on the superlattice architecture. We have fabricated digital superlattices of super cells consisting of N layers of LaMnO$_{3}$ and N layers of SrTiO$_{3}$, N=1, 2, 3, 8, and measured their optical conductivities to probe the way in which the electronic structure depends on N. We found the valence band structure systematically changes, as N decreases, from bulk like band gaps to interface modified band gaps. The temperature dependence of the derived Mn Jahn-Teller/Mott gap in large period superlattices is completely different from the small period, suggesting different spin ordering states reside in them. The electronic structure of two parent materials are coherently blended at the interface, and the spin ordering is strongly modified. This method of producing meta materials is a promising new way of material engineering. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V31.00009: Charge leakage and effective doping of atomically flat (LaMnO$_{3}$)$_{2n}$ / (SrMnO$_{3}$)$_{n}$ superlattice interfaces J.J. Kavich, J.W. Freeland, A. Bhattacharya, M.P. Warusawithana, J.N. Eckstein Atomically flat molecular beam epitaxy (MBE) grown interfaces in high quality digital superlattices provide a unique investigation of the role of substitutional disorder in complex oxide systems. Circularly polarized x-rays are used to measure the electronic and magnetic properties of the random alloy La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ as it is digitally ordered into discrete layers of insulating A-type antiferromagnetic LaMnO$_{3}$ (LMO) and G-type antiferromagnetic SrMnO$_{3}$ (SMO) with superlattice configurations of (LMO)$_{2n}$/(SMO)$_{n}$. The diffusion of carriers across the interface effectively dopes the interface region. The electronic and magnetic structure of the n=1 digital structure is nearly identical to the solid solution of the same doping showing that, on average, the charge distributes uniformly over the entire sample. The ferromagnetic insulating nature of the n = 5 superlattice suggests a finite lengthscale of the diffusion of carriers at the interface. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V31.00010: Structural Basis of Conduction at LaAlO$_{3}$-SrTiO$_{3}$ Interfaces Roy Clarke, D. Kumah, C. Cionca, P.R. Willmott, R. Herger, C.M. Schlepuetz, S.A. Pauli, D. Martoccia, B.D. Patterson, B. Delley, Y. Yacoby We present new results on the application of the x-ray phase retrieval method, Coherent Bragg Rod Analysis (COBRA), to heteroepitaxial interfaces in LaAlO$_{3}$ thin films grown on SrTiO$_{3}$ substrates, a system known to form an interfacial quasi-2D electron gas. We observe a dilated, mixed-valence interface which modifies the electronic band structure, lowering the minimum of the conduction band below the Fermi level and thereby rendering the dilated interface conducting. In particular the COBRA measurements reveal the formation of an interfacial La,SrTiO$_{3}$ layer with an accumulation of trivalent Ti at the interface which is responsible for the lattice dilation and minimizes the electrostatic energy at the TiO$_{2}$-terminated SrTiO$_{3}$ substrate surface. The work presented here establishes a structural basis for the formation of the conducting interface. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V31.00011: Signatures of Enhanced Ordering Temperatures in Digital Superlattices of (LaMnO3)m/(SrMnO3)2m A. Bhattacharya, S. J. May, L. Robertson, P. Ryan, X. Zhai, J. N. Eckstein, S. D. Bader Digital superlattices of (LaMnO3)m/(SrMnO3)2m , for integers m = 1 - 4 have been synthesized with ozone assisted oxide MBE. These superlattices are ordered analogues of La1/3Sr2/3MnO3, an antiferromagnetic insulator. Signatures of enhanced ordering temperatures have been observed in transport and magnetic measurements. We interpret this in terms of the effects of enhanced A-site order on dx2-y2 orbital occupancy. We shall also present evidence in support of our findings with x-ray and neutron scattering. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V31.00012: Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature Cheng Cen, Jeremy Levy, Stefan Thiel, German Hammerl, Christof W. Schneider, Jochen Mannhart, Kristopher E. Anderson, C. Stephen Hellberg We report the creation and erasure of nanoscale conducting regions at the interface between two insulating oxides, LaAlO$_{3}$ and SrTiO$_{3}$. Using voltages applied by a conducting atomic force microscope (AFM) probe, the buried LaAlO$_{3}$/SrTiO$_{3}$ interface is locally and reversibly switched between insulating and conducting states. Persistent field effects are observed using the AFM probe as a gate. Patterning of conducting lines with widths $\sim $3 nm, as well as arrays of conducting islands with densities $>$10$^{14}$/in$^{2}$, are demonstrated. The patterned structures are stable for $>$24 hours at room temperature. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V31.00013: Theory of the Switchable Interfacial Metal-Insulator Transition in LaAlO$_3$ thin films on SrTiO$_3$ C. Stephen Hellberg, Kristopher Andersen, Cheng Cen, Jeremy Levy, Stefan Thiel, German Hammerl, Christof Schneider, Jochen Mannhart Although bulk LaAlO$_3$ and SrTiO$_3$ are both insulating, an electron gas can form at the interface between these compounds. For up to 2 unit cells (uc) of LaAlO$_3$, the interface remains insulating, while for 4 uc and beyond, it is metallic. For 3 uc films, the interface can be reversibly switched from metallic to insulating using voltages applied by an atomic force microscope (AFM). We present first- principles density functional calculations of LaAlO$_3$ films on SrTiO$_3$. We show the AFM induced interfacial metal- insulator transition can be explained by the formation and annihilation of oxygen vacancies on the LaAlO$_3$ surface. [Preview Abstract] |
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