Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y30: Focus Session: LAO/STO Interfaces |
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Sponsoring Units: GMAG DMP Chair: Anand Bhattacharya, Argonne National Laboratory Room: 334 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y30.00001: Coulomb Catastrophe and the Origin of the Sheet Carrier Density at the n-type LaAlO$_3$/SrTiO$_3$ interface: What do band calculations tell us? Zoran Popovic, Sashi Satpathy, Richard Martin Transport measurements of the two-dimensional electron gas (2DEG) at the intrinsic n-type LaAlO$_3$/SrTiO$_3$ interface have found a density of carriers much lower than expected from the ``Coulomb catastrophe" arguments. From a detail density-functional study, we suggest how this discrepancy may be reconciled. We find that electrons occupy multiple subbands at the interface leading to a rich array of transport properties. Some electrons are confined to a single interfacial layer and susceptible to localization, while others with small masses and extended over several layers are expected to contribute to transport. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y30.00002: Quantum nature of two-dimensional electron gas confinement at LaAlO$_{3}$/SrTiO$_{3}$ interfaces Karolina Janicka, Julian Velev, Evgeny Tsymbal Replace this text with your abstract body. The discovery of highly conducting interface between two insulating oxides LaAlO$_{3}$ and SrTiO$_{3}$ has attracted significant interest due to possible applications in all-oxide electronic devices. The two-dimensional electron gas (2DEG) formed at LaAlO$_{3}$/SrTiO$_{3}$ interfaces exhibits extremely high mobility and high density of carriers. Stimulated by this discovery we perform density functional calculations to understand the mechanism controlling the confinement width of the two-dimensional electron gas (2DEG) at LaAlO$_{3}$/SrTiO$_{3}$ interfaces. We find that the 2DEG confinement can be explained by the formation of metal induced gap states (MIGS) in the band gap of SrTiO$_{3}$. These states are formed as the result of quantum-mechanical tunneling of the charge created at the interface due to electronic reconstruction. The penetration depth of the MIGS into the insulator is controlled by the lowest-decay-rate evanescent states of SrTiO$_{3}$, as determined by its complex band structure. Our calculations predict that the 2DEG is confined in SrTiO$_{3}$ within about 1 nm at the interface. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y30.00003: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y30.00004: Effect of Stoichiometry on the Two-Dimensional Electron Gas at the LaAlO$_{3}$/SrTiO$_{3}$ Interface Grown by MBE. Maitri Warusawithana, Charles Brooks, Darrell Schlom, Stefan Thiel, Jochen Mannhart, Nicolas Reyren, Andrea Caviglia, Stefano Gariglio, Jean-Marc Triscone The discovery of a quasi 2-dimensional electron gas (q2-DEG) at the interface between SrTiO$_{3}$ and LaAlO$_{3}$ has enabled a number of exciting developments. So far this q2-DEG has been observed only in films grown by pulsed-laser deposition, which raised a question as to whether this manifestation has a connection with defects that result from the dynamics of the growth scheme employed. We find that a q2-DEG can also be obtained using the more gentle growth technique, molecular-beam epitaxy, and that control of the stoichiometry of the LaAlO$_{3}$ layer is key to its existence. Small changes in the composition of the LaAlO$_{3}$ layer affect the conductivity at the heterointerface. With appropriate stoichiometry the electron gas transitions into a superconducting state below $\sim $200 mK. Interesting possibilities that stem from these findings of composition control on conductivity and the ability to obtain this q2-DEG under the framework of molecular-beam epitaxy will be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y30.00005: Two Dimensional Electron Gas Formed at Inverted SrTiO3-LaAlO3 Interface Mao Zheng, Brian Mulcahy, Xiaofang Zhai, James Eckstein We have grown and tested inverted 2DEG structures consisting of SrTiO3 layers grown on top of thick LaAlO3 films. By engineering the layering, the interface can be made n-type. Contact to the 2DEG is relatively easy in this geometry since the current does not have to pass through the high band gap LAO layer. We have obtained a 2-D carrier density of 2x10\^{}14 electrons/cm\^{}2 at room temperature along with a mobility of 10 cm\^{}2/Vs. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y30.00006: Electrostatic doping in oxide heterostructures. Alexander A. Demkov, Jaekwang Lee, Na Sai Recent experiments on perovskite heterostructures grown by methods ranging from molecular beam epitaxy to pulsed laser deposition suggest the existence of two dimensional electron gas of high mobility at the oxide/oxide interface, and even a possibility of a superconducting state. Both p-type and n-type interfaces have been reported. However, the origin of charge in these insulating materials is still under debate. We report a first-principles study of several heterostructures where we employ the internal filed in a polar oxide LaAlO$_{3}$ to demonstrate the possibility of the electrostatic doping, an effect similar to a well known polar catastrophe in e.g., III-V semiconductors. $_{ }$We use density functional theory at the LDA+U level. We mainly focus on the electronic structure of the oxide/oxide junctions. The results of our calculations suggest that once the critical thickness of the aluminate layer is reached the internal electric field is sufficient to produce the electrostatic doping. We will discuss simple estimates for the temperature of the superconducting transition and the role of oxygen-related defects such as vacancies in the electronic structure and thermodynamic stability of these fascinating oxide structures. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y30.00007: Nanoscale analysis of high-mobility electron gases at SrTiO$_{3}$ interfaces and surfaces Invited Speaker: Electronic reconstructions or defects localized next to an interface between two oxides may lead to dramatic modifications of their physical properties. One intriguing example of such phenomena is the formation of high-mobility two-dimensional electron gases (2DEG) at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ (STO), two insulating dielectric perovskite oxides. To characterize crucial aspects of these 2DEG such as the spatial extension as well as carrier density profiles, sophisticated techniques are required. Here I explain how we used the synergetic combination of different advanced characterization tools including depth-resolved positron annihilation spectroscopy, conductive-tip atomic force microscopy [1], electron energy loss spectroscopy [2] or low-temperature high-magnetic field measurements [3, 4], to characterize with nanometric space resolution high-mobility electron gases at STO interfaces and surfaces. Our results emphasize the relevance of using interface/surface characterization tools that resolve spatially the physical properties and also detect spatial tiny changes of stoichiometry at the interface/surface of complex oxide structures. \\[4pt] [1] M. Basletic et al., Nature Materials, 7, 621 - 625 (2008) \\[0pt] [2] J.-L. Maurice et al., Europhys. Lett. 82, 17003 (2008) \\[0pt] [3] G. Herranz et al., Phys. Rev. Lett. 98, 216803 (2007) \\[0pt] [4] G. Herranz et al., Phys. Rev. B 73, 064403 (2006) [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y30.00008: Band offsets between SrTiO$_3$ and LaAlO$_3$ Kristopher E. Andersen, C. Stephen Hellberg Although separately SrTiO$_3$ and LaAlO$_3$ are both band insulators, together a highly mobile, quasi-2D electron gas can form at their interface. Several mechanisms have been proposed to produce this electron gas, including the electrostatic divergence within LaAlO$_3$. A critical property in understanding this divergence is the valence band offset between SrTiO$_3$ and LaAlO$_3$. However, because the electrostatic potential can diverge, it is not clear \emph{where} the valence band offset should be defined; an issue that may affect experimental band offset measurements. In this talk, the band offsets between SrTiO$_3$ and LaAlO$_3$ are presented within the framework of density functional theory. Both the layer projected density of states and macroscopically averaged potential are used to find the valence band offset for thin films and multilayers. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y30.00009: Transport and band profiles of MBE grown LaAlO$_{3}$/ SrTiO$_{3}$ Yaron Segal, J.H. Ngai, J.W. Reiner, F.J. Walker, C.H. Ahn Previously reported properties of the LaAlO$_{3}$/SrTiO$_{3}$ system showed strong dependence on growth parameters. This makes it difficult to identify the key physical quantities. To isolate the role of oxygen content, we grew LaAlO$_{3}$ films on SrTiO$_ {3}$ using MBE. The thermal evaporation of MBE minimizes potential damage to the substrate and interface. The samples were then annealed at low temperature for a prolonged time, thus raising the oxygen content without damaging the structural integrity of the film. Transport and X-ray photoemission measurements were performed before and after annealing on films grown on both terminations of the substrate. Transport measurements show the in-plane conductivity decreases by several orders of magnitude upon annealing. A dependence on film thickness appears in certain oxygen content regimes, where it can be interpreted as being controlled by oxygen diffusion. Photoemission measurements reveal an intriguing band structure in the LaAlO$_{3}$ film. The termination of the SrTiO$_ {3}$ determines the direction of apparent band bending, for which we discuss possible models. Our results imply that while transport behavior of this system is dominated by oxygen diffusion, the atomic details of the interface have a substantial impact on band structure. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y30.00010: Resonant Anomalous Synchrotron X-Ray Studies of LaAlO$_{3}$ Films on SrTiO$_{3}$(001) Dillon Fong, Tim Fister, Marie-Ingrid Richard, Stephan Hruszkewycz, Jeffrey Eastman, Paul Fuoss, Sung Seok Seo, Ho Nyung Lee The high conductivity present at the interface between LaAlO$_{3}$ and TiO$_{2}$-terminated SrTiO$_{3}$(001) has been attributed to an electronic reconstruction [1] or atomic intermixing [2], both induced by the polar discontinuity. LaAlO$_{3}$ films with thicknesses equal to or thinner than a critical thickness (three unit cells [3]), however, can maintain the interface dipole, and no reconstruction (electronic or atomic) is expected. In this study, we employ resonant anomalous x-ray scattering at the Ti K-edge to investigate the structure and properties of the LaAlO$_{3}$ films both above and below the critical thickness. For films thicker than the critical thickness, an interfacial layer is observed to form. The structure and chemical properties of the interfacial layer as determined by both resonant scattering and x-ray spectroscopy will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y30.00011: Oxygen vacancy, charge doping, and polarization screening in LaAlO$_3$/SrTiO$_3$ interface Yun Li, Jaejun Yu A high mobility electron gas has been observed in the $n$-type (TiO$_2$)/(LaO) interface between two insulators: non-polar SrTiO$_{3}$ (STO) and polar LaAlO$_{3}$ (LAO) and the mechanism of conductivity and dimensionality of electron gas at the interface have been intensively investigated in various experiments. There are two mechanisms suggested for the observed conductivity at the interface: electronic reconstruction and oxygen vacancy. We carried out density-functional-theory calculations to investigate the distribution of electron carriers for the $n$-type LAO/STO interfaces with and without oxygen vacancy. When no oxygen vacancy is present, the critical thickness of LAO film for conducting interface was found to be consistent with experiments. The induced carrier density at the interface without oxygen vacancy turns out to be an order of magnitude smaller than the one expected from the electronic reconstruction. This implies that the lattice polarization takes a significant role in charge screening. On the other hand, when oxygen vacancies are present, the vacancy-induced states are found to affect the carrier doping as well as the screening of polar electric field of LAO film. From the results, we propose that the upper limit of carrier doping should be 0.375 electrons per unit cell. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y30.00012: The effect of strain on the SrTiO3/LaAlO3 heterointerface C. Tyler Diggans, Kristopher E. Andersen, C. Stephen Hellberg Recent experiments have shown that it is possible to form a highly mobile, quasi-2D electron gas at the interface between SrTiO$_{3}$ and LaAlO$_{3}$. Although the origin of this effect is still debated, there is growing consensus that under certain growth conditions (e.g. high oxygen pressures) it is caused by the diverging electric potential within LaAlO$_{3}$ -the so-called polar catastrophe. One aspect of this system that has not been previously considered is the electric polarization of SrTiO$_{3}$, which can be effectively tuned by strain. This polarization can partially or fully compensate the diverging LaAlO$_{3}$ potential, and must be considered if SrTiO$_{3}$/LaAlO$_{3}$ is to be grown on a substrate such as Si. In this talk, an electrostatic model is presented to show how the polar catastrophe is modified by polarization within strained SrTiO$_{3}$. This model is supported by first-principles calculations on strained SrTiO$_{3}$/LaAlO$_{3}$ multilayers of varying thickness. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y30.00013: Polarization effects and the source of electrons in two-dimensional electron gas at insulating oxide heterointerfaces H.W. Jang, D.A. Felker, C.M. Folkman, D.L. Profitt, S.H. Baek, M.S. Rzchowski, C.B. Eom, K. Janicka, Y. Wang, M.K. Niranjan, E.Y. Tsymbal The discovery of a two-dimensional electron gas (2DEG) at the heterointerface between insulating perovskite oxides LaAlO$_{3}$ and SrTiO$_{3}$ has stimulated intensive theoretical and experimental studies on the origin of the 2DEG. Empirically, the electron density has been found to be strongly dependent on the oxygen partial pressure during growth and the thickness of the polar LaAlO$_{3}$ layer. Understanding and controlling the source of the electrons in 2DEGs at oxide heterointerfaces is important for the optimization of their performance. In this talk, we will discuss electrical properties of heterointerfaces between various polar perovskites and SrTiO$_{3}$ fabricated by pulsed laser ablation. The importance of built-in electric fields due to polarization-induced charges, bandgap energy, and oxygen vacancies in the polar layers is highlighted using a simple analysis based on electrostatics. The experimental Hall data are then compared with the theoretical result from first-principles calculations. [Preview Abstract] |
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