Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B37: Focus Session: Complex Oxide Thin Films -- LaAlO3/SrTiO3 Interfaces |
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Sponsoring Units: DMP GMAG Chair: Chris Bell, University of Tokyo Room: E147-E148 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B37.00001: Effect of stoichiometry on the LaAlO$_{3}$/SrTiO$_{3}$ 2-D electron gas grown by MBE M.P. Warusawithana, A.A. Pawlicki, T. Heeg, D.G. Schlom, C. Richter, S. Paetel, J. Mannhart, M. Zheng, B. Mulcahy, J.N. Eckstein, W. Zander, J. Schubert We find that through careful control of the stoichiometry in molecular-beam epitaxy grown LaAlO$_{3}$/SrTiO$_{3}$ samples, a 2-dimensional electron gas occurs at the interface between the two insulating oxides as extensively reported in samples grown by pulsed-laser deposition. Our results eliminate many extrinsic effects suggested as possible mechanisms of conductivity and are consistent with the polar catastrophe mechanism being responsible for the conductivity in our MBE-grown samples. We further show that the cation stoichiometry of the LaAlO$_{3}$ layer is key to the existence of the 2-dimensional electron gas and that a La/Al ratio less than or equal to 0.97 $\pm $ 0.03 is a necessary condition to obtain a conducting interface in this system. [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B37.00002: Transport properties of MBE grown LaAlO$_{3}$/SrTiO$_{3}$ interfaces and the effect of stoichiometry A.A. Pawlicki, M.P. Warusawithana, T. Heeg, D.G. Schlom, C. Richter, S. Paetel, J. Mannhart, M. Zheng, B. Mulcahy, J.N. Eckstein, W. Zander, J. Schubert We report on electronic transport properties on MBE grown LaAlO$_{3}$/SrTiO$_{3}$ 2-dimensional electron gas samples measured at cryogenic temperatures in perpendicular magnetic fields. We find that the electronic properties of this system are strongly dependent on the stoichiometry of the LaAlO$_{3}$ layer: A 2-dimensional electron gas is observed only when the La/Al ratio is less than 1. Hall measurements at low-temperatures reveal that the mobility and the carrier concentration in these samples are $\sim $250 cm$^{2}$V$^{-1}$s$^{-1}$ and 10$^{13}$cm$^{-2}$ respectively. The 2-dimensional electron gas superconducts at around 200 mK. We will discuss how the low-temperature electronic properties of this oxide-oxide interface are affected by the LaAlO$_{3}$ stoichiometry. [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B37.00003: Role of stoichiometry in the interfacial metal-insulator transition in LaAlO$_{3}$/SrTiO$_{3}$ C. Stephen Hellberg, Kristopher Andersen The observed metal-insulator transition in thin films of LaAlO$_{3}$ on SrTiO$_{3}$ depends critically on the stoichiometry of the film: metallic interfaces are found for Al-rich films, while growing even slightly La-rich films results in insulating interfaces. Using first-principles density functional calculations, we examine the effects of changing the stoichiometry of the films. We find that Al will substitute for La, but La will not substitute for Al. Instead, the excess La sits on the surface of the film. The combination of Al vacancies in the film and excess La on the surface screens the diverging electrostatic potential responsible for forming a metallic interface. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B37.00004: Polarity compensation mechanisms in LaAlO$_{3}$ (001) - oriented thin films Hosung Seo, Alexander Demkov Lanthanum aluminate LaAlO$_{3}$ (LAO) is a polar perovskite oxide widely used as a substrate in oxide epitaxy. Thin films of LAO are of particular interest in view of a recent discovery of the two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ interface. In the (001) direction, the LAO thin film can be regarded as a stack of alternating charged LaO and AlO$_{2}$ planes. Therefore, an unreconstructed LAO thin film would have a macroscopic electric field built up and is expected to have a highly unstable surface. In this talk, we discuss theoretically possible microscopic surface reconstructions compensating the electric field. We use the density functional theory approach within the local density approximation. We find that lanthanum vacancies on the LaO-terminated surface or oxygen vacancies on the AlO$_{2}$-terminated surface can effectively compensate the electric field in the LAO(001) thin films. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B37.00005: Structural and dynamical studies of the LaAlO$_3$/SrTiO$_3$ interface X. Wang, G. You, Q-H. Xu, M. Motapothula, M. B. H. Breese, T. Venkatesan, A. Ariando, J. Huijben, H. Hilgenkamp Oxide thin-film heterostructures offer unique opportunities for combining materials with various functionalities, providing a versatile pathway to create novel oxide electronic devices. In particular, studies have recently shown high-mobility conducting planes emerge at the interface between insulating oxides of LaAlO$_3$ and SrTiO$_3$. To fully understand the mechanisms underlying the interface characteristics, and to identify ways to optimize them for device applications, detailed investigations on the structural and electronic properties with all currently available techniques are being pursued. Rutherford backscattering (RBS) is a powerful technique for studying interface composition and interface roughness, which could shed light on the important aspects, such as oxygen vacancies and inter-diffusion in these systems. We present recent RBS studies combined with XRD. Furthermore the real-time dynamical studies of charge carriers out of equilibrium can give detailed microscopic information about electronic correlations at these interfaces, which can be probed with time-resolved optical spectroscopy on a sub-picosecond time scale. We will discuss our experiments using femtosecond laser pulses to investigate the real-time charge dynamics of in the LaAlO$_3$/SrTiO$_3$ interfaces. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B37.00006: Spatial Charge Distribution in the LaAlO$_3$-SrTiO$_3$ Interface Measured by Angle Resolved Soft X-ray Absorption A. Ariando, A. Rusydi, X. Wang, T. Venkatesan, J. Huijben, H. Hilgenkamp, J. C. Lee, S. Smadici, P. Abbamonte At the interface between complex insulating oxides, novel phases with interesting properties occur. In particular, studies have recently shown high-mobility two-dimensional conducting planes emerge at the interface between insulating oxide heterostructures of LaAlO$_3$ and SrTiO$_3$. Although this state has been predicted and reported to be confined at the interface, transport studies alone cannot measure the charge distribution since mobility may also vary spatially. A way to measure the charge distribution independent of mobility is needed. Here, we present for the first time a direct mapping of the spatial charge density distribution of this system through oxygen vacancy mapping about the interface between LaAlO$_3$/SrTiO$_3$ layers prepared at various oxygen deposition partial pressures using angle resolved soft x-ray absorption. We find that, depending on specific growth protocols, the spatial extension of the oxygen vacancies (charges) can be varied from a 3d-like to a 2d-like distribution at the LaAlO$_3$/SrTiO$_3$ interface. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B37.00007: Electronic properties of coupled interfaces in LaAlO$_3$/SrTiO$_3$ heterostructures G.W.J. Hassink, J.A. Boschker, G. Koster, G. Rijnders, D.H.A. Blank The electron density at the LaAlO$_3$//SrTiO$_3$ interface is a function of the separation between the doped interface and a second interface [Nat.Mat. 5, 556-560]. Depending on the nature of the second interface, either \textit{n}-type LaO//TiO$_2$ or \textit{p}-type AlO$_2$//SrO, the electron doping decreases resp. increases with increasing interface separation. This observation can be explained by assuming a \textit{p}-type interface acts as an electron sink, while a \textit{n}-type interface acts as an electron source. Here we extend the research to coupled interfaces with two \textit{n}-type interfaces fabricated using pulsed laser deposition. Applying a microscopic dipole model to the polar discontinuity inherent to the system allows for the extraction of the binding energy for both cases. The positive value for \textit{p}-type interfaces shows that electrons from the primary \textit{n}-type interface are indeed trapped, while the negative value for the secondary \textit{n}-type interfaces indicates that electrons are doped away from the donor interface. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B37.00008: Parallel electron-hole bilayer conductivity from electronic interface reconstruction: Theory Katrin Otte, Rossitza Pentcheva, Warren E. Pickett The polar discontinuity in oxide heterostructures and thin films can lead to novel electronic states even if simple band insulators such as LaAlO$_3$ and SrTiO$_3$ are involved. Density functional theory calculations [1] show that a strong lattice polarization allows several layers of LaAlO$_3$ to remain insulating before an electronic reconstruction takes place at around 4 monolayers (MLs) [2]. Here we demonstrate that a capping SrTiO$_3$ layer can trigger the insulator-to-metal transition already at two MLs of LaAlO$_3$. A surface O 2$p$ state is identified as the origin of this additional band shift. Altogether, a SrTiO$_3$-capping layer represents an alternative pathway to tune the electronic reconstruction of the system and to realize an electron-hole bilayer in the search, e.g., for novel excitonic phases. \newline [1] R. Pentcheva and W.E. Pickett, Phys. Rev. Lett. {\bf 102}, 107602 (2009). \newline [2] S.Thiel et al., Science {\bf 313}, 1942 (2006). [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B37.00009: Parallel electron-hole bilayer conductivity from electronic interface reconstruction: Experiment Alexander Brinkman, Mark Huijben, Josee Kleibeuker, Jeroen Huijben, Hans Boschker, Daan Kockmann, Wolter Siemons, Gert Koster, Harold Zandvliet, Guus Rijnders, Dave Blank, Hans Hilgenkamp Electronic reconstruction at polar interfaces is the change in electronic properties to compensate an otherwise diverging electric potential. For the perovskite oxide SrTiO$_3$-LaAlO$_3 $ model system we show that an additional capping SrTiO$_3$ layer prevents structural or chemical reconstruction at the LaAlO$_3$ surface and provides a possibility to accommodate holes. Magnetotransport, scanning tunneling microscopy and in situ ultraviolet photoelectron spectroscopy provide evidence for electronic reconstruction and for the presence of two spatially separated sheets with electron and hole carriers, that are as close as 1 nm, forming an excitingly versatile system to realize and study 2D excitonic phenomena. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B37.00010: Inhomogeneity at the LaAlO$_{3}$/SrTiO$_{3}$ interface T. Claeson, A. Kalabukhov, R. Gunnarsson, D. Winkler, J. Borjesson, N. Ljustina, E. Olsson, V. Popok, Yu. Boikov, I. Serenkov, V. Sakharov High electrical conductivity has been reported for the interface between two wide-band gap insulators, LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO). It occurs above a critical thickness of LAO and can be tuned by an electric field. The conduction has been attributed to i) ``polar catastrophe'' , where the electrostatic charge at the interface is compensated by the transfer of half an electron per unit cell to the interface, ii) oxygen vacancies in the STO, and iii) cation intermixing, which may result in the formation of metallic La$_{1-x}$Sr$_{x}$TiO$_{3}$ layer. The relation between microstructure and electrical properties is crucial for understanding the origin of electrical conductivity. We have investigated the interface composition using medium-energy ion spectroscopy, high resolution electron microscopy, and Kelvin probe force microscopy. We find a correlation between cationic intermixing at the interface and electrical properties and inhomogeneities of the interface conductivity that may support a percolation model. Work supported by Swedish VR {\&} KAW, Russian ISTC 3743, EC NANOXIDE [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B37.00011: Role of polarization screening and oxygen vacancy in LaAlO$_3$ overlayer on SrTiO$_3$ (001) Yun Li, Jaejun Yu The observation of a high mobility electron gas in the $n$-type (LaO)/(TiO$_{2}$) interface between two band-gap insulators LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) has generated intense research activities. We investigate the role of polarization screening and oxygen vacancy in determining induced carrier density at the $n$-type interface of LAO overlayer on STO by carrying out density-functional-theory calculations. When no oxygen vacancy is present, the detailed balance between the lattice polarization and the charge transfer is found to be crucial for the carrier at the $n$-type interface of LAO overlayers on STO(001). When the LAO overlayers are over a critical thickness, the charge transfer from the LAO surface to the interface is compensated by the electrostatic screening due to the polarization distortions across the LAO layers. To identify the role of oxygen vacancy, we calculated the electronic structure and formation energy of the oxygen vacancy at various locations. The potential screening and induced carrier are strongly affected by the vacancy density. We discuss the mechanism of carrier generation by the oxygen vacancy at the interface. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B37.00012: Localization and Magnetotransport at the LaAlO$_{3}$/SrTiO$_{3}$ Heterointerface Franklin Wong, Yuri Suzuki Low-temperature electrical transport experiments of the metallic LaAlO$_{3}$/SrTiO$_{3}$ heterointerface have revealed a vast array of physical phenomena, including superconductivity, weak localization, and possible magnetic ordering. Through a study of LaAlO$_{3}$ films ranging from 1.6 to 15 nm grown on TiO$_{2}$-terminated (001) SrTiO$_{3}$ substrates, we show that there is an evolution of transport that converges when the film thickness reaches to 5-6 nm. In general, thicker samples have larger carrier densities, but lower low-temperature mobility values. In addition, field-dependent magnetoresistance data show that higher carrier density samples exhibit antilocalization effects. Together these results suggest greater disorder-induced elastic scattering and strong spin-orbit coupling, which may arise from large electric fields at the heterointerface. In contrast, lower carrier density samples exhibit large in-plane negative magnetoresistance and no features of antilocalization. Carrier density and disorder play prominent roles in determining the field-dependent carrier transport in the metallic conduction regime. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B37.00013: Normal state and superconducting transport at the LaAlO$_{3}$ / SrTiO$_{3}$ interface C. Bell, S. Harashima, M. Kim, Y. Kozuka, B.G. Kim, Y. Hikita, H.Y. Hwang The conduction at the LaAlO$_{3}$/SrTiO$_{3}$ interface [1] provides a playground for controlling metallicity and superconductivity in a thin electron gas. This can be achieved by varying the growth of the LaAlO$_{3}$ layer, the thickness [2], and by field effect modulation [3]. A crucial question is whether the variation in conductivity is due to changes in the carrier density or mobility. Using magnetotransport studies of a gated structure, we find that the mobility variation is almost five times that of the sheet carrier density. Superconductivity can be suppressed at both positive and negative gate bias [4]. These results indicate that the relative disorder strength strongly increases across the superconductor-insulator transition. \\[4pt] [1] A. Ohtomo and H.Y. Hwang, Nature \textbf{427} 423 (2004) \\[0pt] [2] C. Bell \textit{et al.}, Appl. Phys. Lett. \textbf{94}, 222111 (2009) \\[0pt] [3] A. Caviglia \textit{et al.}, Nature \textbf{456} 624 (2008) \\[0pt] [4] C. Bell \textit{et al.}, Phys. Rev. Lett. (in press) [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B37.00014: Negative electronic compressibility at the LaAlO$_3$/SrTiO$_3$ interface Lu Li, Stefan Thiel, Christoph Richter, Jochen Mannhart, Ray Ashoori The interface LaAlO$_3$/SrTiO$_3$ is a potential candidate for a high mobility two-dimensional electron system with novel electronic properties. An essential step for device applications is establishing the electric field effect. Metallic gates were fabricated on the top of the conductive interface to tune the carrier density at the oxide interface with electric gating fields. Varying the top gate voltage and monitoring the capacitance, we are able to change the charge carrier density and establish that we can completely deplete the metallic interface using the top gates. Moreover, the capacitance between the interface and the top gate is enhanced greatly at low carrier densities, rather than being simply determined by the geometry. For some devices, the enhancement of capacitance is as high as 40\% of the geometric capacitance. For the same electron densities, field penetration measurements show that the oxide interface significantly overscreens applied electric fields. The observations are attributed to a negative thermodynamic density of states, or ``negative electronic compressibility'', a characteristic property of free two- dimensional electronic systems so far only observed in Si-based or GaAs-based high-mobility devices. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B37.00015: Can an oxygen vacancy lead to the Kondo resistance minimum observed at the LaAlO$_3$/SrTiO$_3$ interface? Sashi Satpathy, Birabar Nanda Recently a Kondo resistance minimum has been observed at the interface between LaAlO$_3$ and SrTiO$_3$[1]. It has been suggested that the effect is due to the scattering of interface electrons from magnetic centers just like in the original Kondo effect; however, the origin of such magnetic centers is not understood. In this work, we evaluate the idea of whether an oxygen vacancy in SrTiO$_3$ might produce a magnetic center. We focus on an isolated vacancy in bulk SrTiO$_3$ from density-functional calculations and provide evidence that of the two electrons released to the system by the oxygen vacancy, one becomes localized near the vacancy site, while the other forms a delocalized state. If there are sufficient vacancies, the delocalized electrons could form the conduction electrons, scattering off of the localized vacancy states behaving as Kondo scatterers. The detail results for this scenario will be discussed. \\ \\ $[1]$ A. Brinkman {\it et al.}, Nature Mater. {\bf 6} 493 (2007). [Preview Abstract] |
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