Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A34: Focus Session: Interfaces in Complex Oxides - LaAlO3/SrTiO3 Transport |
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Sponsoring Units: DMP Chair: Anand Bhattacharya, Argonne National Laboratory Room: C141 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A34.00001: Tailoring a two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ (001) interface by epitaxial strain David Felker, C.W. Bark, Y. Wang, Y. Zhang, H.W. Jang, C.M. Folkman, J.W. Park, S.H. Baek, X.Q. Pan, E.Y. Tsymbal, M.S. Rzchowski, C.B. Eom Recently a two-dimensional electron gas (2DEG) was discovered at the interface between insulating oxides LaAlO$_{3}$ and SrTiO$_{3}$. Properties of this 2DEG have attracted interest due to its potential applications in nanoelectronics. Control over the carrier density and mobility is essential for applications of these novel systems, and may be achieved by epitaxial strain. The relationship between the strain and electrical properties of this 2DEG remains largely unexplored. We use different lattice constant single crystal substrates to produce LaAlO$_{3}$/SrTiO$_{3}$ interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile strained SrTiO$_{3}$ destroys the conducting 2DEG, while compressively strained SrTiO$_{3}$ retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO$_{3}$/SrTiO$_{3}$ interface. We have also found that the critical LaAlO$_{3}$ overlayer thickness for 2DEG formation increases with SrTiO$_{3}$ compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO$_{3}$ layer is responsible for this behavior. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A34.00002: The effect of epitaxial strain and R$^{3+}$ magnetism on interfaces between RAlO$_{3}$ and SrTiO$_{3}$ Mark C. Monti, Shirin Mozaffari, John T. Markert We have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We are studying the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO$_{2}$ terminated SrTiO$_{3}$ (STO) substrates and have grown epitaxial thin films of LaAlO$_{3}$ (LAO), LaGaO$_{3}$ (LGO), and EuAlO$_{3}$ (EAO) on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R$^{3+}$ magnetism on the interface between RAlO$_{3}$ and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 10$^{14}$ cm$^{-2}$ and 4.5 x 10$^{14}$ cm$^{-2}$, respectively. Surprisingly, we find that even good epitaxial interfaces between EAO/STO are insulating. We will investigate the effect of strain by growing La$_{x}$Y$_{1-x}$AlO$_{3}$ on STO: for example La$_{0.4}$Y$_{0.6}$AlO$_{3}$ mimics the lattice size of EAO. We will systematically vary the magnetism of the RAlO$_{3}$ thin films for R = Ce, Pr, Nd, Sm, Eu, Gd, Tb, La$_{x}$Eu$_{1-x}$, ect. This work was supported by: Texas Advanced Research Program 003658-0126, The Robert A. Welch Foundation F-1191, and the National Science Foundation DMR-0605828. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A34.00003: High mobility interface electron gas by defect engineering in a modulation doped oxide heterostructure Guus Rijnders, Mark Huijben, Gertjan Koster, Hajo Molegraaf, Sander Wenderich, Josee Kleibeuker, Alexander Brinkman, Hans Hilgenkamp, Dave Blank, Michelle Kruize, A. McCollam, V.K. Guduru, Uli Zeitler, Jan Cees Maan The manifestation of quantum behavior in two dimensional electron gases in semiconducting heterostructures and their progressive complexity towards fractional quantum Hall effect went hand-in-hand with the efforts to remove the effect of impurity scattering. For oxide materials, history is repeating itself and to date sample quality is reaching levels where quantum behavior starts to become accessible. To really understand the ground state of two dimensional electron gases in oxide systems, where electron-electron correlation effects seem more important, a step towards modulation doping is necessary, removing dopants away from a conduction channel. We will show that the impurity scattering of a 2DEG at the LaAlO3/SrTiO3 interfaces can be significantly suppressed by defect engineering, allowing the observation of quantum transport in a modulation doped oxide system. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A34.00004: Effect of stoichiometry on the interface conductivity of MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ heterostructures M.P. Warusawithana, J. Ludwig, P. Roy, A.A. Pawlicki, T. Heeg, D.G. Schlom, C. Richter, S. Paetel, J. Mannhart, L. Fitting Kourkoutis, J. Mundy, D.A. Muller, M. Zheng, B. Mulcahy, J.N. Eckstein, W. Zander, J. Schubert Through careful control of the stoichiometry in molecular-beam epitaxy grown LaAlO$_{3}$/SrTiO$_{3}$ samples, we find that a 2-dimensional electron gas occurs at the interface between the two insulating oxides as reported in samples grown by pulsed-laser deposition. In this talk, I will discuss the controlled experiments that we have carried out, which effectively eliminate the extrinsic effects that have been suggested as possible mechanisms of conductivity, for the conductivity observed in our MBE-grown samples. We find that the cation stoichiometry of the La$_{(1-x)}$Al$_{(1+x)}$O$_{3}$ layer is key to the existence of the interface 2-dimensional electron gas and that a La/Al ratio, (1-x)/(1+x) 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 21, 2011 8:48AM - 9:00AM |
A34.00005: Importance of defects and stoichiometry in the interfacial metal-insulator transition in LaAlO3 thin films on SrTiO3 C. Stephen Hellberg The observed metal-insulator transition in thin films of LaAlO3 on SrTiO3 depends critically on the stoichiometry of the film: metallic interfaces are found for Al-rich films, while growing even slightly La-rich 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, Al-vacancy structures occur in La-rich films. The Al vacancies can migrate to the interface, screening the potential divergence and preventing a metallic interface from forming. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A34.00006: Interfacial superconductivity and its magnetic field dependence in MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ heterostructures P. Roy, J. Ludwig, E. Steven, A. Kiswandhi, A.A. Pawlicki, J. Brooks, M.P. Warusawithana, D.G. Schlom, C. Richter, J. Mannhart In our MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ samples we find the interface to be conducting and sometimes even superconducting when the La/Al ratio is less than or equal to 0.97 $\pm $ 0.03. Here, we report on the superconducting behavior observed in some samples with La/Al ratio above 0.84 $\pm $ 0.03 and below 0.97 $\pm $ 0.03. The superconducting critical temperature is found to be between 160 -- 235 mK on different samples. We measure the magnetic field dependence of superconductivity and find that the critical magnetic field required to quench superconductivity depends on the direction of the applied magnetic field. The strong anisotropy in the critical field suggests that the superconductivity in these MBE-grown samples is confined to a thin layer at the interface. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A34.00007: Low temperature, high magnetic field magnetoresistance and Hall measurements on MBE-Grown LaAlO$_{3}$/SrTiO$_{3}$ interfaces J. Ludwig, P. Roy, E. Steven, A. Kiswandhi, A.A. Pawlicki, J. Brooks, M.P. Warusawithana, D.G. Schlom, C. Richter, J. Mannhart We have measured MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ samples at temperatures ranging from room temperature to 20mK and at magnetic fields up to 18 Tesla. The La$_{(1-x)}$Al$_{(1+x)}$O$_{3}$ films studied were grown with a stoichiometry gradient (varying x). We report on the low-temperature sheet carrier density and mobility of the conducting samples -- samples with La/Al ratio less than or equal to 0.97 $\pm $ 0.03. We discuss the dependence of sheet carrier density and mobility on stoichiometry by using samples grown on the same substrate and then isolated by using a wire saw. In the devices we measured, the low-temperature sheet carrier densities are on the order of 1x10$^{13}$ cm$^{-2}$ with an approximate variation of 2x10$^{12}$cm$^{-2}$ form device to device. The mobilities observed are on the order of 1x10$^{3}$ cm$^{2}$V$^{-1}$s$^{-1}$. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A34.00008: Two-dimensional quantum oscillations of the conductance at the LaAlO$_{3}$/SrTiO$_{3}$ interface Andrea Caviglia, Stefano Gariglio, Nicolas Reyren, Claudia Cancellieri, Alexandre Fete, Benjamin Sacepe, Marc Gabay, Alberto Morpurgo, Jean-Marc Triscone Electronic states with unusual properties can be promoted at interfaces between complex oxides. A striking example is the interface between the band insulators LaAlO$_{3}$ and SrTiO$_{3}$, which displays conductivity with high mobility and 2D superconductivity. We report on a study of magnetotransport in LaAlO$_{3}$/SrTiO$_{3}$ interfaces characterized by mobilities of the order of several thousands cm$^{2}$/Vs. We observe Shubnikov-de Haas oscillations whose period depends only on the perpendicular component of the magnetic field. This observation directly indicates that the electron gas is two-dimensional and originates from quantum confinement at the interface. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass $m* \simeq$ 1.45$m_{e}$. We discuss the relevance of spin effects on the observed phenomenology. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A34.00009: Direct Magnetization Measurement of the LaAlO$_3$/SrTiO$_3$ heterostructure Lu Li, Christoph Richter, Jochen Mannhart, Ray Ashoori The LaAlO$_3$/SrTiO$_3$ heterostructure is a potential candidate for a high mobility two-dimensional electron system with novel electronic and magnetic properties. Although LaAlO$_3$ and SrTiO$_3$ are both large-gap band insulators, the interface is conductive and even superconducts below 200 mK. Magnetic ordering has been proposed to arise from the polarization-driven charge transfer, but the magnetization of this system has not previously been studied, likely due to the small volume of the interface. Using torque magnetometry, we detect directly the magnetic moment of the interface system. Control experiments with samples without LaAlO$_3$ display a background signal two orders of magnitude smaller, indicating that the observed magnetic moment arises from the deposition of LaAlO$_3$. The measured equilibrium $M-H$ curve resembles that of a soft ferromagnet. Our results indicate the existence of a magnetic ordering at the two-dimensional conductive interface. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A34.00010: Quantum Oscillations at the LaAlO$_3$/SrTiO$_3$ Interface A. McCollam, V.K. Guduru, U. Zeitler, J.C. Maan, M.K. Kruize, S. Wenderich, H. Hilgenkamp, D.H.A. Blank Under certain growth and preparation conditions, the interface between the perovskite oxides LaAlO$_3$ and SrTiO$_3$ can support a 2-dimensional electron gas (2deg) with diverse and remarkable electronic properties. When the mobility of this 2deg becomes high enough, quantum oscillations appear in the magnetoresistance and provide important information about the origin of the electronic behavior. Here we present an angle-dependent magnetotransport study of a high mobility LaAlO$_3$/SrTiO$_3$ interface, at millikelvin temperatures and in magnetic fields of up to 30~T. Large quantum oscillations are observed, with a complex dependence on the applied magnetic field and its orientation with respect to the plane of the interface. We propose that the unusual properties of the oscillations have their origin in the multi-subband character of the 2deg, and present a simple model, based on two-dimensional conductivity, which supports the scenario that several spin-split subbands, with field and angle-dependent occupancy, are contributing to the quantum transport in this system. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A34.00011: Magnetotransport in the 2DEG at Interface Between LaAlO$_{3}$ and Thin Film SrTiO$_{3}$ T. Hernandez, D.A. Felker, C.W. Bark, C.B. Eom, M.S. Rzchowski Transport properties of the 2DEG formed at the heterointerface between LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) grown on Si and (LaAlO$_{3}$)$_{0.3}$-(Sr$_{2}$AlTaO$_{3}$)$_{0.7}$ (LSAT) were compared to those of the LAO on single crystal STO interface. The STO layer on Si was grown by molecular beam epitaxy and on LSAT by pulsed laser deposition (PLD). In all cases, the LAO overlayers were grown using PLD. Mobility, carrier concentration, and magnetoresistance (MR) were measured over the range 3-300K and magnetic fields of 0-8T. The transport properties were similar at room temperature for the different structures. However, at low temperatures, the structures on single crystal STO showed metallic behavior and positive MR, constant in temperature in the 3-20K regime, whereas the ones on Si and LSAT substrates showed a temperature dependence consistent with Mott-type variable range hopping and negative MR with power law behavior in temperature. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A34.00012: Hysteretic magneto-resistance at the LaAlO$_{3}$-SrTiO$_{3}$ interface - interplay between superconducting and ferromagnetic properties Manan Mehta, Dimitry Dikin, Chung Wung Bark, Chad Folkman, Chang-Beom Eom, Venkat Chandrasekhar The conducting interface formed between LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) has been shown to have both magnetic and superconducting properties. The behaviour can be tuned from one to the other by changing the applied gate voltage, thus changing the density of carriers at the interface. We will present magneto-transport data on a Hall-bar geometry patterned LAO/STO interface, with 10 unit cells LAO thickness. The longitudinal magneto-resistance shows strong hysteretic behaviour, indicating a ferromagnetic state, at negative gate voltages; the transverse magneto-resistance being linear. However, the hysteresis survives even into the superconducting state, and also shows up in the transverse magneto-resistance. This suggests an interplay between the superconducting and ferromagnetic order parameters of this system. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A34.00013: Oxygen vacancies at the LaAlO$_3$/SrTiO$_3$ interface: formation energies and metal-insulator transition Hanghui Chen, Alexie Kolpak, Sohrab Ismail-Beigi The intriguing transport properties observed at the LaAlO$_3$/SrTiO$_3$ interface have stimulated numerous studies in the past few years. However, the microscopic mechanism that leads to the formation of the two-dimensional conducting electron gas at the interface remains elusive, partly due to the fact that both intrinsic and extrinsic factors can contribute. We report first principles results on the formation energies of oxygen vacancies on the LaAlO$_3$ thin film surface as a function of coverage and film thickness. In addition to electrostatic contributions to the formation energy due to the polar field in LaAlO$_3$, structural distortions also play an important role in the energetics. We build a simple analytical model to describe our findings which allows us to determine the critical thickness for an oxygen vacancy-induced metal-insulator transition. We discuss the relation of these predictions to the experimental results on this interfacial system. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A34.00014: Electronic Phase Separation at the LaAlO$_3$/SrTiO$_3$ Interface A. Ariando, X. Wang, Z.Q. Liu, J.B. Yi, A. Annadi, A. Roy Barman, A. Rusydi, S. Dhar, Y.P. Feng, J. Ding, T. Venkatesan, G. Baskaran, J. Huijben, H. Hilgenkamp Among the wealth of electronic and magnetic properties exhibited by complex oxides, electronic phase separation (EPS) is one of those whose presence can be linked to many types of exotic behavior, such as colossal magnetoresistance, metal- insulator transition and high-temperature superconductivity. Recently, the oxide community has once again been energized by the observation of a variety of new and unusual electronic phases at the interfaces between the complex oxides, in particular between two nonmagnetic insulators LaAlO$_3$ and SrTiO$_3$. However, no EPS has been observed thus far in this system despite a theoretical prediction. Here, we will show the observation of a ferromagnetic phase and its coexistence with a paramagnetic or a giant diamagnetic phase below 60 K at the interface between LaAlO$_3$ and SrTiO$_3$. The ferromagnetic phase persists even above room-temperature. The coexistence of these multiple magnetic phases along with the interface quasi- 2D electron gas suggests that EPS exists in this system, which can be explained on the basis of selective occupancy of interface sub-bands derived from the nearly degenerate $t_{2g}$- orbitals of Ti $3d$-states in the SrTiO$_3$. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A34.00015: Electronic Phases and Phase Separation in the Hubbard-Holstein Model of a Polar Interface Birabar Nanda, Sashi Satpathy From a mean-field solution of the Hubbard-Holstein model, we show that a rich variety of different electronic phases can result at the interface between two polar materials such as LaAlO$_3$/SrTiO$_3$. Depending on the strengths of the various competing interactions, viz., electronic kinetic energy, electron-phonon interaction, Coulomb energy, and electronic screening strength, the electrons could (i) either be strongly confined to the interface forming a 2D metallic or an insulating phase, (ii) spread deeper into the bulk making a 3D phase, or (iii) become localized at individual sites forming a Jahn-Teller polaronic phase. In the polaronic phase, the Coulomb interaction could lead to unpaired electrons resulting in magnetic Kondo centers. Under appropriate conditions, electronic phase separation may also occur resulting in the coexistence of metallic and insulating regions at the interface. [Preview Abstract] |
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