Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P30: Focus Session: Oxide Surfaces and Interfaces |
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Sponsoring Units: DMP GMAG Chair: Jian Shen, Oak Ridge National Laboratory Room: 334 |
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P30.00001: Structural Stabilization of Antiferromagnetism at the Surface of a Layered Manganite E.W. Plummer, V.B. Nascimento, R.G. Moore, H. Liu, M.H. Pan, D. Mazur, J.W. Freeland, K.E. Gray, R.A. Rosenberg, H. Zheng, J.F. Mitchell, R. Saniz, A.J. Freeman, J. Rundgren Here, we present evidence to support the idea that the distinct surface electronic and magnetic state seen in the double-layered manganites (La$_{2}-_{2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, with dopings of 0.3 $< \quad x \quad <$ 0.4) is driven solely by a subtle change in the lattice at the surface, which is consistent with the strong coupling between the lattice, charge, and spin degrees of freedom in these doped transition-metal oxides. A combined experimental and theoretical approach was essential in understanding the origin of the nonmagnetic surface phase. The new measurements and calculations give an insightful explanation of the previous x-ray and tunneling data, which indicated a nonmagnetic and insulating surface bilayer. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P30.00002: Applying the extended Drude model for the mid-infrared complex conductivity tensor of SrRuO$_3$ M.-H. Kim, G. Acbas, M.-H. Yang, C. T. Ellis, M. Eginligil, J. Cerne, P. Khalifah, I. Ohkubo, H. Christen, D. Mandrus, Z. Fang The complex longitudinal ($\sigma_{xx}$) and Hall ($\sigma_{xy} $) conductivities in a SrRuO$_3$ film are determined using Faraday and Kerr measurements in the mid-infrared (100 $\sim$ 1000 meV) energy and 10 K $\sim$ 300 K temperature ranges. The extended Drude model (EDM) for ($\sigma_{xx}$) is used to subtract the ordinary part of $\sigma_{xy}$. Using the EDM parameters, the ordinary contribution to $\sigma_{xy}$ is found to be significantly smaller than the anomalous contribution at most temperatures and wavelengths, decreasing strongly as the wavelength decreases. We suggest that the paramagnetic part of the $\sigma_{xy}$ results from the unsaturated magnetization, not from the ordinary part of $\sigma_{xy}$. This work was supported by the Research Crop. Cottrell Scholar Award (UB), NSF-CAREER-DMR0449899 (UB), and an instrumentation award from the CAS (UB). Oak Ridge Natl. Lab. Is managed by UT- Battelle, LLC, for the U.S. DOE (contract DE-ACO5-00OR22725). [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P30.00003: Antiferromagnetism and insulating nature of ultrathin films of SrRuO3 Priya Mahadevan, F. Aryasetiawan, A. Janotti, T. Sasaki Metallic oxides form an integral part of oxide-based technologies, constituting the connecting electrode material. In this context SrRuO3 is a material that has been widely studied as it in addition to being metallic is also ferromagnetic. Recent experiments have found that ultrathin films of SrRuO$_3$ are insulating and hence 5-6 monolayers are required before metallic character is observed. In this work we theoretically examine the origin of the insulating state with first principle GGA+U calculations. The value of U is calculated from first principles. Ru has a formal configuration of d$^4$ in SrRuO$_3$. In bulk SrRuO$_3$ this translates into a low spin state with an electronic configuration of $t_{2g\uparrow}t_{2g\downarrow}$. Hence at the surface/ultrathin film limit one expects the observed insulating nature to come from a transition into the nonmagnetic state with the lowest crystal-field levels contributed by $d_{yz}$ and $d_{xz}$ orbitals being completely filled. However one finds that the system undergoes an unusual structural distortion which is accompanied by a spin state transition. This spin state transition is accompanied by a transition into an antiferromagnetic state which drives the system insulating. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P30.00004: Structure and Transport in ultrathin SrRuO$_{3}$ Arthur P. Baddorf, Junsoo Shin, Von Braun Nascimento, Albina Y. Borisevich, Sergei V. Kalinin, Vincent Meunier, Peter Maksymovych SrRuO$_{3}$ (SRO) is a common electrode material in oxide thin film growth, and is also representative ``bad'' metal. Although bulk SRO is orthorhombic, a typical study grows a pseudocubic film on SrTiO$_{3}$(100) substrate. We have measured electrical conductivity and structure of in-situ grown SRO films using scanning probe microscopy, low energy electron diffraction (LEED) and scanning transition electron microscopy. The films remain conducting down to the thickness of one layer. LEED displayed sharp (1x1) patterns at 80 and 300K up to 5 layers, but revealed strong half-order spots with weak intensity at quarter-order by 20 layers. STM images show the surfaces of thick films to have many vacancies, which first principles theory suggests are missing SrO molecules. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P30.00005: Atomic control and characterization of surface defect states of TiO$_{2}$ terminated SrTiO$_{3}$. A. Kareev, M. Kareev, S. Prosandeev, J. Liu, C. Gan, J.W. Freeland, Min Xiao, J. Zhang, L. Brillson, J. Chakhalian By using a new wet-etch procedure$^{1}$ we have obtained high-quality atomically flat TiO$_{2}$ terminated surfaces of STO (100) single crystals with the surface morphology equivalent or better to that of the conventional routes. By applying a combined power of CL and PL, RHEED, AFM, and resonant XAS, we are able to identify and monitor the complex evolution of oxygen defect states and Ti ion valency at the surface and near-surface regions. Our data revealed a high level of local defects resulting in the presence of the Ti$^{3+\delta }$ states at the surface in the conventionally treated STO surface. We have developed an efficient method to control the defect states capable of a marked reduction of the defect concentration. We have demonstrated that the PL, CL and XAS are able to distinguish the surface-related Ti states from oxygen vacancies trapping charge transfer vibronic excitons. $^{1}$M. Kareev et al., Appl. Phys. Lett. 93, 061909 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:36AM |
P30.00006: Polar and non-polar oxide interfaces: charge and spin behaving badly Invited Speaker: Interfaces between two dissimilar materials are well known to lead to new behavior and often useful properties. Whereas covalent semiconductors have been studied and used for decades, and the interfaces of magnetic metals also have assumed great importance, the use of oxides in such juxtaposed systems is much more recent. Oxides add the huge impact of ionicity, and the correlated electron behavior that occurs if open shells are present. Even without correlation effects, finite overlayers (slabs) involving a polar discontinuity can sustain a surprisingly large separation of charge, as will be illustrated with calculational results on LaAlO$_3$ slabs on SrTiO$_3$ substrates: a strong {\it polar distortion}, uniform over several unit cells, creates the necessary screening. The most recent results on the mechanism and character of the insulator-to-metal transition with thickness will be discussed. Polar discontinuities are not necessary to create exotic behavior, as we illustrate with rutile-structure VO$_2$/TiO$_2$ multilayers, where a topologically protected zero-gap two-dimensional half-metal arises in a thickness regime between thin insulating and thick conducting VO$_2$ slabs. Work done in collaboration with R. Pentcheva, V. Pardo, and K. Otte. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P30.00007: Prediction of a switchable two-dimensional electron gas at KNbO3/ATiO3 (A = Sr, Ba, Pb) interfaces Yong Wang, Manish Niranjan, Sitaram Jaswal, Evgeny Tsymbal The demonstration of a quasi-two dimensional electron gas (2DEG) at the (LaO)$^{+}$/(TiO$_{2})^{0}$ interface in LaAlO$_{3}$/SrTiO$_{3}$ heterostructure has fuelled intense research activity in recent years. The 2DEG has a high carrier mobility and electron density that are promising for applications in all-oxide electronic devices. For such applications it is desirable to have the ability to control the properties of the 2DEG by external stimulus, e.g., by an electric field. In this study we use density functional calculations to explore a ferroelectric KNbO$_{3}$/SrTiO$_{3 }$heterostructure for this purpose. The polar discontinuity at the (NbO$_{2})^{+}$/(SrO)$^{0 }$interface in KNbO$_{3}$/SrTiO$_{3}$ heterostructure is similar to that at the (LaO)$^{+}$/(TiO$_{2})^{+}$ interface in LaAlO$_{3}$/SrTiO$_{3}$ heterostructure. Our results suggest that a 2DEG is created at the (NbO$_{2})^{+}$/(SrO)$^{0 }$interface due to the electronic reconstruction with properties strongly determined by the orientation of the electric polarization. We further explore the formation of 2DEG at the interfaces of all ferroelectric KNbO$_{3}$/BaTiO$_{3}$ and KNbO$_{3}$/PbTiO$_{3}$ heterostructures and its dependence on polarization orientation. Finally, we discuss how the properties of 2DEG in aforementioned heterostructures are influenced by the compensation of polarization charges by free carriers, rendering it switchable. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P30.00008: A Novel Quantum Well of Embedded SrTiO$_3$ in LaAlO$_3$ Hanghui Chen, Alexie Kolpak, Sohrab Ismail-Beigi Inspired by the novel behavior of $\textrm{LaAlO}_3/\textrm{SrTiO}_3$(001) heterointerfaces, we propose new sets of interfaces in which one TiO$_2$ or SrO layer is embedded in LaAlO$_3$. These interfaces can form quantum wells which trap electrons or holes in a single atomic layer of TiO$_2$ or SrO, respectively, when the ``polar catastrophe'' occurs. This narrow confinement, in contrast to the situation at the $\textrm{LaAlO}_3/\textrm{SrTiO}_3$(001) interfaces, sheds light on the still uncertain origin of the charge carriers and provides ideas for engineering their properties. In addition, we study the field effect on these systems to predict the critical external electric field required to induce an insulating-to-metallic transition. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P30.00009: Resonant Tunneling Through a Two-Dimensional Electron Gas in All-Oxide Heterostructure Tunnel Junctions J.D. Burton, E.Y. Tsymbal, J.P. Velev Oxide heterostructures exhibit a variety of very interesting physical properties and have tremendous potential for new types of multifunctional device applications. For example, very recently it was discovered that a two-dimensional electron gas (2DEG) is formed at the (001) interface between two perovskite oxides that are otherwise insulating in the bulk. We examine, within the framework of first-principles density functional theory, the effect of a complex SrTiO3-LaO-SrTiO3 barrier forming a 2DEG on conductance and TMR in all-oxide magnetic tunnel junctions. The replacement of one SrO atomic layer by LaO in the otherwise pure SrTiO3 barrier can be understood as precision substitutional doping of trivalent La for divalent Sr, leading to the formation of a 2DEG. Such precision atomic layering is within the reach of current experimental fabrication techniques. Our calculations reveal that compared to the pure SrTiO3 barrier, the tunneling conductance can be substantially enhanced due to resonant tunneling through the 2DEG. However, this effect is sensitive to lattice polarization effects in the SrTiO3 barrier as well as the choice of electrode material. We will discuss these effects with the goal of stimulating experimental studies. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P30.00010: Modulation of the low-temperature magnetoresistance of Ar-irradiated SrTiO$_3$ via field-effect gate doping J.H. Ngai, Y. Segal, J. Hoffman, F.J. Walker, C.H. Ahn Recent experiments have shown that irradiating single crystal SrTiO$_3$ with Ar ions can create an amorphous surface layer with a quasi-2-dimensional electron gas (Q2DEG) below. We present low-temperature magnetotransport measurements of this Q2DEG system, as a function of gate doping. The magnetoresistance can be tuned as n-type carriers are doped into the interface between the amorphous and crystalline SrTiO$_3$ layers. Anisotropy in the magnetoresistance is also measured with respect to the direction of the applied magnetic field. These results will be compared with the magnetotransport properties of LaAlO$_3$/ SrTiO$_3$ heterostructures, where the possibility of novel magnetic behavior will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P30.00011: Nanoscale Electrical Properties of Oxide Heterostructures Revealed Via Introspection Cheng Cen, Stefan Thiel, Jochen Mannhart, Jeremy Levy Previous work shows that conductive regions can be formed via lateral nanoscale confinement of a quasi-two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ interface$^{2}$. Here we demonstrate how structures constructed in this method serve not only as novel nanoelectronic devices but also as tools for studying fundamental physics in the underlying material system. Nanowires, tunnel junctions, field effect transistors (FETs), together with associated phenomena that we observed such as negative differential resistance, provide insight into the mechanism responsible for the existence and spatial confinement of the interfacial metal-insulator transition. We discuss several examples of nanodevices and the constraints they place on models and mechanisms that govern their properties. $^{2}$Cen et al, Nature Materials 7, 298 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P30.00012: A metal-insulator transition tunable by lattice deformation in LaTiO$_{3}$ thin films Franklin Wong, Seung-Hyub Baek, Ho Won Jang, Rajesh Chopdekar, Virat Mehta, Chang-Beom Eom, Yuri Suzuki Strong electron-electron and electron-lattice correlations play critical roles in electronic transitions of complex oxides. Since LaTiO$_{3}$ is a narrow bandgap Mott insulator on the verge of a metal-insulator transition, substrate-induced lattice distortions offer a route to tuning its electronic properties. We have observed metallic to insulating behavior in LaTiO$_{3}$ films depending on choice of (001) substrates: SrTiO$_{3}$, LSAT, and LaAlO$_{3}$. Tetragonal distortions induced by epitaxial in-plane compression from the SrTiO$_{3}$ substrates result in metallicity in LaTiO$_{3}$ films, while films on LSAT substrates exhibit a range of electronic properties depending on the degree of lattice relaxation. Whereas thinner LaTiO$_{3}$ films on LSAT exhibit ``semimetallic'' behavior, in thicker films, the out-of-plane lattice parameters surprisingly converge to values greater than the bulk lattice constant, and the films become more insulating. We will discuss the profound consequences thin-film lattice deformation has on electrical transport. We speculate that stabilization of lattice distortions via epitaxy may open a new avenue for materials engineering of oxides through careful control of structural perturbations. [Preview Abstract] |
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