Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T37: Focus Session: Complex Oxide Thin Films -- Interfaces and Superlattices |
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Sponsoring Units: DMP GMAG Chair: R. Ramesh, University of California, Berkeley Room: E147-E148 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T37.00001: Electronic Structure and Transport Properties of LaNiO$_{3}$/LaAlO$_{3}$ Heterostructures Benjamin Gray, Jian Liu, Mikhail Kareev, Rossitza Pentcheva, Daniil Khomskii, Philip Ryan, J.W. Freeland, Jak Chakhalian In the bulk, the ground state of LaNiO$_{3}$ manifests itself as a correlated metal without magnetic ordering. Recent advances in fabrication of ultra-thin oxides allow for the synthesis of interface-controlled heterostructures with properties not attainable in their bulk counterparts. Towards this end, we fabricated high-quality LaNiO$_{3}$/LaAlO$_{3}$ superlattices epitaxially grown on SrTiO$_{3}$ and LaAlO$_{3}$ substrates by laser MBE. During the talk we present the experimental findings about effects of strain and proximity to the interface on the electronic and orbital structure of Ni, Al and O deduced from synchrotron soft x-ray spectroscopy, XRD, and bulk electric and thermal measurements. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T37.00002: Jahn-Teller physics and orbital control in oxide heterostructures: a first-principles study of LaNiO$_3$/LaXO$_3$ (X= B, Al, Ga, In) Myung Joon Han, Chris A. Marianetti, Andrew J. Millis Generalized Gradient Approximation (GGA) density functional calculations are used to show that different choices of spacer layer $LaXO_3$ in LaNiO$_3$/LaXO$_3$ heterostructures lead to different relative occupancies of Ni $3z^2-r^2$ and $x^2-y^2$ orbitals, with the sign of the orbital polarization even reversing as the B-site atom in the spacer layer is changed from $X=B$ through $Al,Ga$ to $In$. The crucial role played by the hybridization of the apical oxygen to the spacer layer (LaXO$_3$) is demonstrated. Extensions to other systems and implications for many-body calculations are discussed. [Preview Abstract] |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T37.00003: An atomic resolution view at oxidation states in transition metal oxide heterostructures M. Varela, S.J. Pennycook, T.J. Pennycook, W. Luo, S.T. Pantelides, J. Garcia-Barriocanal, F.Y. Bruno, C. Leon, J. Santamaria In LaMnO$_{3}$/SrTiO$_{3}$ (LMO/STO) superlattices the LMO/STO relative layer thickness ratio changes the degree of epitaxial strain within the layers and dramatically affects the physical properties of the system, which can be tuned from insulating, mild ferromagnetic, to metallic ferromagnets. Such behaviors are ultimately related to the oxygen - 3d metal bonds and hence to the occupancies of the partially filled 3d bands, which define their oxidation state. In this talk we will review procedures to measure oxidation states in manganites from electron energy loss spectra obtained in the aberration corrected scanning transmission electron microscope, and apply these techniques to imaging subtle O displacements and measure their effects on the electronic properties of LMO/STO interfaces. These results will be discussed and combined with density functional theory, in connection with the magnetotransport properties. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T37.00004: Resonant Phonon Coupling in Epitaxial La$_{1-x}$Sr$_{x}$MnO$_{3}$/SrTiO$_{3 }$(001) Heterostructures Yaron Segal, C.A.F. Vaz, J.D. Hoffman, F.J. Walker, C.H. Ahn The rich physical phenomena observed in complex manganites stem from strong electron-electron and electron-phonon correlations, which are at the origin of the strong interplay between charge, spin, and orbital degrees of freedom in these materials. In this work, we examine electron-phonon interactions in epitaxial ultrathin films of La$_{1-x}$Sr$_{x}$MnO$_{3}$ grown by molecular beam epitaxy (MBE) on single-termination SrTiO$_{3}$(001) substrates as a function of chemical doping, film thickness, electrostatic gate doping, and applied magnetic field. For Sr dopings near the ferromagnetic-antiferromagnetic (FM-AFM) phase transition, a sharp feature in transport and magnetization is observed, coinciding with the phonon softening that occurs in SrTiO$_{3}$ at the phase transition near 100 K. We show that this effect can be modeled by the increase in the population of the F$_{2u}$ phonons in SrTiO$_{3}$, which couple to the phonon modes of the LSMO. This coupling leads to a large modification in the electron hopping rates, with attendant changes in electronic, transport and magnetic behavior. We also discuss the connection between the orbial ordering in the AFM phase and the strong electron-phonon and phonon-magnon coupling. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T37.00005: Influence of the Termination Layer on the Electronic Properties of LaMnO$_{3}$ / SrTiO$_{3}$ Interfaces Jacobo Santamaria, J. Garcia-Barriocanal, F.Y. Bruno, Z. Sefrioui, A. Rivera-Calzada, N. M. Nemes, C. Leon, M. Garcia-Hernandez, M. Varela, Stephen Pennycook We examine charge leakage at the interface between two oxide materials with different B- site cations: SrTiO$_{3}$ (STO), a band insulator, and LaMnO$_{3}$ (LMO)- a Mott insulator-, or La $_{0.7}$Sr $_{0.3}$MnO$_{3}$ (LSMO) a metal. This system incorporates a polar discontinuity at the interface, as in SrTiO$_{3}$ (STO) /LaAlO$_{3}$ (LAO), which is modulated by the Sr content. Using combined atomic column resolution imaging and spectroscopy we provide direct evidence for a change of the Ti oxidation state from 4$^{+}$ to 3$^{+}$ at the interface in LMO/STO, resulting from the transfer of electrons from the LMO manganite into the titanate layer. This change is also observed for samples with LSMO and appears to be dependent on the layer thickness ratio as in the polarity conflict scenario. Possible explanations are discussed in terms of the composition of the termination plane of the manganite. Work at UCM supported by MAT2008 6517. Research at ORNL sponsored by US DOE [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T37.00006: Excitonic Physics in Oxide Multilayers Andrew Millis Transition metal oxide heterostructures involving Mott insulating components offer an attractive possibility for creating novel excitonic states. The bandgaps of the Mott materials can be small and relatively tunable, making the addition of both electrons and holes feasible. The polar-discontinuity physics offers a promising route to inducing high density interface gasses while the short length scales mean that spatial separation between electron and hole gasses may be small. The strong many body effects occurring in oxides raises the possibility of new kinds of states. This talk will summarize our understanding of the issue, outlining issues involved, listing possible candidate materials and presenting phase diagrams of model systems. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 4:18PM |
T37.00007: Atomic-Scale Chemical Imaging of Composition and Bonding at Perovskite Oxide Interfaces Invited Speaker: Scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) has proven to be a powerful technique to study buried perovskite oxide heterointerfaces. With the recent addition of 3$^{rd}$ order and now 5$^{th}$ order aberration correction, which provides a factor of 100x increase in signal over an uncorrected system, we are now able to record 2D maps of composition and bonding of oxide interfaces at atomic resolution [1]. Here, we present studies of the microscopic structure of oxide/oxide multilayers and heterostructures by STEM in combination with EELS and its effect on the properties of the film. Using atomic-resolution spectroscopic imaging we show that the degradation of the magnetic and transport properties of La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_3$ multilayers correlates with atomic intermixing at the interfaces and the presence of extended defects in the La$_{0.7}$Sr$_{0.3}$MnO$_3$ layers. When these defects are eliminated, metallic ferromagnetism at room temperature can be stabilized in 5 unit cell thick manganite layers, almost 40\% thinner than the previously reported critical thickness of 3-5 nm for sustaining metallic ferromagnetism below T$_c$ in La$_{0.7}$Sr$_{0.3}$MnO$_3$ thin films grown on SrTiO$_3$.\\[4pt] [1] D.A. Muller, L. Fitting Kourkoutis, M. Murfitt, J.H. Song, H.Y. Hwang, J. Silcox, N. Dellby, O.L. Krivanek, Science 319, 1073-1076 (2008). [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T37.00008: Lattice instabilities suppress cuprate-like Fermi surfaces in oxide heterostructures James Rondinelli, Nicola Spaldin Progress in the layer-by-layer growth of transition metal oxide films motivated the intriguing recent suggestion that oxide heterostructures could be engineered to have band structures close to those of the high-$T_c$ cuprates. Although theoretical candidate materials have been identified, experimental realization has not yet been achieved. In this work, using first-principles density functional theory calculations, we explore the Fermi surface behavior of thin layers of metallic, orbitally degenerate, $d^4$ SrFeO$_3$ confined between the $d^0$ dielectric SrTiO$_3$ in a superlattice geometry. We show that the conventional heteroepitaxial constraint, which requires the film and substrate to have identical in-plane lattice constants, splits the electronic degeneracy and combines with the two-dimensionality of the superlattice to produce a metallic cuprate-like band structure. We find, however, that the band structure is drastically changed by the existence of lattice instabilities which occur in the superlattice, yet are stable in each bulk component. We show that these enhanced electron-lattice instabilities are strongly sensitive to the superlattice periodicity and compete with the formation of a cuprate-like Fermi surface. We suggest our results provide a plausible explanation for the absence of metallic behavior in ultra-thin orbitally degenerate oxide superlattices that are predicted to be superconducting. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T37.00009: Modulation-doped ferromagnetism in digitally synthesized manganite superlattices T.S. Santos, B. Kirby, S.J. May, B. Maranville, S. te Velthuis, J. Zarestky, A. Bhattacharya We have digitally synthesized ordered analogs of La$_{1-x}$Sr$_x$MnO$_3$ by interleaving single unit cell layers of LaMnO$_3$ and SrMnO$_3$ with atomic layer precision using ozone-assisted molecular beam epitaxy. In our neutron diffraction experiments on these epitaxial superlattices near $x=0.5$, we confirmed the $A$-type antiferromagnetic spin structure with an enhanced ordering temperature. We found that in superlattices with composition at the ferromagnetic/antiferromagnetic phase boundary, inserting an additional single unit cell layer of LaMnO$_3$ causes a significant increase of the net magnetic moment while still retaining the $A$-type spin structure. Our polarized neutron reflectometry experiments revealed a highly modulated moment commensurate with the structural periodicity of the superlattice, with higher moment in the region of the extra LaMnO$_3$ layer. Thus, introducing a single La dopant layer results in a localized enhancement of double exchange along the c-axis and a canted moment in an otherwise antiferromagnetic structure. PNR analysis reveals the length scale over which these modulation-doped charges extend normal to the interfaces. Supported by DOE, Basic Energy Sciences, contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T37.00010: Thermopower of [LaNiO$_3$/LaAlO$_3$] Superlattices J. L. Cohn, N. Prasai, M. Kareev, Jian Liu, B. Gray, V. Kunets, J. Chakhalian, J. Freeland Thermopower measurements for $4{\rm K}\leq {\rm T}\leq 330 {\rm K}$ will be reported for LaNiO$_3$ films and [$m$LaNiO$_3$/$n$LaAlO$_3$] superlattices ($4\leq m\leq 10$, $n=3$ are layer thicknesses in unit cells) grown on (100)-oriented LaAlO$_3$ and SrTiO$_3$ substrates. The influence of in-plane, epitaxial strain (both compressive and tensile) on the magnitude of the thermopower and its relation to changes in the electronic structure will be discussed. The possible role of phonon drag in a sharp maximum at T$\simeq 30 {\rm K}$ and the use of thermopower to distinguish correlation and disorder effects in the upturn in the low-T resistivity, will also be addressed. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T37.00011: Magnetic interaction at an interface between manganite and other transition-metal oxide S. Okamoto, P. Yu, R. Ramesh A general consideration is presented for the magnetic interaction at an interface between a perovskite manganite and other transition-metal oxide. The latter is specified by the electron number $n$ in the $d_ {3z^2-r^2}$ level as $(d_{3z^2-r^2})^n$. Based on the molecular orbitals formed at the interface and the generalized Hund's rule, the sign of the magnetic interaction is rather uniquely determined. The exception is when the $d_{3z^2-r^2}$ orbital is stabilized in the interfacial manganite layer neighboring to a $(d_{3z^2-r^2})^1$ or $(d_{3z^2-r^2})^2$ system. In this case, the magnetic interaction is sensitive to the occupancy of the Mn $d_{3z^2-r2}$ orbital. It is also shown that the magnetic interaction between the interfacial Mn layer and the bulk region can be changed. Based on this consideration, we discuss the magnetic and orbital couplings at manganite/titanate, manganite/cuprate, and manganite/ferrite interfaces. [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T37.00012: Structure and transport of LaFeO$_{3}$ - Sm$_{2}$CuO$_{4}$ superlattices Flavio Y. Bruno, A. Rivera-Calzada, C. Leon, J. Garcia-Barriocanal, M. Varela, S.J. Pennycook, J. Santamaria When materials with different work functions are stacked to form a superlattice, charge transfer occurs until the electrostatic potential due to charge build up compensates the difference in work functions. This interfacial charge transfer process is a new route to dope materials. We have grown fully epitaxial superlattices consisting of Sm$_{2}$CuO$_{4}$ (SCO), the parent compound of the electron doped superconducting cuprates, and LaFeO$_{3}$ (LFO) an antiferromagnetic insulator. A detailed structural characterization by means of x-ray diffraction, scanning transmission electron microscopy, and atomic force microscopy, demonstrates the high structural quality of our samples. We will show electronic transport measurements of the superlattices, and SCO and LFO individual thin films supporting the possibility of electron doping the Sm$_{2}$CuO$_{4}$ in these samples. [Preview Abstract] |
Wednesday, March 17, 2010 5:18PM - 5:30PM |
T37.00013: Novel Multifunctional Properties Induced by Interface Effects in Perovskite Oxide Heterostructures Kuijuan Jin, Huibin Lu, Kun Zhao, Chen Ge, Meng He, Guozhen Yang Multilayers may lead to interesting artificial materials with novel properties. In this meeting we will show that the introducing of interfaces into perovskite oxides can induce a series of novel properties including an unusual positive magnetoresistance, great enhancement of lateral photovoltage in La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrNb$_{0.01}$Ti$_{0.99}$O$_{3}$, and an electrical-modulation of the magnetoresistance in multi-$p-n$ heterostructures of SrTiO$_{3-\delta }$/La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrTiO$_{3-\delta }$/La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/Si. The novel positive magnetoresistance is attributed to the creation of a space charge region at the interface where the spin of carriers is anti-parallel with the spin of carriers in the region far from the interface of manganese oxide in the heterostructures [1]. \\[4pt] [1] Kui-juan Jin, Hui-bin Lu, Kun Zhao, Chen Ge, Meng He, and Guo-zhen Yang, Adv. Matter. 21, (2009 in press). [Preview Abstract] |
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