Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B13: Complex Oxide Films and Heterostructures - Growth and Properties |
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Sponsoring Units: DCMP Chair: Scott Chambers, Pacific Northwest National Laboratory Room: 007D |
Monday, March 2, 2015 11:15AM - 11:51AM |
B13.00001: Topological properties and correlation effects in oxide heterostructures Invited Speaker: Satoshi Okamoto Transition-metal oxides (TMOs) have long been one of the main subjects of material science because of their novel functionalities such as high-$T_c$ superconductivity in cuprates and the colossal magnetoresistance effect in manganites. In recent years, we have seen tremendous developments in thin film growth techniques with the atomic precision, resulting in the discovery of a variety of electronic states in TMO heterostructures. These developments motivate us to explore the possibility of novel quantum states of matter such as topological insulators (TIs) in TMO heterostructures. In this talk, I will present our systematic theoretical study on unprecedented electronic states in TMO heterostructures. An extremely simple but crucial observation is that, when grown along the [111] crystallographic axis, bilayers of perovskite TMOs form buckled honeycomb lattices of transition-metal ions, similar to graphene. Thus, with the relativistic spin-orbit coupling and proper band filling, two-dimensional TI states or spin Hall insulators are anticipated. Based on tight-binding modeling and density-functional theory calculations, possible candidate materials for TIs are identified. By means of the dynamical-mean-field theory and a slave-boson mean field theory, correlation effects, characteristics of TMOs, are also examined. I will further discuss future prospects in topological phenomena in TMO heterostructures and related systems. \\ The author thanks D. Xiao, W. Zhu, Y. Ran, R. Arita, Y. Nomura and N. Nagaosa for their fruitful discussions and collaboration. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B13.00002: Effect of band filling and symmetry breaking on the electronic ground state in (La$X$O$_{3})_{2}$/(LaAlO$_{3})_{4}$(111) ($X=$3d) superlattices David Doennig, Warren E. Pickett, Rossitza Pentcheva Structural patterns, e.g. a buckled honeycomb lattice, realized in (111)-oriented perovskite bilayers may lead to exotic electronic ground states such as a Dirac-point Fermi surface [1-3]. Based on density functional theory calculations including a Hubbard $U$ term, we explore systematically the effect of band filling in [111]-oriented (La$X$O$_{3})_{2}$/(LaAlO$_{3})_{4}$ superlattices with $X$ spanning the series of open shell \textit{3d} ions. The interplay of charge, spin, orbital, and lattice degrees of freedom reveals some regularities over the series, but also several unexpected symmetry lowering reconstructions that can guide the design of artificial materials of desired spin-charge-orbital order in conjunction with size of the energy gap and the possibility for emergent topological character. \\[4pt] [1] D. Xiao, et al., Nat. Commun. \textbf{2}, 596 (2011).\\[0pt] [2] D. Doennig, W. E. Pickett, and R. Pentcheva, Phys. Rev. Lett. \textbf{111}, 126804 (2013).\\[0pt] [3] D. Doennig, W. E. Pickett, and R. Pentcheva, Phys. Rev. B \textbf{89}, 121110(R) (2014). [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B13.00003: Raman Scattering in La$_{0.2}$Sr$_{0.8}$FeO$_{3-\delta}$ thin film: annealing-induced reduction and phase transformation Mohammad Islam, Yujun Xie, Mark Scafetta, Steven May, Jonathan Spanier Raman scattering in thin film La$_{0.2}$Sr$_{0.8}$FeO$_{3-\delta }$ on MgO(001) collected at 300 K following different stages of annealing at selected temperatures (300 K \textless T \textless 543 K, to 10 h.) and analysis reveal changes in spectral characteristics due to loss of oxygen, onset of oxygen vacancy-induced disorder, and activation of Raman-inactive modes that are attributed to symmetry transformation. The interpretation is further supported by carrier transport measurements under identical conditions showing orders of magnitude increase in the resistivity induced by oxygen loss. After prolonged annealing in air, evolution of the spectrum is consistent with the appearance of a topotactic transformation of the crystal structure from that of the rhombohedral \textit{AB}O$_{3}$ perovskites to that of Brownmillerite-like structure consisting of octahedrally and tetrahedrally coordinated Fe atoms. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B13.00004: Strain control of oxygen stoichiometry in epitaxial perovskites Ho Nyung Lee, Tricia Meyer, Jonathan Petrie, Shinbuhm Lee, John Nichols, S.S. Ambrose Seo, John Freeland Many physical properties of transition metal oxides (TMOs) are critically dependent upon the oxidation state of transition metals. Thus, a precise control of oxygen stoichiometry is critical to unambiguously understand many intriguing properties and functionalities. Based on a recent discovery of TMO-based oxygen sponges that can shed or absorb oxygen at highly reduced temperatures as low as 200 $^{\circ}$C [Jeen \textit{et al}., Nature Mater. \textbf{12}, 1057 (2013) and Choi \textit{et al}., Phys. Rev. Lett. \textbf{111}, 097401 (2013)], we have explored various complex oxide materials to control the oxygen stoichiometry and, thereby, the critical physical properties. The latter include superconductivity in doped La$_{2}$CuO$_{4}$, metal-insulator transition in VO$_{2}$, and electronic and ionic conductivity as well as magnetism in SrCoO$_{3-d}$. In particular, by tuning strain systematically via lattice mismatching, we found that the epitaxial strain is a great tool to create functional defects that are critical in discovering new functionalities and/or improving the performance of materials especially for electronic and ionic conduction in complex oxides. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B13.00005: Epitaxial Growth of BaSnO$_{3}$ using Hybrid Molecular Beam Epitaxy Abhinav Prakash, Tianqi Wang, Christian M. Schlep\"utz, Bharat Jalan Using co-deposition of a chemical precursor for Sn, a solid source for Ba and an RF plasma source for oxygen, we have extended the hybrid MBE approach for the growth of stoichiometric BaSnO$_{3}$. First, we present a detailed growth study of SnO$_{2}$ on r-plane sapphire as a function of Sn flux, oxygen pressure and substrate temperature. High-resolution x-ray diffraction (HRXRD) and AFM showed single phase, epitaxial SnO$_{2}$ films and smooth surfaces, respectively. Three growth regimes were identified: reaction-, flux- and desorption-limited with increasing substrate temperature. Further study at constant substrate temperature revealed growth rate increases first and then becomes constant with increasing tin flux. We will then present a comprehensive study of the growth of phase-pure, smooth epitaxial BaSnO$_{3}$ films on SrTiO$_{3}$. HRXRD of 5nm BaSnO$_{3}$ on SrTiO$_{3}$ using synchrotron radiation suggest that films grow mostly coherent with out-of-plane lattice parameters of 4.20-4.25{\AA} for different cation flux ratios, remarkably similar to the calculated value of 4.26{\AA} using elastic tensors assuming BaSnO$_{3}$ grows coherent on SrTiO$_{3}$. Strain relaxation, stoichiometry control and their roles on the electronic transport will be discussed. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B13.00006: Structure and Electronic Transport in BaSnO$_{3}$ Deposited \textit{via} High Pressure Oxygen Sputtering Koustav Ganguly, Palak Ambwani, Jong Seok Jeong, K. Andre Mkhoyan, Peng Xu, Chris Leighton, Bharat Jalan We present structural and electronic transport properties of oxygen vacancy-doped BaSnO$_{3}$ films grown on SrTiO$_{3}$(001) and MgO(001) using the high pressure oxygen sputtering technique. High-resolution x-ray diffraction (HRXRD), combined with scanning transmission electron microscopy (STEM), confirms phase-pure epitaxial BaSnO$_{3}$(001) films on both substrates. The out-of-plane lattice parameter obtained from wide-angle x-ray diffraction is used as a sensitive probe for cation stoichiometry and strain relaxation. Irrespective of growth parameters, the out of plane lattice parameter remains unchanged, close to that of bulk (4.116 {\AA}). A detailed thickness-dependent structural study using HRXRD and STEM suggests the formation of misfit dislocations as the primary mechanism for strain relaxation. We further show that as-grown, insulating BaSnO$_{3}$ films can be made conductive with n-type carriers (with typical room temperature concentrations and Hall mobilities being 10$^{19}$ cm$^{-3}$ and 12 cm$^{2}$V$^{-1}$s$^{-1})$ via high temperature (900 $^{\circ}$C) vacuum annealing. Analysis of transport data from films with fixed density indicates a significant influence of film thickness, and thus strain relaxation, on the electron mobility. Temperature-dependent transport and magnetotransport studies will be described in detail as a function of annealing conditions, and will be correlated to strain relaxation. This work is supported by NSF through the UMN MRSEC. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B13.00007: A Conductive Polar Interface with high mobility formed between LaInO$_{3}$ and BaSnO$_{3}$ perovskite oxides Useong Kim, Chulkwon Park, Taewoo Ha, Young Mo Kim, Namwook Kim, Chanjong Ju, Jaejun Yu, Jae Hoon Kim, Kookrin Char LaInO$_{3}$/BaSnO$_{3}$ (LIO/BSO) polar interface is the interface between BaSnO$_{3}$ (BSO), a non-polar perovskite oxide with high oxygen stability and electron mobility, and LaInO$_{3}$ (LIO), a polar perovskite oxide with the matched lattice parameters. Once the LIO/BSO interface forms, the conductance in the interface is significantly enhanced. The high oxygen stability of BSO enables dopant-controlled transport experiments by ruling out the involvement of oxygen vacancies in the transport phenomena. The conductance enhancement at the LIO/BSO interface was monitored while varying the doping rate of La dopants in the BSO layer. As a result, we found that the La doping rate was a dominant factor determining the extent to which the conductance was enhanced. It implies that the electronic reconstruction at the polar interface depends critically on the initial position of the Fermi level in the BSO side. The high electron mobility of BSO enables the exploitation of the conductive LIO/BSO interface at room temperature. We fabricated a field effect transistor utilizing such interface. At room temperature the device shows outstanding performances in terms of three device parameters: field effect mobility higher than 90 cm$^{2}$/Vs, on/off ratio as high as 10$^{7}$, and subthreshold swing as low as 0.65 V/dec. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B13.00008: Enhanced electrical mobility in the (La,Ba)SnO$_{3}$ film grown on BaSnO$_{3}$ (001) substrate Kee Hoon Kim, Hyung Joon Kim, Woong-Jhae Lee, Tai Hoon Kim, Egon Sohn, Ju-Young Park, Ki-Young Choi Doped BaSnO$_{3}$ (BSO) systems with a perovskite structure are drawing increasing interests because of their high electrical mobility ($\approx $300 cm$^{2}$V$^{-1}$s$^{-1})$, wide optical band gap ($\ge $ 3.1 eV) and superior oxygen stability. In order to realize a semiconducting device with high speed based on the doped BSO films, an insulating substrate made of the BSO single crystal will be indispensable to the realization of trunly epitaxial films without structural defects. Here, we report the successful growth of an insulating BSO single crystal by using the cupric-oxide-based flux growth method with an oxidizer. After preparing the BSO(001) substrate with one side polished, we deposited epitaxial La doped BSO films on the BSO substrate (BLSO/BSO(001)) by using the pulsed laser deposition. The electrical mobility ($\mu )$ of BLSO/BSO(001) films are found to be $\approx $70-100 cm$^{2}$V$^{-1}$s$^{-1}$ in the low (10$^{19}$ cm$^{-3})$ to high ($\ge $10$^{20}$ cm$^{-3})$ doping ranges, which are clearly larger than those grown on SrTiO$_{3}$ (STO) substrate ($\approx $15-60 cm$^{2}$V$^{-1}$s$^{-1})$. We'll also show some of our recent efforts to realize the field effect transistor based on the BSO single crystal substrate. The present results show that the single crystal BSO substrate can offer various opportunities to realize practical electronic devices based on the doped BSO films. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B13.00009: Interfacial engineering of optical absorption in epitaxial LaCrO$_{3}$-SrTiO$_{3}$ superlattices Ryan Comes, Tiffany Kaspar, Steve Heald, Mark Bowden, Scott Chambers SrTiO$_{3}$ (STO) is a wide-gap semiconductor well suited for photocatalytic H$_{2}$ production due to the alignment of its band edges with the half-cell energies of the H$_{2}$O redox reactions. However, the wide optical gap of STO (3.3 eV) makes the material an inefficient light absorber in the visible spectrum, preventing formation of electron-hole pairs needed for photocatalysis. Superlattice films comprised of alternating layers of band insulator SrTiO$_{3}$ and Mott insulator LaCrO$_{3}$ (LCO) have been theoretically predicted to offer intriguing optical properties due to the broken symmetry between the unoccupied Ti d$_{\mathrm{xy}}$ and Ti d$_{\mathrm{xz}}$ and d$_{\mathrm{yz}}$ orbitals. In this work, we examine the properties of LCO-STO superlattices grown with various periodicities on (La,Sr)(Al,Ta)O$_{3}$ (LSAT) (001) substrates using oxide molecular beam epitaxy. Films were characterized via \textit{in situ} x-ray photoelectron spectroscopy to measure valence band structure and interfacial band bending. Polarized Ti and Cr K-edge x-ray absorption near edge spectroscopy was used to examine the bonding anisotropy. Spectroscopic ellipsometry measurements show the presence of interfacially-induced visible light absorption not found in either STO or LCO. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B13.00010: Tuning the physical properties in strontium iridate heterostructures John Nichols, Tricia Meyer, Ho Nyung Lee Strontium iridate (Sr$_{\mathrm{n+1}}$Ir$_{\mathrm{n}}$O$_{\mathrm{3n+1}})$ has received lots of attention recently for its potential to reveal novel physical phenomena due to strong spin-orbital coupling with an interaction energy comparable to that of the on-site Coulomb interaction and crystal field splitting. The coexistence of fundamental interactions has created an exotic J$_{\mathrm{eff}} \quad =$ 1/2 antiferromagnetic insulating ground state in Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$. In particular, it is known that this system can be driven into a metallic state with the simultaneous increase in dimensionality (n) and strain. We have investigated the effects of electron confinement by interfacing strontium iridates with other perovskite oxides. We have synthesized thin film heterostructures, SrIrO$_{\mathrm{3}}$/AMO$_{\mathrm{3}}$ (A $=$ Sr, La; B$=$ Ti, Mn, Rh), layer-by-layer with pulsed laser deposition equipped with reflection high-energy electron diffraction. Based on investigations with x-ray diffraction, \textit{dc} transport, SQUID magnetometry, and various spectroscopic measurements, we will present that the physical properties of the heterostructures are strongly dependent on spatial confinement and epitaxial strain. *This work was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B13.00011: Strain-controlled easy axis orientation of epitaxial CoFe$_2$O$_4$ films by He implantation Andreas Herklotz, Antony T. Wong, Stefania F. Rus, Thomas Z. Ward Heteroepitaxial strain engineering is an essential tool in the strongly correlated systems for investigating fundamental coupling effects and for more practical control of thin film properties. Here, we use strain doping by He implantation as an alternative technique to control thin film functionalities. We demonstrate the tuning of the magnetic anisotropy of CoFe$_2$O$_4$ (CFO) films through He implantation. Compressively strained thin films of CFO are grown coherently on MgO substrates and show pronounced out-of-plane magnetic anisotropy. Successive doping of the CFO films with He using a commercial ion gun results in an expansion of the out-of-plane lattice parameter while maintaining in-plane epitaxial lock to the substrate. We observe a continuous rotation of the magnetic easy axis towards the film plane with increasing unit cell tetragonality. The results are in agreement with the strain-induced change of the magnetic anisotropy due to the large negative magnetostriction of CFO and demonstrate that strain doping via He implantation is an elegant path to tune desired characteristics of transition metal oxide thin films. This work was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B13.00012: Atomic structure, electronic properties, and band offsets of SrRuO$_{3}$/TiO$_{2}$ heterojunctions Naheed Ferdous, Elif Ertekin Photocatalytic water splitting by sunlight can in principle be an environmentally green approach to hydrogen fuel production, but at present photocatalytic conversion efficiencies remain too small. In titanium dioxide (TiO$_{2})$, the most commonly used photocatalyst, the biggest limitation arises from poor absorption of visible light. One way to increase the visible light absorption is to create a composite heterojunction by integrating TiO$_{2}$ with a strongly light absorbing material. Inspired by experimental results demonstrating good light absorption in the correlated metal oxide Strontium Ruthenate (SrRuO$_{3})$, as well as enhanced photocatalytic activity of SrRuO$_{3}$/TiO$_{2}$ heterojunctions [1], we have carried out electronic structure calculations based on density functional theory to explain and improve on the observed properties of such heterojunctions. Our calculations present that this heterojunction exhibits type-II band alignment which is necessary to transport optically excited electrons from the SrRuO$_{3}$ to the TiO$_{2}$, with calculated work functions in good agreement with experimental measurements. Also, DFT calculations help to explain the origin of large light absorption in the correlated metal oxide, which arises from electronic excitations from O 2p levels into the Ru d-orbital quasiparticle states in the material. The use of correlated metal oxide/ TiO$_{2}$ heterojunctions is a potentially interesting approach to improved photocatalytic activity. [1] Lee, S, Apgar, B A, {\&} Martin, L W. (2013). Strong visible-light absorption and hot-carrier injection in tio2/srruo3heterostructures. Advanced energy materials, 3(8), 1084-1090. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B13.00013: ABSTRACT WITHDRAWN |
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