Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L37b: Complex Oxide Interfaces and Heterostructures - Nickelates, Vanadates and VO2Focus
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Sponsoring Units: DMP DCMP Chair: Anand Bhattacharya, Argonne National Laboratory Room: 384 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L37b.00001: Probing momentum-resolved electronic structure of buried artificial graphenelike Mott crystal NdNiO3 [111] with soft x-ray angle-resolved photoemission Arian Arab, Weibing Yang, Ravini Chandrasena, Srimanta Middey, Vladimir Strokov, Kristjan Haule, Jak Chakhalian, Alexander Gray Transition metal oxides (TMO) exhibit a wide variety of potentially advantageous strongly-correlated electronic phenomena such as metal-insulator transitions, high-Tc superconductivity, half-metallicity, etc. Until now, however, most of the work has been focused on synthesizing and investigating systems that are grown along the [001] pseudocubic direction. Here we utilize soft x-ray angle-resolved photoemission spectroscopy to investigate the momentum-resolved valence-band electronic structure of artificial graphenelike Mott crystal NdNiO3 grown along the [111] direction. Our measurements reveal broken six-fold symmetry of the Ni 3d eg states hosted on a buckled honeycomb lattice. This engineered electronic structure is unique to the ultrathin (2 u.c.) quazi-2D crystal, and cannot be realized either in the bulk or in the thin-film Nickelate grown along the conventional [001] direction. Our findings open the door for engineering novel polarized Mott-electronic ground states in rare-earth Nickelates, as well as other strongly-correlated transition-metal oxides. [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L37b.00002: The role of the defects in the structural transitions in NdNiO$_{\mathrm{3}}$ (NNO)/GaNdO$_{\mathrm{3}}$ films Zhen Wang, Srimanta Middey, Jing Tao, Yimei Zhu, Jak Chakhalian, Jiandi Zhang, E.W. Plummer The metal-insulator transition (MIT) in bulk NNO is observed to be associated with a structural phase transition. However, it was reported that the structural transition, probed by x-ray scattering techniques, is not necessary to in the MIT taking place in the NNO thin films. In order to the origin of the MIT in the NNO thin films, we employ the transmission electron microscopic (TEM) techniques to further characterize the crystal structures in real-space. Ruddlesden-Popper (R-P) planar faults are a special type of defects existing in many films. We found that the density of the R-P defects is different in the NNO thin films with distinct film thickness. Particular, few R-P defects were observed in the very thin NNO film (thickness \textasciitilde 6 nm), while significant amount of R-P defects can be found in the film about 20 nm thick. Our structural observations clearly indicate that both the shift of A-site (Nb) ions positions and Ni-O-Ni tilt angles are suppressed near the R-P planar faults compared to the bulk NNO. More structural characterizations, such as the strain due to the substrate and elemental mapping at the interfaces/defects, will be discussed to demonstrate the possible mechanisms of the structural symmetry anomaly in the NNO thin films. [Preview Abstract] |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L37b.00003: Ionic Potential and Band Narrowing as a Source of Orbital Polarization in Nickelate/Insulator Superlattices Alexandru B. Georgescu, Ankit S. Disa, Divine P. Kumah, Sohrab Ismail-Beigi, Frederick J. Walker, Charles H. Ahn Nickelate interfaces display complex, interacting electronic properties such as thickness dependent metal-insulator transitions. One large body of effort involving nickelates has aimed to split the energies of the Ni 3d orbitals (orbital polarization) to make the resulting band structure resemble that of cuprate superconductors. The most commonly studied interfacial system involves superlattices of alternating nickelate and insulating perovksite-structure layers; the resulting orbital polarization at the nickelate-insulator interface is understood as being due to confinement or structural symmetry breaking. By using first principles theory on the NdNiO$_{3}$/NdAlO$_{3}$ superlattice, we show that another important source of orbital polarization stems from electrostatic effects: the more ionic nature of the cations in the insulator (when compared to the nickelate) can shift the relative orbital energies of the Ni. We use density functional theory (DFT) and add electronic correlations via slave-bosons to describe the effect of correlation-induced band narrowing on the orbital polarization. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L37b.00004: Conductivity and local structure in LaNiO$_{3}$ Jennifer Fowlie, Marta Gibert, Giulio Tieri, Alexandre Gloter, Jorge Íñiguez, Alessio Filippetti, Sara Catalano, Stefano Gariglio, Odile Stéphan, Jean-Marc Triscone In this study we approach the thickness-dependence of the properties of LaNiO$_{3}$ films along multiple, complementary avenues: sophisticated ab initio calculations, scanning transmission electron microscopy and electronic transport. Specifically, we find an unexpected maximum in conductivity in films of thickness 6 - 10 unit cells (3 nm) for several series of LaNiO3 films. Ab initio transport based on the detailed crystal structure also reveals a maximum in conductivity at the same thickness. In agreement with the structure obtained from scanning transmission electron microscopy (STEM), our simulated structures reveal that the substrate- and surface-induced distortions lead to three types of local structure (heterointerface, interior-layer, surface). Based on this observation, a 3-element parallel conductor model neatly reproduces the trend of conductivity with thickness. This study addresses the question of how structural distortions at the atomic scale evolve in a thin film under the influence of the substrate and the surface. This topic is key to the understanding of the physics of heterostructures and the design of functional oxides. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L37b.00005: Interfacial effects on the electronic structure of LaNiO3 films Zhigang Gui, Anderson Janotti LaNiO$_3$ (LNO) is an interesting and unique oxide in the family of perovskite nickelates. For instance, bulk LNO remains metallic and paramagnetic all the way to low temperatures, with no signature of the metal-insulator transition (MIT) and long-range magnetic order as commonly seen in other bulk nickelates. However, MIT has been reported to occur in oxygen-deficient or extremely thin LNO films, the cause of which has been widely debated. Using density functional calculations we study the effects of excess electrons or holes on the electronic and structural properties of LNO bulk and thin epitaxial films. Special attention is paid to interfacial effects, where electrons are transferred to or from LNO thin films through the interfacial termination of the perovskite oxide substrate. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:51PM |
L37b.00006: The electronic structure and properties of negative charge transfer gap and mixed valent Oxides Invited Speaker: George Sawatzky In high oxidation state oxides like the trivalent Nickel oxides, tetravalent Co and Fe oxides as well as the parent superconductors BaBiO3 and SrBiO3 the cation electron affinity in the formal valence could end up larger than the O 2- ionization potential leading to a so called negative charge transfer gap. If the charge transfer energy is strongly negative, then we should really adopt starting electronic configurations such as Ni2$+$ rather than 3 $+$ or Bi 3$+$ rather than 4$+$ with compensation holes in the O 2p valence band for charge neutrality. This leads to very different electronic structures and descriptions of the physical properties and the interpretation of spectroscopies than when starting from the formal oxidation state picture. We demonstrate that with this in mind we can very well explain many of the properties and phases of the Nickelates and the Ba(Sr)Bi O3 perovskite oxides as well as their spectroscopic properties. We discuss the general problem of treating such systems resulting in inverted crystal field [pictures and low spin rather than high spin states even for modest crystal fields. We also use a cluster exact diagonalization calculation to show that the x ray spectroscopy results now agree very well with experiment which has been a problem for decades. In the intermediate range of negative charge transfer gap systems we have to deal with mix valent starting points for which SmB6 is a typical example but also may include 3d transition metal compounds. We describe how these systems often termed as Kondo Lattice problems can be described in terms of mixed valent in momentum space when they are in their coherent low temperature state. Mixed valent in moment space we argue requires a strongly momentum dependent self energy in the description of the quasi particle spectral functions. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L37b.00007: (LaTiO$_3$)$_n$/(LaVO$_3$)$_n$ as a model system for unconventional charge transfer and polar metallicity Yakui Weng, Jun-Jie Zhang, Bin Gao, Shuai Dong At interfaces between oxide materials, lattice and electronic reconstructions always play important roles in exotic phenomena. In this study, the density-functional theory and maximally localized Wannier functions are employed to investigate the (LaTiO$_3$)$_n$/(LaVO$_3$)$_n$ magnetic superlattices. By considering lattice distortion and dimensional effect, many interesting interfacial physics have been found in the $n=1$ superlattice, e.g. magnetic phase transition, unconventional charge transfer, and metal-insulator transition. In addition, the compatibility among the polar structure, ferrimagnetism, and metallicity is predicted in the $n=2$ superlattice. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L37b.00008: Metal-Insulator crossover in SrVO$_{\mathrm{3}}$ thin film Gaomin Wang, Zhen Wang, Mohammad Saghayezhian, Chen Chen, Lina Chen, Hangwen Guo, Yimei Zhu, Jiandi Zhang Paramagnetic metallic oxide SrVO$_{\mathrm{3}}$ (SVO) represents a prototype system for the study of the mechanism behind thickness-induced metal-to-insulator transition (MIT) or crossover due to its simple structure and itinerancy. Here SrVO$_{\mathrm{3}}$ thin films with different thicknesses were obtained through the layer-by-layer growth by laser Molecular Beam Epitaxy on SrTiO$_{\mathrm{3}}$ (001) surface. Ultraviolet Photoemission Spectroscopy and Scanning Tunneling Spectroscopy measurements confirm a MIT at the thickness of \textasciitilde 3 unit cell, while atomically resolved Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy analysis reveal the depletion of Sr, change of V-valence and expansion of the out-of-plane lattice constant in the first three unit cell above the interface, thus different from the rest of the films. The existence of significant amount of oxygen vacancies is proposed, which is also supported by X-ray Photoelectron Spectroscopy, therefore providing a possible explanation of MIT. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L37b.00009: Mapping growth windows in strongly-correlated quaternary perovskite oxide systems by hybrid molecular beam epitaxy matthew brahlek, Lei Zhang, Jason Lapano, Haitian Zhang, Roman Engel-Herbert Metal-insulator transitions, high-temperature superconductivity and colossal magnetoresistance represent a few of the many phenomena that emerge in the solid solution $A$'$_{\mathrm{1-x}}A_{\mathrm{x}}B$O$_{\mathrm{3}}$. Growing these in thin film form is, however, a challenge due to the precise control required for the composition, $x$, as well as the stoichiometry ($A+A')$:$B$. The hybrid metal-organic \quad molecular beam epitaxy (hMBE) technique has been shown to exactly control stoichiometry, but requires understanding how to interpolate the growth conditions between the end members $A$'$B$O$_{\mathrm{3}}$ and \textit{AB}O$_{\mathrm{3}}$. Using the example of La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$VO$_{\mathrm{3}}$, the two-dimensional growth parameter space spanned by the flux of the metal-organic precursor vanadium oxytriisopropoxide and composition, $x$, can be mapped quickly with a single calibration sample using \textit{in situ} reflection high-energy electron (RHEED), which is corroborated by X-ray diffraction and atomic force microscopy.[1] This strategy enables the identification of growth conditions that allow the deposition of stoichiometric perovskite oxide films with random $A$-site cation mixing. In particular, at the quantum critical point that separates the Mott-insulator (LaVO$_{\mathrm{3}})$ from a strongly-correlated Fermi-liquid (SrVO$_{\mathrm{3}})$ this ability to produce ultrahigh quality films allows the novel competition between disorder-effects and electron-electron interactions to be revealed. This work was supported by the Dept. of Energy (DE-SC0012375). [1] M. Brahlek, \textit{et al} Appl. Phys. Lett. 109, 101903 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L37b.00010: Chemical strain engineering of magnetism in PrVO3 thin films Wilfrid Prellier, Olivier Copie, Julien Varignon, Helene Rotella, Gwladys Steciuk, Philippe Boullay, Alain Pautrat, Adrian David, Bernard Mercey, Philippe Ghosez Transition metal oxides having a perovskite structure present a wide range of functional properties ranging from insulator-to-metal, ferroelectricity, colossal magnetoresistance, high-temperature superconductivity and multiferroicity. Such systems are generally characterized by strong electronic correlations, complex phase diagrams and competing ground states. In addition, small perturbation induced by external stimuli (electric or magnetic field, temperature, strain, pressure..) may change structure, and ultimately modify the physical properties. Here, we synthetize an orthorhombic perovskite praseodymium vanadate (PrVO$_3$), which is grown on strontium titanate substrate. We show that the control of the content of oxygen vacancies, the so-called chemical strain, can indeed result in unexpected properties. We further demonstrate that the Néel temperature can be tuned using the same substrate in agreement with first-principles calculations, and demonstrate that monitoring the concentration of oxygen vacancies through the oxygen partial pressure or the growth temperature can produce a substantial macroscopic tensile strain of a few percents. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L37b.00011: Intentional cation intermixing at VO2/TiO2 interfaces: Why a metallic phase with V-V dimers can exist Louis Piper, Nicholas Quackenbush, Hanjong Paik, Darrell Schlom In its bulk form, VO2 displays an abrupt metal-to-insulator transition (MIT) accompanied by a structural phase transition involving the evolution of V-V dimers.~ One can modulate electron correlation effects by preferentially filling the d sub orbitals of VO2 by growing epitaxial films on isomorphic TiO2 substrates i.e. atomically abrupt interfaces. [1] ~ Here we report our Synchrotron spectroscopy studies of intentional cation intermixing at the VO2/TiO2 interface.~ Although the~resultant film was metallic with no evidence of a MIT, the V-V dimer intensity did vary with temperature i.e. a structural transition persisted.~ We explain our findings within a hole doping model, whereby the high concentration of Ti4$+$ ions replacing V4$+$ ions within the structure heavily reduces the V 3d electron count i.e. away from the Mott criterion.~The fact that the distortion of the lattice is still thermodynamically favored at lower temperatures indicates that electron correlations are not a prerequisite for the structural phase transition of VO2.[2]~ Taken together with our previous strain studies,[1] our data elegantly shows the importance of orbital occupancy on the MIT of VO2. [1] Phys. Rev. B 93, 241110(R) (2016) [2] to be published (2016) [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L37b.00012: Mechanical Control of Metal-Insulator Transition in Epitaxial Vanadium Dioxides Yogesh Sharma, Changhee Sohn, John Nichols, Qian Li, Nina Balke, Panchapakesan Ganesh, Janakiraman Balachandra, Olle Heinonen, Ho Nyung Lee Strongly correlated vanadium dioxide (VO$_{2})$ is one of the most promising materials exhibiting a temperature-driven metal-insulator transition (MIT) in the vicinity of room temperature. Thus, establishing control over the MIT of VO$_{2}$ by means of external stimuli, such as strain, temperature, and electric field, is an important task to realize VO$_{2}$-based advanced electronic devices. Herein, we explore the mechanical control of the conductivity and hence the MIT in epitaxial VO$_{2}$ thin films using electronic transport and scanning probe microscopy approaches. We found that the mechanical pressure could modulate VO$_{2}$ film's conductivity. This result is attributed to the piezochemical effect as oxygen stoichiometry is strongly influenced by the applied mechanical pressure. This piezochemical coupling is further realized by observed shifts in electrochemical potential of oxygen vacancies as a function of applied pressure and electric field. We relate our results to calculations of oxygen vacancy formation energy as function of pressure. Such strain-induced changes in electrochemical potential and conductance of VO$_{2}$ films indicate the local pressure-induced MIT, and can be explained based on the flexoelectric effect and/or Vegard strain effect. Our findings provide the basis for piezochemical control of MIT in VO$_{2}$ thin films.~ [Preview Abstract] |
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