Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T13: Focus Session: Nickelate and Vanadate Heterostructures |
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Sponsoring Units: DMP Chair: Yuri Suzuki, Stanford University Room: 007D |
Thursday, March 5, 2015 11:15AM - 11:51AM |
T13.00001: Geometry and heterointerface engineered phases of nickelates Invited Speaker: Jak Chakhalian Deterministic control over the spatial arrangement atoms in a crystal is the backbone of its properties that, along with the interactions, defines its ground state. Following this notion, several theoretical proposals exist to utilize a few unit cells of a correlated oxide heterostructured along the pseudo-cubic (111) direction. This geometrically engineered motif relies on the presence of correlated carriers placed on a buckled honeycomb (i.e., graphene-like) lattice, or dice lattices for bilayers and trilayers of ABO$_3$ perovskites. The guiding principle is to use strong electronic correlations combined with quantum confinement and symmetry-breaking interfaces to enable access to new electronic band structures that may activate novel or latent quantum phases. In this talk, the current status of research in this field will be reviewed. The experimental challenges in realization and characterization of such heterostructures will be exemplified by rare earth nickelates heterostructures. Several promising examples of such geometrically engineered artificial Mott materials will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T13.00002: Tunable Charge and Spin Order in PrNiO3 Thin Films and Superlattices Matthias Hepting, Matteo Minola, Alex Frano, Georg Cristiani, Meng Wu, Martin Bluschke, Yi Lu, Hans-Ulrich Habermeier, Gennady Logvenov, Eva Benckiser, Mathieu Le Tacon, Bernhard Keimer The rich phase diagram of transition metal oxides such as rare-earth nickelates \textit{R}NiO$_{3}$ (\textit{R} = rare-earth) results from the interplay between charge, orbital, spin, and lattice degrees of freedom. Recent progresses in the growth of epitaxial heterostructures have allowed the design of new phases, distinct from the ones existing in bulk materials. We have used polarized Raman scattering in conjunction with resonant soft x-ray diffraction to probe charge and magnetic ordering in epitaxially strained PrNiO$_{3}$ films, and in superlattices (SLs) of PrNiO$_{3}$ with the insulator PrAlO$_{3}$. As a function of epitaxial strain and spatial confinement we have identified three different states, only two of which were observed before in bulk \textit{R}NiO$_{3}$: an insulating state with robust spin and charge order in films and SLs under tensile strain, and a metallic state with neither form of order in films under compressive strain; the SLs under compressive strain however exhibit a weakly metallic state with fully developed spin order and without presence of charge order. This novel pure spin-density wave ground state has been predicted by theory but experimental evidence remained elusive so far. [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T13.00003: The effect of the interface termination on the atomic and electronic structure of LaNiO$_3$/PbZr$_x$Ti$_{1-x}$O$_3$ Andrei Malashevich, Matthew S. J. Marshall, Ankit S. Disa, Frederick J. Walker, Charles H. Ahn, Sohrab Ismail-Beigi Thin film metal oxide/ferroelectric interfaces can exhibit dependence of conductivity on the polar state of the ferroelectric layer. This property has potential for technological applications in non-volatile field-effect devices. Recently, we demonstrated that ferroelectric PbZr$_{0.2}$Ti$_{0.8}$O$_3$ (PZT) can be used to modulate conductivity of the (001)-oriented LaNiO$_3$/PZT interface. We found that changes in conductivity result primarily from large mobility changes in the interfacial channel region. In this study, we investigate the effect of the LaNiO$_3$ film termination (LaO vs NiO$_2$) on the atomic structure and electronic properties of LaNiO$_3$/PZT. We present the results of the first-principles calculations of the atomic structure of the related LaNiO$_3$/PbTiO$_3$ interface for both LaNiO$_3$ terminations. For each termination, we analyze the dependence of the atomic structure and electronic properties on the ferroelectric PbTiO$_3$ polar state and compare the results to available experimental observations. [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T13.00004: Strain Control of Electronic Phase in Rare Earth Nickelates Zhuoran He, Andrew Millis In this work, we use DFT+$U$ methods to study the effects of strain on the electronic states and lattice structure of thin films of LuNiO$_3$. We model the effects of a substrate-induced strain by fixing the in-plane lattice parameter and relax both the $c$-axis lattice parameter and all internal coordinates. Both compressive and tensile strain destroy the charge order and create a metallic state. Tensile strain induces a staggered Jahn-Teller order. The staggered Jahn-Teller state is shown to compete with the charge-ordered state. The transitions are found to be first order, but the insulating gap in the charge-ordered phase varies substantially with applied strain. Implications for experiments are discussed. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T13.00005: Comparative study of LaNiO$_{3}$/LaAlO$_{3}$ heterostructures grown by oxide MBE and PLD techniques Friederike Wrobel, Gennady Logvenov, Georg Christiani, Eva Benckiser, Alison F. Mark, Wilfried Sigle, Peter A. van Aken, Bernhard Keimer The physical properties of functional oxides can be altered by, e.g., dimensionality, strain, interfacial interaction and doping. Ozone assisted molecular beam epitaxy (oxide MBE) is a technique that gives wide access to all tuning parameters while having a low deposition energy. We succeeded in growing high-quality heterostructures based on LaNiO$_{3}$ with oxide MBE and pulsed laser deposition (PLD) and compared crystallinity, resistivity, x-ray absorption (XAS), orbital polarization and high-resolution transmission electron microscopy (HRTEM) images. Despite the difference in growth conditions, the samples show essentially the same physical properties: By reducing the layer thickness, LaNiO$_{3}$ turns from a paramagnetic metal into an antiferromagnetic insulator. XAS confirms the nickel $+$III oxidation state and that the orbital polarization is mainly controlled through substrate strain. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T13.00006: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T13.00007: Orbital engineering of nickelates in three-component heterostructures Ankit Disa, Divine Kumah, Andrei Malashevich, Hanghui Chen, Sohrab Ismail-Beigi, Fred Walker, Charles Ahn, Eliot Specht, Dario Arena The orbital configuration of complex oxides dictates the emergence of a wide range of properties, including metal-insulator transitions, interfacial magnetism, and high-temperature superconductivity. In this work, we experimentally demonstrate a novel method for achieving large and tunable orbital polarizations in nickelates. The technique is based on leveraging three-component, atomically layered superlattices to yield a combination of inversion symmetry breaking, charge transfer, and polar distortions. In the system we studied, composed of LaTiO$_3$/LaNiO$_3$/LaAlO$_3$, we use synchrotron x-ray diffraction and spectroscopy to characterize these properties and show that they lead to fully broken orbital degeneracy in the nickelate layer consistent with a single-band Fermi surface. Furthermore, we show that this system is widely tunable and enables quasi-continuous orbital control unachievable by conventional strain and confinement-based approaches. This technique provides an experimentally realizable route for accessing and studying novel orbitally dependent quantum phenomena. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T13.00008: Soft X-ray ARPES investigation of the nickelate Fermi surface in exchange biased LaNiO$_{3}$-LaMnO$_{3}$ superlattices Flavio Bruno, S. McKeown Walker, A. de la Torre, A. Tamai, M. Gibert, S. Catalano, J-M. Triscone, Z. Wang, F. Bisti, V. Strocov, F. Baumberger We investigate (111)-oriented superlattices consisting of paramagnetic LaNiO$_{3}$ (LNO) and ferromagnetic LaMnO$_{3}$ (LMO). The field dependence of the magnetization in these heterostructures was measured at 5 K after cooling the sample in the presence of a 0.4 T field. Surprisingly, a shift of 15 mT in the magnetization loop towards negative fields along the magnetic field axis was observed [1]. If the same measurement is repeated in a (111) LMO thin film, no exchange bias is observed which implies that LNO is the driving force for the biasing effect exhibited by the heterostructures. Since LNO is a well-known paramagnetic material, the existence of exchange bias in the superlattices implies the existence of an interface-induced magnetic order. Here we use soft x-ray angle resolved photoemission spectroscopy --SX ARPES- to study the electronic band structure of LNO layers in these heterostructures. Due to the increase in photoelectron escape depth in the 500 -- 1000 eV energy range, we are able to map the LNO Fermi surface below 7 u.c. of LMO. In this talk we will discuss the similarities and differences in the electronic structure between thin films of (111)-LNO and buried LNO-LMO interfaces.\\[4pt] [1] M.Gibert, et al, Nat. Materials, 11, 195 (2012). [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T13.00009: Advanced electron microscopy characterization of tri-layer rare-earth oxide superlattices Patrick Phillips, Ankit Disa, Sohrab Ismail-Beigi, Robert Klie Rare-earth nickelates are known to display complex electronic and magnetic behaviors owed to a very localized and sensitive Ni-site atomic and electronic structure. Toward realizing the goal of manipulating of the energetic ordering of Ni d orbitals and 2D conduction, the present work focuses on the experimental characterization of thin film superlattice structures consisting of alternating layers of LaTiO$_{\mathrm{3}}$ and LaNiO$_{\mathrm{3}}$ sandwiched between a dull insulator, LaAlO$_{\mathrm{3}}$. Using advanced scanning transmission electron microscopy (STEM)-based methods, properties such as interfacial sharpness, electron transfer, O presence, and local electronic structure can be probed at the atomic scale, and will be discussed at length. By combining both energy dispersive X-ray (EDX) and electronic energy loss (EEL) spectroscopies in an aberration-corrected STEM, it is possible to attain energy and spatial resolutions of 0.35 eV and 100 pm, respectively. Focus of the talk will remain not only on the aforementioned properties, but will also include details and parameters of the acquisitions to facilitate future characterization at this level. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T13.00010: Measurement of the quantum capacitance of two-dimensional vanadium dioxide films Zhe Wu, Talbot Knighton, Vinicio Tarquini, Jian Huang, Nelson Sep\'ulveda With a homebuilt ac bridge, we have performed capacitance measurement of quasi two-dimensional vanadium dioxide films grown on silicon-dioxide/p-doped silicon substrate. The out-phase-signal, which corresponds to the resistivity variation, is superior to the four-terminal measurement result of the temperature dependence of the resistivity which varies by four orders of magnitude from 360K to 310K. The hysteretic behavior shows an overlap of two distinctive features that indicate a shifted structural transition relative to the Mott transition. In addition, the quantum capacitance is obtained through the in-phase signals so that d$\mu $/dn, the inverse of the density of states, is determined as a function of temperature. This has resulted in a diverging compressibility below the critical temperature by four orders of magnitude, consistent with a Mott transition influenced by Peierls transition. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T13.00011: Carrier Localization in Confined Vanadate Superlattices Craig Eaton, Lei Zhang, Roman Engel-Herbert Perovskite oxide heterostructures have attracted attention due to the wealth of phenomena emerging at the interface, as well as the presence of strong electron correlations with potential applications as active electronic material for logic application utilizing the metal-to-insulator transition. Successful monolithic integration of perovskite oxides with Si makes them an ideal material choice. Here we present the growth of cubic SrTiO$_{3}$/SrVO$_{3}$/SrTiO$_{3}$ heterostructures on (La$_{0.3}$Sr$_{0.7})$(Al$_{0.65}$Ta$_{0.35})$O$_{3}$ substrates and orthorhombically distorted CaTiO$_{3}$/CaVO$_{3}$/CaTiO$_{3}$ heterostructures on (LaSrAlTa$_{4})$O$_{3}$ substrates by hybrid molecular beam epitaxy, where alkaline earth metals were supplied using conventional effusion cells and the transition metals from the metal-organic precursor titanium-isopropoxide and vanadium oxi-tri-isopropoxide. Here, the interfaces are non-polar and carrier confinement in the correlated vanadate metals (d$^{1}$ configuration, 1 electron per unit cell) is achieved using insulating titanates as barrier material. Growth challenges associated with optimizing conditions for cation and oxygen stoichiometry are discussed. Confined structures down to 2 ML have been studied to demonstrate the potential for tuning incipient 2D Mott transition from 3D correlated metal. Room temperature hall measurements revealed carrier concentration in SrVO$_{3}$ films are 2 $\times$ 10$^{22}$ cm$^{-3}$ in thick films and decreases to 8 $\times$ 10$^{20}$ cm$^{-3}$ at 3 ML confinement, revealing the onset of strong carrier localization. Direct comparison between SrVO$_{3}$ and CaVO$_{3}$ structures are presented to elucidate the role of dimensional confinement and structural distortion. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T13.00012: Emergent conductance and magnetism at metal oxide interfaces via internal charge transfer Hanghui Chen, Andrew Millis, Chris Marianetti Internal charge transfer across the interface of transition metal oxides is proven to be a powerful approach to induce new electronic structure in metamaterials (PRL 111, 116403 (2013); arXiv:1408.0217 (2014)). Here we use ab initio calculations to demonstrate that while SrVO$_3$ is a paramagnetic metal and SrMnO$_3$ is an antiferromagetic insulator, charge transfer in a SrVO$_3$/SrMnO$_3$ superlattice leads to both electronic and magnetic reconstructions on the Mn sites: the Mn $e_g$ states are electron doped and the Mn $t_{2g}$ core spins are ferromagnetically aligned. As a result, net magnetic moments are expected to emerge in the superlattice. Our work shows that charge transfer is a robust route to the design of novel two dimensional half metallic ferromagnets. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T13.00013: DFT+DMFT study of strain and interface effects in $d^1$ and $d^2$ $t_{2g}$-perovskites Gabriele Sclauzero, Krzysztof Dymkowski, Claude Ederer Metal-insulator transitions in thin films of early-transition metal correlated oxides are linked to both epitaxial strain and electronic reconstruction at the film/substrate interface. We separately address these two key factors for LaTiO$_3$ and LaVO$_3$ through density functional theory plus dynamical mean-field theory (DFT+DMFT). We find that mere epitaxial strain suffices to induce an insulator-to-metal transition in LaTiO$_3$ [1], but not in LaVO$_3$, in agreement with recent experiments [2]. We show that this difference can be explained by the combined effect of strain-induced changes in the crystal field splitting of $t_{2g}$ orbitals and different orbital filling in these two materials. The role of the interface is investigated through DFT+DMFT simulations of LaVO$_3$/SrTiO$_3$ heterostructures with varying superlattice periodicities and substrate terminations. Our aim is to assess whether the metallicity observed at the LaVO$_3$/SrTiO$_3$ interface could be driven by pure electronic reconstruction effects, rather than structural or stoichiometric reasons (such as, e.g., O-related defects). [1] Dymkowski and Ederer, Phys. Rev. B \textbf{89}, 161109 (2014). [2] He \emph{et al.}, Phys. Rev. B \textbf{86}, 081401 (2012). [Preview Abstract] |
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