Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X46: Complex Oxide Interfaces & Heterostructures -- Metal-insulator transitions and charge transfer phenomenaFocus
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Sponsoring Units: DMP Chair: Roman Engel-Herbert Room: BCEC 212 |
Friday, March 8, 2019 8:00AM - 8:36AM |
X46.00001: Probing Ultrathin Functional Layers and Buried Interfaces with Advanced X-ray Spectroscopic Techniques Invited Speaker: Alexander Gray Rational design of low-dimensional electronic phenomena at oxide interfaces is considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically-abrupt interface between paramagnetic LaNiO3 and antiferromagnetic CaMnO3 exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO3. A rich cation picture, emerging from the polarity mismatch and electronic reconstruction at the interface, is considered to be the driving factor for this phenomenon. Once fully understood, such emergent functionality could turn this Mott-interface system into a key building block for the above-mentioned future devices. In this talk, I will discuss three recent studies, in which we utilized a combination of x-ray spectroscopic and electron imaging techniques to investigate the electronic-structural origins of this emergent phenomenon. Starting with the building blocks of this heterojunction (CaMnO3 and LaNiO3), we used a combination of hard x-ray photoemission (HAXPES) and x-ray absorption spectroscopy (XAS) to establish a direct link between the in-plane strain and the oxygen-vacancy content in CaMnO3 [1]. Then, by using a combination of XAS and scanning transmission electron microscopy (STEM), we examined the nature of the metal-insulator transition in LaNiO3 in the ultrathin limit (<2 u.c.) [2]. Finally, we utilized a combination of HAXPES and standing-wave photoemission spectroscopy (SW-XPS) to demonstrate a depth-dependent charge reconstruction at the LaNiO3/CaMnO3 interface [3]. Our findings suggest a new strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states. |
Friday, March 8, 2019 8:36AM - 8:48AM |
X46.00002: Resistive switching in tunnel junctions with a single-crystal La2NiO4 electrode Shida Shen, Morgan Williamson, Gang Cao, Jianshi Zhou, Maxim Tsoi We study the resistive switching in tunnel junctions with single-crystal La2NiO4 electrodes. Such electro-resistive devices are promising candidates for future nonvolatile memory and reconfigurable logic applications thanks to their simple structure, scalability and endurance. Our tunnel junctions were prepared by painting a spot of conductive silver epoxy on the surface of La2NiO4 single crystal. The interface between the silver and the semiconducting single crystal served as a natural barrier forming planar normal metal/insulator/semiconductor (N-I-S) tunnel junctions with resistances ranging from a few tens to tens of thousands of Ohms. The current-voltage measurements performed on such junctions at room temperature demonstrated a bias-driven resistive switching with ratios above 1000% and high endurance. In situ measurements with two junctions (N-I-S-I-N) demonstrate the polarity-dependent resistive switching of the two (N-I-S and S-I-N) junctions and show no contribution from the bulk of the La2NiO4 crystal. Such an interfacial nature of the switching phenomenon is promising for fabrication of thin-film planar devices to be used in nonvolatile memory and logic. |
Friday, March 8, 2019 8:48AM - 9:00AM |
X46.00003: Spontaneous epitaxy of VO and stabilization of interface phase of V2O3 during vanadium metal deposition on SrTiO3 in vacuum Agham Posadas, Sunah Kwon, Moon Kim, Alexander Demkov The deposition of vanadium metal on SrTiO3 results in the spontaneous scavenging of oxygen ions from SrTiO3 to oxidize vanadium to VOx, where x = 0.5-1.2, depending on the temperature. At sufficiently high temperature, the VOx forms as an epitaxial film on both SrTiO3 (100) and SrTiO33 (111). Surprisingly, the oxygen scavenging and epitaxy persist for thicknesses over 300 Å with no sign of degradation. Additionally, when vanadium is deposited on SrTiO3(111), an interfacial phase of hexagonal V2O3 with 2x2 surface reconstruction is formed, which is stable for three monolayers, before rocksalt VO(111) starts to be formed. We describe the growth process and layer by layer characterization of the films using in situ reflection high-energy electron diffraction (RHEED), x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), and reflective electron energy loss spectroscopy (REELS), as well as ex situ grazing incidence in-plane x-ray diffraction and cross-sectional scanning transmission electron microscopy. |
Friday, March 8, 2019 9:00AM - 9:12AM |
X46.00004: Tunability of the Metal–Insulator Transition in VO2/TiO2 Heterostructures Guoxiang Hu, Qiyang Lu, Ilkka Kylanpaa, Jaron Krogel, Paul Kent, Olle Heinonen, Ho Nyung Lee, Panchapakesan Ganesh The metal–insulator transition in strongly correlated vanadium dioxide (VO2) has attracted considerable interest. It has been shown that formation of oxygen vacancies (VO) in VO2 can suppress this MIT completely without any structural transition. Since VO can be introduced via tuning the thickness or type of capping oxide layers, in this work, we studied the control of MIT in VO2 by interfacing with a TiO2 capping layer. Using a combined approach with experimental measurements and theoretical calculations, we find that a TiO2 capping layer on an epitaxial VO2 thin film suppresses the MIT in VO2. Density functional theory (DFT) calculations, benchmarked against more accurate many-body quantum Monte Carlo (QMC) calculations, provide information on the VO formation energy profile across the VO2/TiO2 interface. It is found that the VO2/TiO2 interface can facilitate the VO migration from TiO2 to VO2, and it is this migration that is responsible for the suppression of MIT in VO2. Our study highlights the crucial role of VO introduced by a capping layer in tuning the MIT in epitaxial VO2 thin films, which is useful for developing advanced electronic and iontronic devices. |
Friday, March 8, 2019 9:12AM - 9:24AM |
X46.00005: Origin of Suppression of Metal-Insulator Transition in Non-Stoichiometric VO2 Panchapakesan Ganesh, Frank Lechermann, Ilkka Kylanpaa, Jaron Krogel, Paul Kent, Olle Heinonen Rutile (R) phase VO2 is a quintessential example of a strongly correlated bad-metal, which undergoes a metal-insulator transition (MIT) concomitant with a structural transition to a V-V dimerized monoclinic (M) phase below TMIT∼ 340K. In particular, doping vanadia thin-films with oxygen vacancies (VO) has been shown to completely suppress this MIT without any structural transition[1]. We explain this suppression by elucidating the influence of oxygen-vacancies on the electronic-structure of the R phase VO2, explicitly treating strong electron-electron correlations using dynamical mean-field theory (DMFT) as well as diffusion Monte Carlo (DMC) techniques. We show that VO’s tend to change the V-3d filling away from its nominal half-filled value, with the eπg orbitals competing with the otherwise dominant a1g orbital. Loss of this near orbital polarization is associated with a weakening of electron correlations, which removes a charge-density wave (CDW) instability along the V-V dimerization direction above a critical doping concentration, thereby suppressing the metal-insulator transition. Our study also suggests that MIT is predominantly driven by a correlation-induced CDW instability. [1] Phys. Rev. Applied 7, 034008 (2017). |
Friday, March 8, 2019 9:24AM - 9:36AM |
X46.00006: Depth-profiling metal-oxygen hybridization and orbital polarization in isovalent perovskite oxide heterostructures Paul Rogge, Padraic Shafer, Gilberto F L Fabbris, Wen Hu, Elke Arenholz, Mark Dean, Steven J May Heterostructures of complex oxides have been successfully deployed to realize new electronic properties, with much of this work motivated by interfacial charge transfer effects that change the local charge density. Here, we synthesized superlattices of the isovalent perovskite oxides SrFeO3 and CaFeO3 using molecular beam epitaxy to investigate structural-induced changes in the orbital character of carriers across oxide interfaces while retaining the same nominal charge density. Using resonant x-ray reflectivity at the oxygen K-edge, we demonstrate that the Fe-O hybridization in CaFeO3 and SrFeO3 differs and is additionally modified at the interface. Further, using linearly polarized photons we find that the reflectivity at the Fe L-edge is polarization-dependent, which is attributed to strain-induced orbital polarization in the Fe 3d electron orbitals. By modeling the resonant reflectivity, we correlate changes in the Fe orbital polarization with changes in the Fe-O hybridization across the SrFeO3-CaFeO3 interface and reveal the presence of a hybridization superstructure. |
Friday, March 8, 2019 9:36AM - 9:48AM |
X46.00007: Engineering ferroelectricity and metal-to-insulator transitions in metal-oxide heterostructures Summayya Kouser, Saurabh Ghosh, John Brehm, Albina Y Borisevich, Sokrates T Pantelides Structurally designing materials by combining two or more different materials for realizing interesting emergent properties to target a desired functionality reduces the need for serendipity. One of the nascent strategies for pursuing new acentric compounds focuses on layering centrosymmetric materials in a particular fashion that lifts inversion symmetry while retaining other electronic, magnetic or optical functionalities found in the constituent materials, thus enabling new multiferroics or narrow bandgap polar semiconductors. Here, we formulate an approach to functionalize a polar metal i.e., (Ca1_xSrxFeO3)m/(SrFeO3)n into an insulating ferroelectric constructed from metallic oxide components CaFeO3 and SrFeO3. We have investigated several odd and even superlattices with different periodicities (e.g. m x n = 1x1, 2x2, 1x3, 3x1 etc). We find that the trigger for such a metal-to-insulator (M-I) transition is the disproportionation of the oxidation states of the central Fe ions. We find that appropriate periodicity and cation ordering is critical for the desired transition. The present work opens a door for designing ferroelectrics with tunable energy gaps by inducing controlled M-Itransitions. |
Friday, March 8, 2019 9:48AM - 10:00AM |
X46.00008: Atomic Resolutin Study of Charge Transfer at the LaTiO3/LaFeO3 Interface Chenyi Gu, Min Gu, Zhoushen Yuan, Yuefeng Nie, Peng Wang, Xiaoqing Pan Charge transfer at complex oxide interfaces can give rise to a rich variety of exotic two-dimensional phenomena. Recently, an antiferromagnetic to nonmagnetic transition was reported at the LaTiO3/LaFeO3 interface based on the observation of Ti and Fe valence change by X-ray photoelectron spectroscopy. Nonetheless, a direct spatially resolved measurement with atomic resolution to support the charge transfer truly happens at the interface is lacking. Here, we investigate the valence variations of Ti and Fe at the LaTiO3/LaFeO3 interface by a combination of molecular beam epitaxy and aberration corrected scanning transmission electron microscopy. We demonstrate that the charge transfer from Ti to Fe occurs near the interface accompanied by minor cation intermixing, and the charge transfer depth is two unit cells. Our work provides a spatially resolved observation of charge transfer at this interface, and lends significant credence to charge transfer designs in functional oxide interfaces. |
Friday, March 8, 2019 10:00AM - 10:36AM |
X46.00009: Electronic properties of nickelate-based heterostructures Invited Speaker: Marta Gibert Epitaxial heterostructures offer multiple strategies to manipulate the interplay between the different degrees of freedom in transition metal oxides. Interfacial structural and electronic couplings are key to tune the functionalities of these materials and even allow access to novel electronic phases. |
Friday, March 8, 2019 10:36AM - 10:48AM |
X46.00010: Electronic properties of (001)pc and (111)pc SmNiO3/NdNiO3 superlattices Claribel Dominguez Ordonez, Marta Gibert, Jennifer Fowlie, Sara Catalano, Michel Viret, Nicolas Jaouen, Jean-Marc Triscone Rare earth nickelates (RNiO3 (RNO), R = rare earth) stand out for their unique metal to insulator transition (MIT) upon reducing temperature (TMI), accompanied by an unusual antiferromagnetic ordering at TNéel≤TMI. In bulk, NdNiO3 (NNO) exhibits TNéel=TMI, whereas SmNiO3 (SNO) displays TNéel<TMI. It has been shown that the TMI and TNéel of the corresponding films can be tuned over a wide temperature range by means of epitaxial strain. Moreover, by growing along the (111)pc crystallographic direction, one can induce splitting of TMI and TNéel over a temperature range never achieved for films grown in the (001)pc-orientation [1] . Here, we adopt a new strategy to study the interfacial matching constraints in RNiO3-based heterostructures and their impact on the electronic properties by growing (001)pc and (111)pc-oriented (m(SNO)/n(NNO))N superlattices (SLs). We have found that the TMI and TNéel of the whole system depend on the superlattice wavelength (Λ=m+n) with a single TMI for the shortest superlattice period (Λ). In order to understand the evolution of the lattice distortions with the SL periodicities and how interfacial coupling affects TMI and TNéel, theoretical predictions will be shown. |
Friday, March 8, 2019 10:48AM - 11:00AM |
X46.00011: Metal-Insulator transition in thin films and multilayers of early transition metal oxides from DFT+DMFT Sophie Beck, Claude Ederer We study the interplay between several control mechanisms on the emerging functionalities of complex oxide thin films and heterostructures composed of different early transition metal oxides, including correlated metals, Mott insulators and band insulators, using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). |
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