Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V09: Complex Oxide Heterostructures - Multiferroic Effects and Metal-Insulator TransitionsFocus
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Sponsoring Units: DMP Chair: Anderson Janotti, University of Delaware Room: LACC 301A |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V09.00001: Interface magnetism in complex oxides heterostructures and manufactured magnetoelectric coupling Invited Speaker: Er-Jia Guo Novel electronic and magnetic properties can be achieved in materials engineered at nanometer length scales. Examples include conducting or magnetic interfaces between materials that are neither conducting nor magnetic. With an understanding of interface structure, electronic and magnetic degrees of freedom may be controlled, ideally at room temperature, to achieve magnetoelectric coupling in a nanocomposite or possibly to control spin textures in topological materials. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V09.00002: Metal-Insulator Transitions in Freestanding NdNiO3 Films Danfeng Li, Seung Sae Hong, Di Lu, Yasuyuki Hikita, Harold Hwang Complex transition metal oxides, owing to their correlated d electrons, provide an ideal playground for manipulation of charge, spin, orbital and lattice degrees of freedom. The metal-insulator transition (MIT) in rare-earth nickelates is a representative example, where the phases and the ground states of thin films can be tuned through epitaxial strain and geometric design. Recent research interest has been focused on the role of the strain field on the Ni-O-Ni bonds and the corresponding electronic structure, which play a vital role in controlling the MIT. Here we report the fabrication of freestanding single-crystalline NdNiO3 membranes using a new synthesis approach and the examination of their electronic properties. The ability to transfer freestanding NdNiO3 membranes onto other templates opens the door to further access the interplay between structural rotations/distortions and spin/charge ordering in this material family, for instance, by applying external strains to an unprecedented level. This offers an exceptional opportunity to study the MIT in nickelate films and heterostructures in the two-dimensional limit without substrate constraint. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V09.00003: Interfacial Effects on the Electronic Structure of LaNiO3 Films Zhigang Gui, Anderson Janotti LaNiO3 (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 films 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 films. Special attention is paid to interface effects, where electrons are transferred to or from LNO thin films through the interface termination of the perovskite oxide substrate. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V09.00004: Influence of Strain on VO2 Stoichiometry and its Metal-Insulator Transition Panchapakesan Ganesh, Janakiraman Balachandran, Yogesh Sharma, Ilkka Kylänpää, Jaron Krogel, Ho Nyung Lee, Paul Kent, Olle Heinonen VO2 is a canonical example of a strongly correlated system that exhibits a wide range of polymorphs such as monoclinic (M1, M2, T) and rutile (R) with each exhibiting unique electronic and optical properties. VO2 undergoes a metal-insulator transition (MIT) around 340K accompanied by a first order phase transition from high temperature metallic rutile phase to low temperature insulating monoclinic phase. Understanding and controlling this MIT by varying chemical composition, and external stimuli, such as strain, temperature, and electric field is necessary to realize VO2 based devices. In this talk, we will explore the coupling between experimentally realizable strain and oxygen vacancy (VOq) formation as well as diffusion energies in VO2 using beyond-DFT methods (e.g. DFT+U, meta- and hybrid DFT), benchmarked against very accurate many-body Quantum Monte Carlo (QMC) calculations. Theory predicts that strain facilitates electron-doping via creation of oxygen vacancies in VO2, consistent with our experimental findings on the strain control of the MIT in VO2 thin-films. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V09.00005: Artificial two-dimensional polar metal at room temperature Yanwei Cao, Zhen Wang, Se Young Park, Yakun Yuan, Xiaoran Liu, S. Nikitin, H. Akamatsu, Mikhail Kareev, Srimanta Middey, Derek Meyers, P. Thompson, Philip Ryan, Padraic Shafer, Alpha N'Diaye, Elke Arenholz, V. Gopalan, Yimei Zhu, Karin Rabe, Jak Chakhalian Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields favoring the polar structure are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here, we report on the realization of a room temperature two-dimensional polar metal of the B-site type in tri-color (tri-layer) superlattices BaTiO3/SrTiO3 /LaTiO3. A combination of atomic resolution scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS), optical second harmonic generation (SHG), electrical transport, and first-principles calculations have revealed the microscopic mechanisms of periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to a new class of all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum states including coexisting ferroelectric, ferromagnetic, and superconducting phases. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V09.00006: Coexisting first and second-order electronic phase transitions in a correlated oxide Kirk Post, Alex McLeod, Matthias Hepting, Martin Bluschke, Yifan Wang, Georg Christiani, Gennady Logvenov, Aliaksei Charnukha, Matteo Minola, Alexander Boris, Eva Benckiser, Karin Dahmen, Erica Carlson, Bernhard Keimer, Dimitri Basov Understanding and controlling phase transitions is a cornerstone of contemporary physics. Landau provided an invaluable insight by formulating the thermodynamics of complex systems in terms of a local order parameter (Φ), wherein second-order transitions are described by continuous evolution of Φ from zero, whereas in first-order transitions, Φ changes discontinuously. Here we show that the temperature-tuned insulator-to-metal transition (IMT) in the prototypical correlated electron system NdNiO3 defies this established binary classification. By harnessing a nano-scale optical probe of the local electronic conductivity, we observed two physically distinct, yet concurrent phase transitions in different regions of a 7nm NdNiO3 epitaxial film.In the bulk of the material, we resolve a discrete, first-order transition between metal and insulator phases. Meanwhile, we visualize anomalous nano-scale “domains walls” in the insulating state that undergo a continuous IMT, with hallmarks of a second-order transition, distinct from the bulk behavior of our specimen. The accurate reproduction of our experimental findings within Landau theory confirms that interaction between concurrent orders forms a crucial organizing principle in the complex phase transition of NdNiO3. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V09.00007: Engineering the sub-band electronic structure in transition metal oxide quantum wells Jason Kawasaki, Choong Hyun Kim, Jocienne Nelson, Sophie Crisp, Chris Zollner, John Heron, Craig Fennie, Darrell Schlom, Kyle Shen Quantum confinement is an essential tool both for modern technologies as devices become scaled down to only a few atoms thick, and for exploring the fundamental physics of 2D electron systems. In the simplest picture, confinement in the out-of-plane direction results in quantized, two-dimensional sub-bands. In nearly all quantum well systems that have been investigated to date (e.g. semiconductors, noble metals), the in-plane effective mass is nearly independent of the sub-band index. Yet it would be desirable to deliberately and deterministically engineer the effective mass of the sub-bands for technological applications such as quantum cascade lasers, tunnel diodes, and photocatalysis. Here, we demonstrate the ability to deterministically enhance sub-band effective masses by a factor of 5 in atomically thin films of the transition metal oxide IrO2 grown by oxide molecular beam epitaxy (MBE) and studied by angle-resolved photoemission spectroscopy (ARPES). We show that the sub-band effective masses can be deliberately engineered through consideration of the significant long-range, out-of-plane hopping matrix elements, and this approach can be broadly applied to a wide class of other functional electronic materials. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V09.00008: Superconducting Behaviour of Thin Niobium Film Deposited on the Surface-Reconstructed SrTiO3 Akhilesh Singh, MingYuan Song, Tsung-Chi Wu, Wei-Li Lee It has been shown that ultra-high vacuum annealing of the TiO2-terminated SrTiO3 (STO) substrate can lead to several different types of atomic surface reconstructions. By using low energy electron diffraction technique, we clearly identified a special ( atomic reconstruction on the STO surface, which gives rise to a conducting STO surface. In this work, we studied the magnetotransport and superconducting properties of thin niobium (Nb) film with a thickness of a few nanometers deposited on the surface reconstructed STO (SR-STO). The results on Nb/SR-STO films were then compared to that in the control Nb films deposited on insulating TiO2-terminated STO substrate and also on SiO2/Si substrate, where possible proximity effects and enhanced fluctuations on Nb due to SR-STO can be revealed and characterized. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V09.00009: Abstract Withdrawn
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Thursday, March 8, 2018 4:42PM - 4:54PM |
V09.00010: Scaling behavior of the spin cycloid in BiFeO3 films Stuart Burns, Daniel Sando, Bin Xu, Lachlan Russell, Guochu Deng, Jan Seidel, laurent bellaiche, Valanoor Nagarajan, Clemens Ulrich Understanding and manipulating complex spin textures in multiferroics can provide new opportunities in electric field-controlled spintronics. In BiFeO3, a well-known room-temperature multiferroic, competition between various exchange interactions gives rise to non-collinear spin order, manifesting as an incommensurate spin cycloid with period of 64 nm. We report on the stability and scaling behaviour of the cycloid in (110)-oriented epitaxial Co-doped BiFeO3 thin films. Neutron diffraction shows i) this cycloid, despite its out of plane propagation vector, can be stable in films as thin as 50 nm, smaller than the cycloid period itself; ii) the cycloid period increases significantly for the thinnest film of 50 nm, iii) for all film thicknesses investigated, the cycloid has a unique [11-2] cycloid propagation direction (different from the bulk); and iv) the cycloid period increases upon approach to TN. These observations are supported by Monte Carlo theory based on a first-principles effective Hamiltonian method. Our results offer new perspectives for nanoscale magnonic devices. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V09.00011: Disorder-free electrochemical doping of charge carriers into epitaxial La2CuO4+y films Alex Frano, Martin Bluschke, Zhijun Xu, Ronald Marks, Apurva Mehta, Valery Borzenets, Ming Yi, Benjamin Frandsen, Yi Lu, Derek Meyers, Gideok Kim, Georg Christiani, Gennady Logvenov, Eva Benckiser, Bernhard Keimer, Robert Birgeneau Interesting new electronic phases in oxides can be explored by tuning parameters like external fields, chemical composition, epitaxial strain, etc. However, doping oxide heterostructures by chemical substitution or by variation of the oxygen stoichiometry usually comes at the expense of significant disorder which can negatively affect the desired ground state. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V09.00012: Strain-driven attenuation of superconductivity in heteroepitaxial perovskite/YBCO/perovskite thin films Hao Zhang, Anh Nguyen, Chao Zhang, Thomas Gredig, John Wei To distinguish between the effects of strain and magnetism on the superconductivity in c-axis La2/3Ca1/3MnO3/YBa2Cu3O7-δ (LCMO/YBCO) heterostructures, we study perovskite/YBCO/perovskite thin films using either ferromagnetic LCMO or paramagnetic LaNiO3 (LNO) as perovskite. For a lattice-symmetry matched comparison, we also use orthorhombic PrBa2Cu3O7-δ (PBCO) in place of the pseudocubic perovskites. Unlike PBCO/YBCO/PBCO, both LCMO/YBCO/LCMO and LNO/YBCO/LNO trilayers show strong attenuation of the superconducting Tc as YBCO layer thickness is reduced from 21.4 to 5.4 nm. Our results indicate that heteroepitaxial strain, rather than long-range proximity effect, is responsible for the long length scales of Tc attenuation observed in c-axis LCMO/YBCO heterostructures [1]. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V09.00013: Band gap modulation via internal electric field control in Ruddlesden-Popper oxides Yongjin Shin, James Rondinelli Ruddlesden-Popper (RP) oxides are perovskite-derived structures whereby the perovskite “blocks” are partially connected and form a quasi-two-dimensional structure. Recently a density functional theory (DFT) study predicted (LaSr)AlO4 n=1 RP-oxides to show an insulator-metal transition beyond which only band gap variations were previously predicted when A-cation elements are properly ordered. The inequivalent charge states of La3+ and Sr2+ generate internal electric fields, which induces a sub-nanoscale band bending [1]. Although the internal electric field profile of systems with strong ionic character can be predicted by an ionic model, systems with valence d-orbitals easily deviate from the simple prediction. Here, we examine the interplay between d-orbital and internal electric fields created by atomic arrangements based on DFT calculations. We conclude by summarizing our findings into a set of working principles for band gap control, without chemical doping or changes in cation stoichiometry. |
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