Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B64: Nickelate, Ferrite, and Cobaltate-Based Heterostructures: Metal-Insulator Transition, Magnetism, and Orbital OrderingFocus
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Sponsoring Units: DMP Chair: Divine Kumah, North Carolina State University Room: Mile High Ballroom 4E |
Monday, March 2, 2020 11:15AM - 11:27AM |
B64.00001: Effect of ordered oxygen vacancies on the electronic and magnetic structure of perovskite-derived LaNiO3-δ nickelate Yongjin Shin, James Rondinelli Rare-earth nickelates perovskites (RNiO3, with R=rare earth) exhibit a plethora of electronic and magnetic phase transitions owing to the orbital structure of its trivalent Ni cations. Unlike other RNiO3 compounds, LaNiO3 is the only compound lacking a thermal metal-insulator transition. Nonetheless, oxygen-deficient LaNiO3-δ exhibits interesting electronic/magnetic transitions with varying oxygen content δ owing to the reducibility of Ni3+. Specifically, the metal-semiconductor-insulator transition occurs concurrently with paramagnetic (PM)-ferromagnetic (FM)-antiferromagnetic (AFM) transition in bulk materials as δ varies [1]. Here, we explain the LaNiO3-δ phase transitions and their dependencies upon (ordered) oxygen vacancy formation using first-principles calculations. We find oxygen vacancies form along the (110)pc direction and transform NiO6 octahedra to NiO4 square planar units. The resultant NiO4 unit is electronically and magnetically inactive, and this with the change in NiO6 connectivity governs the phase transitions. We conclude with model interpretations of the transient states reported for LaNiO2.5 and LaNiO3, and their connection to experimental observations. |
Monday, March 2, 2020 11:27AM - 11:39AM |
B64.00002: Length Scales and Symmetries in The Physics of Correlated Perovskite Oxides Alexandru Bogdan Georgescu, Claribel Dominguez Ordonez, Jennifer Fowlie, Bernat Mundet, Yajun Zhang, Alain Mercy, Sara Catalano, Duncan T.L. Alexander, Philippe R Ghosez, Antoine Georges, Andrew Millis, Marta Gibert, Jean-Marc Triscone An essential aspect in the study of correlated materials is the interplay of structural motifs and electronic degrees of freedom. This talk will describe how to design an oxide heterostructure composed of two different nickelates in order to provide insight into the length and energy scales involved in the metal insulator transition, as well as the resulting roles of the different structural motifs [1]. Then, I will explain how different aspects of the symmetry of the system can affect the interplay of correlations and kinetic energy. Specific results will be presented for heterostructures composed of different rare earth nicklate materials, and possible generalizations to other materials will be presented. |
Monday, March 2, 2020 11:39AM - 12:15PM |
B64.00003: Effect of interfaces on band hybridization, orbital polarization, and helical magnetism in SrFeO3/CaFeO3 heterostructures Invited Speaker: Steven May The alkaline earth ferrates exhibit an array of interesting physical phenomena such as metal-insulator transitions, strong Fe-O covalency, rapid redox reactions, and helical magnetism. In this talk, I will present how the Fe-O band hybridization and Fe 3d orbital occupancy changes across interfaces in isoelectronic SrFeO3/CaFeO3 superlattices. A consequence of the strong hybridization in these materials is that their electronic configuration can be described as containing both d4L0 and d5L1 contributions, where L indicates a ligand hole. Using resonant soft x-ray reflectivity, we find a modulation of the Fe-O hybridization across the superlattice with SrFeO3 hosting a larger ligand hole density than CaFeO3.[1] We show that the Fe eg orbital polarization is correlated with the degree of d4L0 electronic character present in the layers. Results will also be presented on the stability of the helical magnetic order in (CaFeO3)20/(SrFeO3)n superlattices. Using resonant x-ray diffraction, we show that the helical magnetic order coherently propagates through superlattices with n = 1 but does not extend through the SrFeO3 layer in a superlattice with n = 6.[2] We describe these results in context of recently reported multi-q helimagnetism in metallic SrFeO3, while our results support single-q helimagnetism in insulating CaFeO3. |
Monday, March 2, 2020 12:15PM - 12:27PM |
B64.00004: Charge transfer in iridate-nickelate superlattices Michele Kotiuga, Heung-Sik Kim, David Vanderbilt, Karin M Rabe The construction of superlattices, unit-cell-scale layering of two or more materials at the nanoscale, allows one to access novel properties not possessed by the constituent materials. Here, we show that in superlattices consisting of strontium iridate and lanthanum nickelate, up to one electron can transfer from iridium to nickel at the interfaces, leading to Ni2(d8) and the unusual oxidation state Ir5+(d4). Using density functional theory based calculations including a Hubbard U correction and spin-orbit coupling, we characterize the 1:1 superlattices with both (001) and (111) ordering, allowing us to explore the competition among Hund's coupling, the crystal field splitting and spin-orbit coupling. We find that the spin-orbit coupling present for iridium does not completely dominate the d4 configuration, which would result in a nonmagnetic state with a filled Jeff=3/2 manifold, but competes with the Hund's coupling and crystal field splitting. In these superlattices, the surprising competition in the energy landscape results in Ir5+ with a non-zero magnetic moment. |
Monday, March 2, 2020 12:27PM - 12:39PM |
B64.00005: Origin of orbital polarization in transition metal oxides: the case of Co2+ Alex Lee, Sohrab Ismail-Beigi The degree of broken orbital degeneracy, which can be represented by the term \emph{orbital polarization}, can play a crucial role in the electronic and magnetic properties of transition metal oxides. Since spin, lattice, and orbital degrees of freedom are entangled, the fundamental origin and systematic understanding of the conditions leading to strong orbital polarization is lacking. Recently, we observed the strong orbital polarization of Co2+ in LaCoO3+LaTiO3 (LCO+LTO) superlattice [1]. The orbital polarization of Co is particularly interesting, since Co2+ ion has multiple spin states and t2g or eg character is determined by these spin states. Here we systematically study the origin of the strong orbital polarization of Co2+ by considering the various structural phases of (LCO)1+(LTO)1 superlattice and La2CoTiO6. While the symmetry reduction by forming a superlattice is the sufficient condition to break degeneracy of the eg bands for the low-spin state, the polarization is greatly enhanced by Coulomb U. The sign of the eg polarization depends on U, local octahedral distortion, and strain. For the high-spin state, the origin of the orbital polarization of t2g bands is similar to eg case. |
Monday, March 2, 2020 12:39PM - 12:51PM |
B64.00006: Ferromagnetism in ultrathin double perovskite La2NiMnO6 thin films Gabriele De Luca, Jonathan Spring, Umar Bashir, Anna Zakharova, Claribel Dominguez Ordonez, Marta Rossell, Cinthia Piamonteze, Marta Gibert Double perovskite oxides with chemical formula A2BB’O6 possess the prototypical perovskite structure integrated by two different cations (B, B’) ordered in a rock-salt fashion. In the R2NiMnO6 (RNMO) family, R being a rare-earth cation, the positive superexchange interaction between the Ni2+ and Mn4+ electronic states results in the coexistence of ferromagnetic order with insulating behaviour, a combination of features that is rarely found in nature. La2NiMnO6 (LNMO), in particular, has a bulk ferromagnetic Curie temperature of circa 280K. Although an insulating ferromagnetic oxide with near room temperature TC would be ideal for novel spintronic devices, only a few attempts of growing ultrathin LNMO films have been reported so far. |
Monday, March 2, 2020 12:51PM - 1:03PM |
B64.00007: Probing the electronic ground states of thin film Ruddlesden-Popper (Rn+1NinO3n+1) nickelates Grace Pan, Qi Song, Charles Brooks, Spencer Doyle, James Ehrets, Dan Ferenc Segedin, Hanjong Paik, Julia Mundy The recent discovery of superconductivity in a hole-doped infinite layer nickelate has spurred the reexamination of how nickelate physics may be amenable to stabilizing new superconducting phases [1]. The nickelate identified is isostructural to the superconducting cuprates and lies squarely in the superconducting regime of the simple phase diagram by Zhang [2] but disentangling the electronic from the structural contributions presents a key challenge. We have stabilized, for the first time, the Ruddlesden-Popper nickelates (Ndn+1NinO3n+1) in thin film form up to n = 6. We will discuss how tuning of the Ruddlesden-Popper order alters the electronic ground states of the system including the nickel 3d occupancy and effects on the canonical metal-to-insulator transition. |
Monday, March 2, 2020 1:03PM - 1:15PM |
B64.00008: Band Offset Induced Charge Redistribution in SrNiO3/LaFeO3 Superlattices Scott Chambers, Le Wang, Yang Zhenzhong, Mark Bowden, John William Freeland, Sushko Peter, Du Yingge Charge transfer at oxide interfaces can drive emergent phenomena that do not occur in the bulk, thereby significantly enriching our fundamental understanding of these material. We have synthesized a series of (SrNiO3)1/(LaFeO3)n superlattices (SLs) by oxide molecular beam epitaxy on (LaAlO3)0.3(Sr2AlTaO6)0.7 (001) substrates. Our structural characterization results indicate that cubic perovskite SrNiO3 (SNO), which does not exist in the bulk, can be stabilized as a single unit cell level in SLs with LaFeO3. Our in-situ X-ray photoemission spectroscopy and X-ray absorption spectroscopy measurements indicate that depending on the value of n, the Ni and Fe valences are either 3+ or 4+. The n = 1 SL contains Ni3+ and Fe4+, while Ni4+ and Fe3+ are observed in the n = 5 SL. These results are consistent with valence band offset measurements which indicate that a potential well for holes is present in the LFO (SNO) layers for n = 1 (5). The insights gained from these spectroscopies shed considerable light on the associated two dimensional (2D) semiconducting transport behavior in these SLs, thereby positioning us to utilize these materials systems in new and novel electronic devices. |
Monday, March 2, 2020 1:15PM - 1:27PM |
B64.00009: Role of interface polarity in the electronic reconstruction of infinite-layer vs. perovskite nickelate films on SrTiO3(001) Benjamin Geisler, Rossitza Pentcheva Motivated by the recent observation of superconductivity in the infinite-layer nickelate NdNiO2 on SrTiO3(001) by Li et al. [1], we explore the effect of interface polarity on the electronic properties of NdNiOn/SrTiO3(001) and LaNiOn/SrTiO3(001) thin films (n=2,3) by performing first-principles calculations including a Coulomb repulsion term. For infinite-layer nickelate films (n=2), electronic reconstruction drives the emergence of a 2DEG at the interface due to a strong occupation of the Ti 3d states. This effect is more pronounced than in the paradigmatic LaAlO3/SrTiO3(001) system and accompanied by a substantial reconstruction of the Fermi surface. We contrast our findings with results for the perovskite compounds (n=3). Moreover, we analyze the topotactic reaction from a perovskite to an infinite-layer heterostructure and show why the reduction is confined to the nickelate film, whereas the SrTiO3 substrate remains intact. |
Monday, March 2, 2020 1:27PM - 1:39PM |
B64.00010: Spatial distribution of hole around LaNiO3/LaMnO3 interface Masato Anada, Satoshi Sakaguchi, Kazuki Nagai, Miho Kitamura, Hiroshi Kumigashira, Yusuke Wakabayashi The superlattice of LaMnO3 and LaNiO3 exhibits ferromagnetism as a results of charge transfer between Mn and Ni across the interface[1]. In case of bilayer of LaNiO3 on LaMnO3(NMT) and of LaMnO3 on LaNiO3(MNT) grown on SrTiO3 (001) substrate, the former shows much larger magnetization [2]. Detailed structure and valence distribution of the interfaces were examined by non-resonant and K-edge resonant surface x-ray diffraction method. Bayesian inference was applied to derive the atomic positions [3] to derive local polarization. The spatial distribution of Mn4+ was determined from the energy spectra of scattering. As a results, Mn4+ was found mainly in the Ni-Mn mixture region, and the total amount of transferred electron in NMT(0.8±0.2e-) is larger than MNT(0.5±0.2e-), which is consistent with the larger magnetization in NMT. |
Monday, March 2, 2020 1:39PM - 1:51PM |
B64.00011: Disentangled transitions in artificial rare-earth nickelates Srimanta Middey, Derek Meyers, Mikhail Kareev, Yanwei Cao, Xiaoran Liu, Padraic Shafer, John William Freeland, J.-W. Kim, P. J. Ryan, Jak Chakhalian The observation of simultaneous metal-insulator transition, charge ordering transition, and structural transition in rare-earth nickelates hinders our understanding of the underlying mechanism. We have devised a series of new superlattices by combining EuNiO3 and LaNiO3 to form ultrashort period superlattices, which allow one to disentangle the simultaneous orderings. Tailoring an incommensurate heterostructure period relative to the bulk charge ordering pattern suppresses the charge order transition while preserving metal-insulator and antiferromagnetic transitions. Such selective decoupling of the entangled phases resolves the long-standing puzzle about the driving force behind the metal-insulator transition and points to the site-selective Mott transition as the operative mechanism. |
Monday, March 2, 2020 1:51PM - 2:03PM |
B64.00012: Structural Distortions and the Metal Insulator Transition in (111) LaNiO3 Ultrathin Films Margaret Kane, Lauren Riddiford, Arturas Vailionis, Apurva Mehta, Alpha T. N'Diaye, Elke Arenholz, Yuri Suzuki LaNiO3 (LNO) is unique among rare-earth nickelates in that it doesn’t exhibit a temperature dependent metal-insulator transition (MIT) in bulk. A thickness dependent MIT can be seen in (001)-oriented LNO films at ~8 Å thick and has been ascribed to ligand-holes, oxygen vacancies and charge disproportionation. We observe a similar MIT in (111)-oriented LNO on LaAlO3 in transport measurements, but at a greater critical thickness than in (001) LNO on LAO. Synchrotron x-ray diffraction data and dynamical analysis show an elongation of the out-of-plane lattice parameter to 2.4 Å near the (111) interface. As the film gets thicker, the lattice constant shrinks to 2.25 Å and the film exhibits bulk metallic behavior. This distortion in the [111] direction in the first 8 unit cells is distinct from strain accommodation in (001) films, where oxygen octahedra can more easily rotate to relieve strain. We correlate the insulating behavior to this distortion and an increase in Ni2+, measured via x-ray absorption spectroscopy. Together these results imply that metallicity in LNO films is a balance among the charge, lattice and orbital degrees of freedom. |
Monday, March 2, 2020 2:03PM - 2:15PM |
B64.00013: Bayesian inference of perovskite oxide interface structure based on surface x-ray diffraction data Kazuki Nagai, Masato Anada, Yoshinori Nakanishi-Ohno, Masato Okada, Yusuke Wakabayashi Crystal truncation rod (CTR) scattering method is one of the approaches to determine the structure of ultrathin films. To perform structure refinement from CTR scattering data, a good initial model is needed to obtain the correct structure. Any surface electron density analysis requires to extrapolate the scattering data. |
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