Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D64: Emerging Phenomena & Defects in Transition Metal Oxides and 2D MaterialsFocus
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Sponsoring Units: DMP Chair: Alexandru Bogdan Georgescu, Simons Foundation Room: Mile High Ballroom 4E |
Monday, March 2, 2020 2:30PM - 2:42PM |
D64.00001: Electronic and magnetic state of LaMnO3/SrTiO3 heterostructures: Effect of local correlation and nonlocal exchange Hrishit Banerjee, Oleg Janson, Karsten Held, Tanusri Saha-Dasgupta, Markus Aichhorn Motivated by the puzzling report of a ferromagnetic insulating state in LaMnO3/SrTiO3 heterostructures, we calculate the electronic and magnetic state of LaMnO3, strained to a SrTiO3 square substrate. We use 3 different computational approaches: (a) density functional theory (DFT) with Hubbard U(DFT+U), (b) DFT + dynamical mean-field theory (DMFT), and (c) DFT + Hartree Fock (HF) as a hybrid functional. While the first two approaches include local correlations and exchange at Mn sites, treated in a static and dynamic manner, respectively, the last one takes into account the effect of nonlocal exchange at all sites. We find in all three approaches that the compressive strain induced by the square substrate of SrTiO3 turns LaMnO3 to a ferromagnet with suppressed Jahn-Teller distortion, in agreement with experiment. The hybrid calculations result in a ferromagnetic insulating solution, as observed in experiment. This insulating behavior is found to originate from an electronic charge disproportionation. Our conclusions remain valid when we investigate LaMnO3/SrTiO3 within the experimental setup of a superlattice geometry using hybrid functionals. DMFT calculations show the presence of a paramagnetic insulating state. |
Monday, March 2, 2020 2:42PM - 2:54PM |
D64.00002: Ultimate strength measurements on freestanding SrTiO3 thin films Varun Harbola, Samuel Crossley, Prastuti Singh, Ruijuan Xu, Harold Hwang The last two decades have seen enormous growth in the field of nanoengineering and nanomechanics using thin sheets owing to the variety of 2D materials available to us (2D Mater. 5, 032005 (2018)). With new advances in thin film growth techniques(Nat. Mater. 15, 1255–1260 (2016)), a new class of functional oxide thin films which are freestanding can be produced and readily incorporated in such nanomechanical implementations. However functional oxides are rigid and brittle in bulk, so it becomes imperative to verify the feasibility of nanomechanical deformations by investigating the breaking strength of these thin films. Here we report measurements of the ultimate strength of SrTiO3 thin films using an atomic force microscope (AFM) by impinging upon a freestanding drumhead with an AFM tip. We demonstrate that in the sub-20 nm thickness regime of these thin films, SrTiO3 can withstand an elastic extension of ~ 6% which is more than an order of magnitude higher than that for bulk. Furthermore, we also show that the fracture point of these films with respect to applied force is robust thus demonstrating their potential for use in nanomechanical platforms and devices. |
Monday, March 2, 2020 2:54PM - 3:06PM |
D64.00003: Local modification of superconductivity in few unit cell thick high-TC Bi2Sr2CaCu2O8+δ superconductor Sanat Ghosh, Jaykumar Vaidya, Sawani Datta, Ram Prakash Pandeya, Digambar A. Jangade, Kalobaran Maiti, A. Thamizhavel, Mandar M Deshmukh High temperature superconductors are interesting systems to study from the point of views of basic understanding about the origin and the nature of their superconducting properties as well as their numerous applications. Here we report transport studies on one of the high TC cuprate superconductors namely Bi2Sr2CaCu2O8+δ , an easily exfoliable Van der Waals material having superconducting transition temperature of 85K. In thin layers of this material, we have found that one can tune its superconducting property locally. This can have many potential device applications like patterning 2D superconductors for making high temperature Josephson Junctions and superconducting quantum interference devices. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D64.00004: Proving High Temperature Superconductivity in Monolayer Bi2Sr2-xLaxCuO6+δ Hengsheng Luo, Liguo Ma, Yijun Yu, Peng Cai, Dongjoon Song, Ruidan Zhong, Jian Shen, Genda Gu, Hiroshi Eisaki, Xianhui Chen, Yuanbo Zhang Dimensionality plays a fundamental role in cuprate high-temperature superconductivity; all high-temperature cuprate superconductors adopt a layered crystal structure, and much of high-temperature superconductivity theory is based on purely two-dimensional (2D) models. A monolayer Bi2201 contains only a single layer of CuO2 plane, and therefore represents a cuprate superconductor in the ultimate 2D limit. Here, we exfoliate La doped Bi2201 (Bi2Sr2-xLaxCuO6+δ) single crystals down to monolayer (i.e. half unit cell) for transport measurements. We are able to tune the doping level of Bi2201 by annealing the sample in vacuum or ozone, and study the strongly correlated electronic states in Bi2201 over the entire phase diagram in a single sample. In addition, we probe the electronic structure of monolayer Bi2201 at atomic scale using scanning tunnelling microscopy and spectroscopy. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D64.00005: Single-domain to Multi-domain Transition Due to Phase Separation in (La1-yPry)1-xCaxMnO3 Thin Films Ashkan Paykar, A. Biswas The perovskite manganite (La1-yPry)1-xCaxMnO3 exhibits complex electronic and magnetic behaviors caused by a phase competition between its ferromagnetic metallic (FMM) and charged order insulating (COI) phases. While La0.67Ca0.33MnO3 (x = 0.33, y = 0) undergoes a transition from a paramagnetic insulator to a pure FMM below 250 K, (La0.4Pr0.6)0.67Ca0.33MnO3 (x = 0.33, y = 0.6) shows electronic phase-separation between FMM and COI phases at temperatures approximately below 120 K. Thin films of (La1-yPry)1-xCaxMnO3 (x = 0.33, y = 0, 0.5, 0.6) were grown on (110) NdGaO3 (NGO) using pulsed laser deposition. Among these thin films, (La0.4Pr0.6)0.67Ca0.33MnO3 also shows a single-domain to multi-domain transition in the FMM regions as they percolate at low temperatures. This domain transition is partially due to the in-plane uniaxial magnetic anisotropy caused by the anisotropic substrate strain exerted by (110) NGO. To check if phase separation also plays a role in the domain transition, we measured the temperature dependence of the coercive field (Hc) for y = 0 and y = 0.5 thin films. These films show a monotonic increase in Hc with lowering temperature showing that phase separation and a percolation of the FMM regions is required for the domain transition. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D64.00006: Effects of epitaxial strain and oxygen underdoping on order parameter competition in manganite/cuprate thin-film heterostructures Chao C Zhang, Hao Zhang, Anh Nguyen, Thomas Gredig, Min Gu Kang, Riccardo Comin, John Y.T. Wei To distinguish the role of magnetism from other factors that affect superconductivity in c-axis La2/3Ca1/3MnO3/YBa2Cu3O7-δ(LCMO/YBCO) heterostructures, we grow and characterize various perovskite/YBCO/perovskite trilayers, using ferromagnetic LCMO, paramagnetic LaNiO3 (LNO), and orthorhombic PrBa2Cu3O7-δ (PBCO) as the sandwiching layers. LCMO and LNO trilayers show similarly large superconducting Tc reductions with decreasing YBCO layer thickness, whereas this Tc reduction is not seen in the PBCO trilayers. Our results indicate that epitaxial strain has a stronger effect on the Tc of LCMO/YBCO heterostructures than any long-range magnetic proximity effect present. Using this system, we also study how the Tc reduction varies with oxygen underdoping by deoxygenating the cuprate layer. [1] We discuss our results in the wider context of multiple competing orders, in particular the recent observation by resonant x-ray scattering of robust charge-density-wave order in LCMO/YBCO multilayers. [2] |
Monday, March 2, 2020 3:42PM - 4:18PM |
D64.00007: Creating Emergent Phases in Transition Metal Oxides Invited Speaker: milan radovic Transition Metal Oxides (TMOs) exhibit unique and multifunctional physical phenomena directly related to the spin and orbital degrees of freedom of metal d-states and their interplay with the lattice. Importantly, the iso-structure of TMOs permits realization of hetero-structures generating at their surfaces and interfaces new physical matters that radically differ from those of the constituent bulk materials. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D64.00008: Electrically Controlled Intrinsic Tunneling in Dynamically Phase Separated Manganites Ambika Shakya, A. Biswas
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Monday, March 2, 2020 4:30PM - 4:42PM |
D64.00009: Electronic and magnetic properties in three-component manganite films – the role of ordered interfaces and ionic size effects Caitlin Kengle, Maitri Warusawithana, Dakota Brown, James Payne, Thomas Pekarek The mixed-valent manganite, La1-xSrxMnO3, with x= 1/3 has been widely studied for its colossal magneto-resistive properties arising from the double exchange interaction. This material can be grown in thin-film form as a random alloy or an ordered superlattice. Here we grow films replacing 50% of the La with Y to make La1/3Y1/3Sr1/3MnO3 as both random alloys where La, Sr, and Y randomly occupy the A-site and ordered superlattices where the supercells comprise of single unit cells of LaMnO3, YMnO3, and SrMnO3 stacked in sequence. While electronically La and Y are very similar, as both tend to be in a 3+ oxidation state in the crystal, we find contrastingly different electronic transport. The Y-substituted films exhibit a suppression of the metal-to-insulator transition compared to that of La1/3Sr2/3MnO3. While the suppression is observed in both the ordered superlattice and the random alloy, we find it notably more amplified in the random alloy sample. We will discuss how ionic size effects and ordered interfaces affect the electronic and magnetic properties of these samples. |
Monday, March 2, 2020 4:42PM - 4:54PM |
D64.00010: Superconductivity in a Metal/Quantum Dimer Heterostructure Eli Gerber, Jian-Huang She, Choong Hyun Kim, Craig J Fennie, Michael J Lawler, Eun-Ah Kim Recently we we proposed a new approach to engineering exotic superconductors based on a metal/quantum spin ice heterostructure. However, pyrochlore materials are difficult to grow and current models of their spin fluctuation spectra remain incomplete. In contrast, weakly-interacting spin dimer compounds such as Ba3Mn2O8 are experimentally tractable and their spin correlations can be calculated explicitly. In this work we focus on a new example of such a setup, the spin dimer compound Ba3Mn2O8 in heterostructure with electron-doped Ba3Sb2O8, and predict interfacial p+ip pairing at a few Kelvin. We also provide criteria for materializing the heterostructure using ab initio calculations. Hence we present a concrete proposal based on a controlled calculation predicting p+ip superconductivity in a metal/quantum dimer heterostructure. |
Monday, March 2, 2020 4:54PM - 5:06PM |
D64.00011: Investigation of the Defect-Tolerance of Chalcogenide Perovskites Jiang Luo, ZHAOHAN i ZHANG, Boyang Zhao, Huandong Chen, Jayakanth Ravichandran, Rohan Mishra Chalcogenide perovskites, such as BaZrS3, have been proposed as defect-tolerant materials — that can maintain their electronic properties even in the presence of defects. When combined with their novel electronic structure, they have been proposed as promising materials for application in solar cells and optoelectronic devices; however, a systematic investigation of the defect tolerance of such perovskites is missing. We have used BaZrS3 as a prototype chalcogenide perovskite and investigated its defect tolerance to intrinsic point defects using a combination of density-functional-theory calculations, spectroscopic and transport measurements. Based on the calculations, we find that most defects lead to shallow levels. However, we find that the sulfur vacancies have low formation energy. They lead to a deep transition level that is spatially localized to act as a non-radiative recombination center. Results of transport measurements with specimens annealed with S-rich and S-deficient conditions will be presented. Our work demonstrates that critical control over the chalcogen stoichiometry is necessary to improve the performance of these chalcogen perovskites. |
Monday, March 2, 2020 5:06PM - 5:18PM |
D64.00012: Variation of defect states with the number of layers in 2D materials Dan Wang, Ravishankar Sundararaman The high sensitivity of two-dimensional (2D) materials to surroundings makes layer number an important consideration in designing devices for opto-electronic and quantum applications. However, the theoretical prediction on the variation process of defect level with increasing layer number is challenging due to the high computational cost. Here, we used a recently developed continuum model[1] to explore the layer-dependent effects. The scheme removes the defect-free layers from the density-functional-theory (DFT) calculations and captures their effects on the defect by replacing the electrostatic effect of the defect-free layers with a continuum dielectric function. Application of this method to defects in multilayer hBN (from monolayer to five-layer and bulk) reveals that defect levels become shallower with increasing layer number due to the increased dielectric screening, and the reduction process of ionization energy is highly dependent on the level of theory (DFT or many-body perturbation theory) as a result of the presence of self-interaction error and the absence of non-local screening effect in DFT. |
Monday, March 2, 2020 5:18PM - 5:30PM |
D64.00013: Tunable High-Temperature Superconductivity in Monolayer Bi2Sr2CaCu2O8+δ Yijun Yu, Liguo Ma, Peng Cai, Ruidan Zhong, Cun Ye, Jian Shen, Genda Gu, Xianhui Chen, Yuanbo Zhang Atomically-thin layered van der Waals crystals represent ideal material systems in the ultimate two-dimensional (2D) limit. Among all layered materials, high-temperature superconductors stand out for their fundamental importance in material research and potential impact on future technology. Here, we developed a fabrication process that enabled us to produce intrinsic monolayer crystals of high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here monolayer refers to a half unit cell) and probe their transport properties. The lack of dimensionality effect on superconducting transition temperature Tc defies expectations from Mermin-Wagner theorem; it also sharply contrasts with much reduced Tc in conventional superconductors in 2D limit. Being atomically thin, monolayer Bi-2212 offers unprecedented tunability over its doping and disorder level across the superconductor-insulator transition (SIT). Finite size scaling analysis at SIT provides a unified picture for disparate observations in cuprate systems. Our results establish monolayer Bi-2212 as an ideal platform for studying high-temperature superconductivity and other strongly correlated phenomena in 2D. |
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