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Kim, Young-Min,Pennycook, Stephen J.,Borisevich, Albina Y. Elsevier 2017 Ultramicroscopy Vol.181 No.-
<P><B>Abstract</B></P> <P>Octahedral tilt behavior is increasingly recognized as an important contributing factor to the physical behavior of perovskite oxide materials and especially their interfaces, necessitating the development of high-resolution methods of tilt mapping. There are currently two major approaches for quantitative imaging of tilts in scanning transmission electron microscopy (STEM), bright field (BF) and annular bright field (ABF). In this paper, we show that BF STEM can be reliably used for measurements of oxygen octahedral tilts. While optimal conditions for BF imaging are more restricted with respect to sample thickness and defocus, we find that BF imaging with an aberration-corrected microscope with the accelerating voltage of 300kV gives us the most accurate quantitative measurement of the oxygen column positions. Using the tilted perovskite structure of BiFeO<SUB>3</SUB> (BFO) as our test sample, we simulate BF and ABF images in a wide range of conditions, identifying the optimal imaging conditions for each mode. We show that unlike ABF imaging, BF imaging remains directly quantitatively interpretable for a wide range of the specimen mistilt, suggesting that it should be preferable to the ABF STEM imaging for quantitative structure determination.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An aberration-corrected BF STEM imaging at an accelerating voltage of 300kV provides a wide range of defocus-thickness imaging parameters and specimen misorientation for precisely locating oxygen positions in a tilted perovskite structure. </LI> <LI> BF STEM imaging for the measurements of oxygen octahedral tilts has quantitatively compared with ABF STEM imaging by systematic image simulations. </LI> <LI> BF STEM imaging working at 300kV shows superior accuracy for the measurements of the octahedral tilt angles as compared with ABF STEM imaging. </LI> </UL> </P>
Efficiency criteria for optimization of separation cascades for uranium enrichment
Georgy Sulaberidze,Shi Zeng,Andrey Smirnov,Anton Bonarev,Valentin Borisevich,Dongjun Jiang 한국원자력학회 2018 Nuclear Engineering and Technology Vol.50 No.1
As it is known, uranium enrichment is carried out on industrial scale by means of multistage separationfacilities, i.e., separation cascades in which gas centrifuges (GCs) are connected in series andparallel. Design and construction of these facilities require significant investment. So, the problem ofcalculation and optimization of cascade working parameters is still relevant today. At the same time,in many cases, the minimum unit cost of a product is related to the cascade having the smallestpossible number of separation elements/GCs. Also, in theoretical studies, it is often acceptable toapply as an efficiency criterion the minimum total flow to supply cascade stages instead of theabovementioned minimum unit cost or the number of separation elements. In this article, cascadeswith working parameter of a single GC changing from stage to stage are optimized by two of theabovementioned performance criteria and are compared. The results obtained allow us to make aconclusion about their differences.
Point Defect Configurations of Supersaturated Au Atoms Inside Si Nanowires
Oh, Sang Ho,Benthem, Klaus van,Molina, Sergio I.,Borisevich, Albina Y.,Luo, Weidong,Werner, Peter,Zakharov, Nikolai D.,Kumar, Dhananjay,Pantelides, Sokrates T.,Pennycook, Stephen J. American Chemical Society 2008 NANO LETTERS Vol.8 No.4
Yin, Y. W.,Burton, J. D.,Kim, Y-M.,Borisevich, A. Y.,Pennycook, S. J.,Yang, S. M.,Noh, T. W.,Gruverman, A.,Li, X. G.,Tsymbal, E. Y.,Li, Qi Nature Publishing Group 2013 NATURE MATERIALS Vol.12 No.5
The range of recently discovered phenomena in complex oxide heterostructures, made possible owing to advances in fabrication techniques, promise new functionalities and device concepts. One issue that has received attention is the bistable electrical modulation of conductivity in ferroelectric tunnel junctions (FTJs) in response to a ferroelectric polarization of the tunnelling barrier, a phenomenon known as the tunnelling electroresistance (TER) effect. Ferroelectric tunnel junctions with ferromagnetic electrodes allow ferroelectric control of the tunnelling spin polarization through the magnetoelectric coupling at the ferromagnet/ferroelectric interface. Here we demonstrate a significant enhancement of TER due to a ferroelectrically induced phase transition at a magnetic complex oxide interface. Ferroelectric tunnel junctions consisting of BaTiO<SUB>3</SUB> tunnelling barriers and La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>MnO<SUB>3</SUB> electrodes exhibit a TER enhanced by up to ~ 10,000% by a nanometre-thick La<SUB>0.5</SUB>Ca<SUB>0.5</SUB>MnO<SUB>3</SUB> interlayer inserted at one of the interfaces. The observed phenomenon originates from the metal-to-insulator phase transition in La<SUB>0.5</SUB>Ca<SUB>0.5</SUB>MnO<SUB>3</SUB>, driven by the modulation of carrier density through ferroelectric polarization switching. Electrical, ferroelectric and magnetoresistive measurements combined with first-principles calculations provide evidence for a magnetoelectric origin of the enhanced TER, and indicate the presence of defect-mediated conduction in the FTJs. The effect is robust and may serve as a viable route for electronic and spintronic applications.
Kim, Ki Sung,Kim, Young-Min,Mun, Hyeona,Kim, Jisoo,Park, Jucheol,Borisevich, Albina Y.,Lee, Kyu Hyoung,Kim, Sung Wng Wiley (John WileySons) 2017 Advanced Materials Vol.29 No.36
<P>Structural defects often dominate the electronic- and thermal-transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic-scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti1-xHfxNiSn1-ySby half-Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti1-xHfxNiSn1-ySby alloys containing inherent atomic-scale defect disorders are produced in one hour by a newly developed process of temperature-regulated rapid solidification followed by sintering. The collective atomic-scale defect disorder improves the zT to 1.09 +/- 0.12 at 800 K for the Ti0.5Hf0.5NiSn0.98Sb0.02 alloy. These results provide a promising avenue for improving the TE performance of state-of-the-art materials.</P>
Multiferroic tunnel junctions and ferroelectric control of magnetic state at interface (invited)
Yin, Y. W.,Raju, M.,Hu, W. J.,Burton, J. D.,Kim, Y.-M.,Borisevich, A. Y.,Pennycook, S. J.,Yang, S. M.,Noh, T. W.,Gruverman, A.,Li, X. G.,Zhang, Z. D.,Tsymbal, E. Y.,Li, Qi American Institute of Physics 2015 Journal of Applied Physics Vol.117 No.17
Mishra, Rohan,Kim, Young-Min,He, Qian,Huang, Xing,Kim, Seong Keun,Susner, Michael A.,Bhattacharya, Anand,Fong, Dillon D.,Pantelides, Sokrates T.,Borisevich, Albina Y. American Physical Society 2016 Physical Review B Vol.94 No.4
<P>The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a (LaFeO3)(8)/(SrFeO3)(1) superlattice film capped with four layers of LaFeO3. Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from Fe3+ in the bulk to Fe2+ at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen.</P>
Kim, Young-Min,He, Jun,Biegalski, Michael D.,Ambaye, Hailemariam,Lauter, Valeria,Christen, Hans M.,Pantelides, Sokrates T.,Pennycook, Stephen J.,Kalinin, Sergei V.,Borisevich, Albina Y. Nature Publishing Group, a division of Macmillan P 2012 NATURE MATERIALS Vol.11 No.10
Oxygen vacancy distributions and dynamics directly control the operation of solid-oxide fuel cells and are intrinsically coupled with magnetic, electronic and transport properties of oxides. For understanding the atomistic mechanisms involved during operation of the cell it is highly desirable to know the distribution of vacancies on the unit-cell scale. Here, we develop an approach for direct mapping of oxygen vacancy concentrations based on local lattice parameter measurements by scanning transmission electron microscopy. The concept of chemical expansivity is demonstrated to be applicable on the subunit-cell level: local stoichiometry variations produce local lattice expansion that can be quantified. This approach was successfully applied to lanthanum strontium cobaltite thin films epitaxially grown on substrates of different symmetry, where polarized neutron reflectometry revealed a strong difference in magnetic properties. The different vacancy content found in the two films suggests the change in oxygen chemical potential as a source of distinct magnetic properties, opening pathways for structural tuning of the vacancy concentrations and their gradients.
Kim, Young-Min,Morozovska, Anna,Eliseev, Eugene,Oxley, Mark P.,Mishra, Rohan,Selbach, Sverre M.,Grande, Tor,Pantelides, S. T.,Kalinin, Sergei V.,Borisevich, Albina Y. Nature Publishing Group, a division of Macmillan P 2014 NATURE MATERIALS Vol.13 No.11
The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.