Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B32: Focus Session: Transition Metal Oxides I |
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Sponsoring Units: GMAG DMP Room: 207B |
Monday, March 2, 2015 11:15AM - 11:51AM |
B32.00001: Probing functional perovskites through scanning transmission electron microscopy and first-principles theory Invited Speaker: Stephen Pennycook The aberration-corrected scanning transmission electron microscope (STEM) can provide real space imaging and spectroscopy at atomic resolution with a new level of sensitivity to structure, bonding, elemental valence and even spin state. Coupled with first-principles theory, this represents an unprecedented opportunity to probe the functionality of complex nanoscale systems. A number of examples will be shown, including the microscopic origin of the barrier to O vacancy transport across grain boundaries in Y-stabilized ZrO$_{2}$, the strain stabilized generation of a spin state superlattice in La$_{0.5}$Sr$_{0.5}$CoO$_{3-x}$ (LSCO) [1], the unexpected ferromagnetism in ultrathin, insulating LaCoO$_{3-x}$ (LCO) films [2] due to a vacancy superlattice (Fig. 1), and finally, the origin of the 2D electron gas at a LaAlO$_{3}$/SrTiO$_{3}$ interface is shown to be not due to vacancies but to the polar nature of the substrate [3].\\[4pt] Work performed in collaboration with J. Gazquez, N. Bi\v{s}kup, J. Salafranca, C. Cantoni, M. Varela and S. T. Pantelides. \\[4pt] [1] J. Gazquez, et al., Nano Lett, \textbf{11}, 973 (2011).\\[0pt] [2] N. Bi\v{s}kup, et al., Phys. Rev. Lett. \textbf{112}, 087202 (2014).\\[0pt] [3] C. Cantoni, et al., Adv. Mater. \textbf{24}, 3952 (2012). [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B32.00002: Spin-state blockade in Te$^{6+}$-substituted electron-doped LaCoO$_{3}$ Keisuke Tomiyasu, Shun-Ichi Koyama, Masanori Watahiki, Mika Sato, Kazuki Nishihara, Mitsugi Onodera, Kazuaki Iwasa, Tsutomu Nojima, Yuuichi Yamasaki, Hironori Nakao, Youichi Murakami Perovskite-type LaCoO$_{3}$ (Co$^{3+}$: $d^{6})$ is a rare inorganic material with sensitive and characteristic responses among low, intermediate, and high spin states. For example, in insulating nonmagnetic low-spin states below about 20 K, light hole doping (Ni substitution) induces much larger magnetization than expected; over net 10$\mu_{B}$/hole (5$\mu_{B}$/Ni) for 1$\mu_{B}$/hole (1$\mu_{B}$/Ni), in which the nearly isolated dopants locally change the surrounding Co low-spin states to magnetic ones and form spin molecules with larger total spin [1-4]. Further, the former is isotropic, whereas the latter exhibits characteristic anisotropy probably because of Jahn-Teller distortion [2]. In contrast, for electron doping, relatively insensitive spin-state responses were reported, as in LaCo(Ti$^{4+})$O$_{3}$, but are not clarified, and are somewhat controversial. Here, we present macroscopic measurement data of another electron-doped system LaCo(Te$^{6+})$O$_{3}$ and discuss the spin-state responses. -- Refs. [1] S. Yamaguchi et al., PRB (1996). [2] K. Tomiyasu et al., PRB (2013). [3] A. Podlesnyak et al., PRL (2008). [4] Y. Ju et al., J. Supercond. Nov. Magn. (2013). [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B32.00003: Crystal Fields as a Probe of the Valence Transition in Pr-based Cobaltites D. Phelan, S. Rosenkranz, D.M. Pajerowski, C. Leighton Upon cooling, a unique first-order metal-insulator transition (MIT) has been observed in cobaltite perovskites at certain compositions with Pr on the A-site; this is strongly suspected as a consequence of electron transfer from Pr$^{3+}$ ions to hybridized Co-O orbitals. Here we discuss the crystal field levels of Pr$^{3+}$ and Pr$^{4+}$ ions which we have investigated using inelastic neutron spectroscopy as a function of temperature for three samples: one insulating, one metallic, and one that undergoes the MIT. We show that the insulating compound, PrCoO$_{3-\delta}$, contains Pr$^{3+}$ ions (J=4) whose ground state multiplet is split into nine singlets, and we extract its crystal field parameters. In the hole-doped metal, Pr$_{0.7}$Ca$_{0.3}$CoO$_{3-\delta}$, which also contains Pr$^{3+}$ ions, the crystal field levels maintain the same energy-level structure but are noticeably broadened. The scattering from the Pr$^{3+}$ crystal fields, though present, is diminished in the sample that undergoes the MIT (Pr$_{0.63}$Y$_{0.07}$Ca$_{0.3}$CoO$_{3-\delta}$), consistent with a decrease in the concentration of Pr$^{3+}$; moreover, the levels are severely broadened. The experiments illustrate the inhomogeneous charge distribution that occurs around the Pr sites upon substitution. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B32.00004: Spatial Magnetic Phase Separation in the Oxygen Deficient Perovskite SrCoO$_{3-y}$ Z.H. Zhu, F.J. Rueckert, J.I. Budnick, W.A. Hines, B.O. Wells, Ch. Niedermayer, B. Dabrowski We report here the unique local structure of the magnetically phase separated perovskite SrCoO$_{3-y}$ by means of transverse field (TF) and zero field (ZF) muon spin rotation. Previously we reported samples of SrCoO$_{3}$ and SrCoO$_{2.875}$ have unique magnetic and crystallographic phases, whereas samples of intermediate composition have multiple magnetic transitions but a single structural phase (\textit{Appl. Phys. Lett}. \textbf{99}, 052503 (2011)). Muon measurements establish that the intermediate compounds show spatially separated, distinct magnetic phases -- true magnetic phase separation. In addition, the two magnetic phases evolve with temperature in an unusual way indicating that the two phases remain coupled. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B32.00005: Atomic-resolution scanning transmission electron microscopy study of the valence state transition in (Pr$_{0.85}$Y$_{0.15}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ Robert Klie, Ahmet Gulec, Daniel Phelan, Chris Leighton The observation of a first-order magnetic/electronic transition in certain Pr-based perovskite cobaltites, such as Pr$_{0.5}$Ca$_{0.5}$CoO$_{3}$, has attracted significant attention. A simultaneous metal to insulator transition, a sharp drop in the magnetic moment and a change in the electronic structure has been reported to occur below $T_{MIT}$. It was suggested that the low-temperature phase is stabilized by a shift of the mixed valence Co$^{3+}$/Co$^{4+}$ toward pure Co$^{3+}$, enabled by a valence change of Pr$^{3+}$ to Pr$^{4+}$. We present an atomic-scale study of (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ using atomic-resolution imaging, electron energy-loss spectroscopy and in-situ cooling experiments in a scanning transmission electron microscope. The valence state transition in (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ occurs at a transition temperature $T_{MIT}$ $\sim$ 135K for $y = $0.15 and the in-situ cooling experiments are conducted at 90 K. At room temperature, we find oxygen vacancy ordering associated with a Co valence state ordering and we will demonstrate that the electron transfer occurs from Pr to Co below the transition temperature. The oxygen vacancy ordering disappears as a result of the Co valence state transition. The effects of oxygen mobility, sample homogeneity and the impact on the observed transition will be discussed. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B32.00006: Brownmillerite CaCoO$_{2.5}$: Synthesis, Re-entrant Structural Transitions and Magnetic properties Junjie Zhang, Hong Zheng, Christos Malliakas, Jared Allred, Yang Ren, Qing'an Li, Tianheng Han, John Mitchell Cobalt oxides attract both fundamental and technological attention due to their physical properties including thermoelectricity, giant magnetoresistance, superconductivity and multiferroicity [1]. Here we report the first synthesis of CaCoO$_{2.5}$ single crystals using a high pressure optical-image floating zone technique. We find that it is an ordered oxygen-deficient perovskite of the brownmillerite type, and it undergoes an unprecedented re-entrant structural phase transitions (Pcmb$\to $ P2/c11$\to $ P12$_{1}$/m1$\to $ Pcmb) with decreasing temperature. We describe its temperature-dependent structural, thermal, and magnetic properties, including AFM ordering near 240 K, with a weakly spin canted ferromagnet ground state below 140 K. The magnetic response of CaCoO$_{2.5}$ depends markedly on the cooling rate and field history. Magnetization data also imply the potential of a distinct, field-induced phase arising uniquely from the P12$_{1}$/m1 structure, revealed as kinetically trapped by a rapid-cooling protocol.\\[4pt] [1] Raveau, B.; Seikh, M. M. Cobalt Oxides: From Crystal Chemistry to Physics; Wiley-VCH: Weinheim, 2012. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B32.00007: Neutron Scattering Study on the Spin-1/2 Triangular-Lattice Antiferromagnet Ba$_{3}$CoSb$_{2}$O$_{9}$ Jie Ma, Y. Kamiya, Tao Hong, H.B. Cao, G. Ehlers, Z.L. Dun, W. Tian, C.D. Batista, H.D. Zhou, M. Matsuda Ba$_{3}$CoSb$_{2}$O$_{9}$ is a spin-1/2 triangular-lattice antiferromagnet with uniform bond length that has attracted a lot of attention in the past decade. This compound is recognized as an ideal material to study the interplay between frustration, low-dimensionality, and strong quantum fluctuations. Both neutron diffraction and inelastic neutron scattering measurements were performed on Ba$_{3}$CoSb$_{2}$O$_{9}$ are obtained by the neutron scattering technique. According to our unpolarized and polarized neutron diffraction measurements, the Co$^{2+}$ magnetic moments form a 120 degree structure in the easy ab-plane. The intra- and inter-plane exchange interactions were determined by fitting the measured spin-wave dispersion with an S$=$1/2 XXZ model. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B32.00008: Anisotropic dispersion of charge stripe fluctuations in La$_{1.75}$Sr$_{0.25}$NiO$_4$ John Tranquada, Ruidan Zhong, Genda Gu, Dmitry Reznik, Barry Winn It has recently been demonstrated that charge stripe fluctuations can be detected in La$_{2-x}$Sr$_x$NiO$_4$ by inelastic neutron scattering at temperatures close to the charge-ordering transition [1]. The next step is to characterize the dispersion of these excitations. To do this, we have studied a crystal with $x=0.25$ using the HYSPEC instrument at the Spallation Neutron Source. The clearest picture has been obtained at a temperature of 160 K, where spin order is absent while the charge order is weak but finite. The effective observation window is limited to $E < 8$ meV, as acoustic phonons dispersing from neighboring Bragg peaks obscure the weak signal of interest at higher energies. Measuring about the charge-order peak at wave vector (4.4,3,0), where $a^*=1.159$~\AA$^{-1}$, we observe a dispersion with a velocity of $\sim20$ meV-\AA\ along the stripe-modulation direction, but no clear dispersion in the orthogonal direction. The detected velocity has the scale of lattice, rather than purely electronic, excitations. \par\noindent [1] S. Anissimova {\it et al.}, Nat. Commun. {\bf 5}, 3467 (2014). [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B32.00009: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 1:27PM - 1:39PM |
B32.00010: Low-energy description of rare-earth nickelates Oleg Peil, Alaska Subedi, Antoine Georges We propose a simple low-energy theory of rare-earth nickelates that involves only two $e_g$ orbitals per nickel site. We show that this theory, in particular, captures all important features of the metal-insulator transition of nickelates. In the monoclinic insulating state, bond-length disproportionation splits the manifold of $e_g$ bands, leading to a modulation of the effective on-site energy. We show that, when subject to a local Coulomb repulsion $U$ and Hund's coupling $J$, the resulting bond-disproportionated state is a paramagnetic insulator for a wide range of interaction parameters. Furthermore, we find that when $U-3J$ is small or negative, a spontaneous instability to bond disproportionation takes place for large enough $J$. In the metallic phase, on the other hand, this negative effective coupling results in the suppression of the orbital polarization of $e_g$ states, consistent with experiments and earlier calculations. This minimal theory emphasizes that a small or negative charge-transfer energy, a large Hund's coupling, and a strong coupling to bond-disproportionation are the key factors determining the physics of nickelates. Finally, we discuss particular experimental consequences of this theoretical picture. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B32.00011: Exchange and Magnetic Anisotropic Interactions of Magnetic Ions in Antiferromagnetic Materials, Nd$_{2}$CuO$_{4}$ Alexander Bazhan Investigations of magnetic ions interactions, based on theory of crystallographic and magnetic symmetry, which indicate quadratic forms of thermodynamic potentials, presented in irreducible representations of interacting magnetic moments, described by antiferromagnetic and magnetic vectors, which determine antiferromagnetic orderings, are caring out using vector v.s.magnetometer, in example, of non collinear antiferromagnetic orderings of four Cu ions in magnetic unit cells in Nd$_{2}$CuO$_{4}$ of tetragonal symmetry, applying in HTS. Magnetic field dependencies of three separate components of samples magnetic moments directly indicate magnetic moments orientations, determined by magnetic ions interactions. Transformations of non collinear antiferromagnetic orderings of Cu ions in magnetic fields H$_{\mathrm{c1}}$ $\sim$ 42kOe and H$_{\mathrm{c2}}$ $\sim$ 8.4kOe, at fields orientations along [100] and [110] axis, T $\sim$ 1.8K, which are discussing using representations, determined by two antiferromagnetic vectors in equations of magnetic states, can be used in investigations, when two dimensional correlated electrons systems are introduced in such materials, before HTS. [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B32.00012: The Discovery of Ferroelectricity in NH4CuCl3, a Gapless Quantum Magnet Jared Kinyon, Eun Sang Choi, Ron Clark, Haidong Zhou, Zhenxing Wang, Kwang-yong Choi, Hans Van Tol, Naresh Dalal Alkali metal trichlorocuprates ($M_3CuCl_3$: $M = K^+,Tl^+$ and $NH_{4}^{+}$) have received considerable attention as a unique system of gapped/gapless quantum magnets. However, their possible multiferroelectricity, as both magnets and ferroelectrics, have not been reported. Such a finding would add a new dimension to their utility as memory storage elements. Here we report the discovery of ferroelectricity in $NH_{4} CuCl_{3}$, and several of its mixed lattices with K. In this talk, we present details of our crystal growth, x-ray diffraction studies of lattice authenticity and purity, dielectric, specific heat with and without fields, magnetic susceptibility, pulsed fields, and EPR characterization of these novel materials. Theoretical studies of the new findings should be fruitful. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B32.00013: ABSTRACT MOVED TO W32.00006 |
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