Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L6: Focus Session: Emergent Properties in Bulk Complex Oxides: Manganites |
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Sponsoring Units: GMAG DMP Chair: Xia Hong, University of Nebraska-Lincoln Room: 108 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L6.00001: Large lattice distortions associated with the magnetic transition in La0.7Sr0.3MnO3 Dmitry Reznik, Frank Weber, Oleg Prokhnenko, Dimitri Arguriou Colossal magnetoresistance (CMR) is associated with the phase transition from a metallic ferromagnetic to insulating paramagnetic phase, which can be controlled by an applied magnetic field. The insulating phase occurs due to trapping of the charge carriers by polaronic lattice distortions, which raise the resistivity. Theories based on local physics predict that the magnitude of the resistivity jump at Tc is determined by how much, on average, the amplitude of these distortions increases at the phase transition. Using neutron scattering, we measured the average distortion amplitude in La0.7Sr0.3MnO3. Surprisingly, its increase from below to above Tc is just as large as in other manganites, which have a much larger resistivity jump. This result suggests that the strength of CMR is determined not by the size of distortions, but by their cooperative nature specific to each compound. Existing theories need to be extended to include correlations between different unit cells to explain and predict the strength of CMR. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L6.00002: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L6.00003: Quantitative analysis of the diffuse scattering from a bilayer manganite S. Rosenkranz, J. Chen, D.J.P. Morris, S.N. Ancona, H. Zheng, J.F. Michell, M. Anitescu, R. Osborn, B.J. Campbell Magnetoresistance in manganese oxides is strongly enhanced by the presence of Jahn-Teller polarons and short range correlations between them. While previous investigations have shown the existence of both local lattice distortions and short-range order, a detailed quantitative description of the local structure, how it evolves as a function of temperature and doping, and how it affects the physical properties is still lacking. Here we present a detailed analysis of the diffuse scattering measured over a large range of temperatures from a bilayer manganite exhibiting colossal magnetoresistance. Focusing on the diffuse scattering around a single Bragg Peak and utilizing a point-defect approximation, we are able to derive a complete picture of the detailed temperature dependence of the Jahn-Teller distorted MnO$_6$ octahedra. These results further serve as a first test of the formalism for the quantitative analysis of the diffuse scattering over a large volume of reciprocal space, including both diffuse scattering from local defects close to Bragg Peaks, as well diffuse scattering due to short-range correlations. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L6.00004: Theoretical studies on the stability of metal-insulator coexistence in the absence of defects in perovskite manganites Keun Hyuk Ahn, Tsezar Seman, Turab Lookman, Alan R. Bishop We examine the stability of large metal-insulator domains in perovskite manganites in the absence of defects, using a model expressed in terms of symmetrized atomic-scale lattice distortion modes. Our results demonstrate that an intrinsic mechanism is responsible for the inhomogeneities in perovskite manganites, which involves long-range interactions between strain fields, the Peierls-Nabarro energy barrier, and complex energy landscapes with multiple metastable states. This is in contrast to an extrinsic mechanism such as chemical randomness or defects. We highlight experimental results which support the intrinsic mechanism rather than the extrinsic mechanism. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L6.00005: Spin Waves and Magnetic Correlations in the Multiferroic Sr0.56Ba0.44MnO3 Jeffrey Lynn, Daniel Pratt, James Mais, Omar Chmaissem, Bogdan Dabrowski Neutron diffraction and inelastic scattering measurements have been carried out on a polycrystalline sample of ferroelectric Sr$_{\mathrm{0.56}}$Ba$_{\mathrm{0.44}}$MnO$_{\mathrm{3}}$ (T$_{\mathrm{F\thinspace }}=$ 350 K) using the BT-7 and SPINS triple-axis spectrometers. The system orders antiferromagnetically at 197 K with an order parameter that varies smoothly with temperature. Inelastic measurements at base temperature reveal an energy gap of 4.6(5) meV, with a continuous distribution of magnetic scattering above the gap that exhibits a weak peak at 7.5 meV. The top of the magnon band was measured to be 43(1) meV. The data were modeled with a simple nearest-neighbor exchange$ J$ and the measured anisotropy gap, which was powder-averaged and fit to the data to yield an exchange constant $J=$4.8(2) meV. Above T$_{\mathrm{N}}$ strong correlations persist, consistent with the determined exchange as the scattering broadens in wave vector. Replace this text with your abstract body. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L6.00006: Doping influence of spin dynamics and magnetoelectric effect in hexagonal Y$_{0.7}$Lu$_{0.3}$MnO$_3$ Wei Tian, Guotai Tan, Liu Liu, Jinxing Zhang, Barry Winn, Tao Hong, Jaime Fernandez-Baca, Chenglin Zhang, Pengcheng Dai Inelastic neutron scattering experiments were performed to study spin waves and their correlation with the magnetoelectric effect in Y$_{0.7}$Lu$_{0.3}$MnO$_3$. The Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect in YMnO$_3$ and LuMnO$_3$. In Y$_{0.7}$Lu$_{0.3}$MnO$_3$, our INS study reveals a much smaller in-plane (hexagonal \textit{ab}-plane) anisotropy gap that coincides with a weaker in-plane dielectric anomaly at $T_N$. Since both the smaller in-plane anisotropy gap and the weaker in-plane dielectric anomaly are coupled to a weaker Mn trimerization distortion in Y$_{0.7}$Lu$_{0.3}$MnO$_3$ comparing to YMnO$_3$ and LuMnO$_3$, we conclude that the Mn trimerization is responsible for the magnetoelectric effect and multiferroic phenomenon in Y$_{1-x}$LuxMnO$_3$. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L6.00007: Ellipsometry studies of the optical phonons in hexagonal manganites $R$MnO$_{3}$ R. Basistyy, T.N. Stanislavchuk, M. Kotelyanskii, N. Lee, X. Wang, S-W. Cheong, G.L. Carr, A.A. Sirenko Optical properties of hexagonal multiferroic oxides $R$MnO$_{3}$, where $R=$ Ho, Er, Tm, Yb, and Lu, have been studied in the far-infrared spectral range between 10 and 4000 cm$^{-1}$ and temperatures between 1.5 K and 300 K. An advanced experimental technique of Muller matrix Spectroscopic Ellipsometry was used at the U4IR beamline of the National Synchrotron Light Source, Brookhaven National Lab. Spectra of the optical phonons will be presented in terms of the temperature dependencies of the phonon frequencies, their oscillator strength, anisotropy, and signatures of the spin-phonon interaction at the antiferromagnetic (AFM) phase transition. The spin-phonon interaction reveals itself as a non-Gr\"{u}neisen behavior of several phonon frequencies below $T_{\mathrm{N}}$ (Mn$^{3+})$. A decrease of the ionic radius for $R^{3+}$ ions between Ho$^{3+}$ and Lu$^{3+}$ resulted in a systematic increase of the optical phonon frequency. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L6.00008: Microwave imaging of charge-ordered phase coexistence in a layered manganite Eric Yue Ma, Ben Bryant, Yusuke Tokunaga, Zhi-Xun Shen Microwave impedance microscopy has been used to probe the co-existence of two charge- and orbital-ordered phases with slightly different in-plane conductivity in the layered manganite Pr(Sr$_{\mathrm{0.1}}$Ca$_{\mathrm{0.9}})$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$. Hysteretic, spatially inhomogeneous transitions between 1) the two charge-ordered phases and 2) the charge-ordered and disordered phases are observed, in which the transition temperature is affected by local strain induced by the presence of microscopic structural twin domains. In addition, conductivity contrast features are observed in the higher temperature, anti-ferroelectric charge ordered phase, which may represent anti-ferroelectric domain walls. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L6.00009: Magnetostructural coupling in the magnetodielectric Mn$_3$O$_4$ Moureen Kemei, Jaye Harada, Matthew Suchomel, Ram Seshadri At room temperature, the spinel Mn$_3$O$_4$ is distorted from cubic symmetry due to Jahn-Teller distortions of octahedral Mn$^{3+}$ and is described by the tetragonal spacegroup $I4_1/amd$. It undergoes ferrimagnetic ordering near 43$\,$K where anomalies in heat capacity and dielectric measurements are also observed. High-resolution variable-temperature synchrotron X-ray powder diffraction reveals a distortion of the long-range crystal structure at the onset of magnetic order. The structure of Mn$_3$O$_4$ below the magnetostructural ordering temperature is described by coexisting tetragonal and orthorhombic phases. A discontinuous change in lattice parameters at the magnetostructural ordering temperature illustrates that this system undergoes a first-order phase transition. Sharp changes in heat capacity at the magnetic ordering temperature are also consistent with a first-order phase transition. We present the complete crystallographic description of this important magnetodielectric spinel and suggest mechanisms behind the spin-driven lattice distortion. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L6.00010: Hybrid quasiparticles within the orthorhombic or hexagonal topology of RMO$_{3}$ (R$=$Nd,Pr,Tm,Er; M$=$Mn,Cr) under strong magnetic fields R. Sopracase, K. Holldack, L. del Campo, N.E. Massa, M.J. Mart\'Inez-Lope, J.A. Alonso We report on magnetoelectric quasiparticles that originate from electronic Coulomb and exchange correlations using a Bruker IFS125-HR interferometer at 0.5 cm$^{-1}$ resolution in the THz beamline of the electron storage ring BESSYII in Berlin. Orthorhombic NdMnO$_{3}$ and hexagonal TmMnO$_{3}$ have quasiparticles at energies of zone center magnons. In both cases, increasing the applied field, the $\sim$ 20 cm$^{-1}$ line matching the lowest energy magnon, has its intensity reduced sharply while bands associated in TmMnO$_{3}$ to magnon--acoustical phonon dispersion crossing and gap opening behave differently. The line at $\sim$ 48 cm$^{-1}$, the higher branch of the phonon gap, shows a Zeeman splitting-like behavior, while the lower branch at $\sim$ 31 cm$^{-1}$ has weak field dependences. The asymmetric envelope peaking at $\sim$ 35 cm$^{-1}$ in NdMnO$_{3}$ weakens, softens, and evolves at 8 T into two unresolved bands suggesting field induced TA$+$magnon coupling materializing a condition for a multiferroic state. Metastable orthorhombic ErMnO$_{3}$ has two bands at 5 K which resembles those of NdMnO$_{3}$. A remarkable 35 cm$^{-1}$ Zeeman splitting at 5 K in PrCrO$_{3}$ is tentatively associated to Cr$^{3+}$ electrons in a distorted polarizable p-d bond. ErCrO$_{3}$ shows such a feature at 50 cm$^{-1}$ as well additional zero field splitting at 8 and 9 cm$^{-1}$ in the spin reorientation phase. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L6.00011: Magnetoelectric orthorhombic and multiferroic hexagonal ErMnO$_{3}$: THz hybrid modes in RMnO$_{3}$(R)$=$ Nd, Er, Tm) N.E. Massa, V. Ta Phuoc, L. del Campo, D. De Sousa Meneses, P. Echegut, K. Holldack, M.J. Mart\'Inez-Lope, J.A. Alonso We report on far- and mid-infrared emission, reflection, and transmission spectra of metastable orthorhombic perovskite (Pbnm-T$_{\mathrm{N}}$ $\sim$ 42 K) and hexagonal (P6$_{3}$cm- T$_{\mathrm{N}}$ $\sim$ 84 K) ErMnO$_{3}$. The number of phonon modes remains constant from 300 K to 4 K. Magnetically disordered electrons in fluctuating orbitals lead to an ambient THz broad reflectivity band. On cooling toward T$_{\mathrm{N}}$ the electrons exhibit increasing charge and magnetic short-range correlations and condense into soft bands that harden at about T$_{\mathrm{N}}$ as magnetic order sets in. However, at difference of NdMnO$_{\mathrm{3}}$ and TmMnO$_{3}$ that show correlation with a gap opening in transverse acoustical phonon dispersion and spin order, hexagonal ErMnO$_{3}$ develops at 5 K a set of four strong hybridized modes centered $\sim$ 70 cm$^{-1}$ in addition to one peaking at $\sim$ 96 cm$^{-1}$ and another weaker at $\sim$ 44 cm$^{-1}$. Orthorhombic ErMnO$_{3}$ develops a 46 cm$^{-1}$ band and a very strong one at 91cm$^{-1}$ that seems to correlate to a weaker 98 cm$^{-1}$ phonon in the short-range only magnetic order environment. We conclude that Er$^{3+}$ paramagnetic fluctuations increases Mn spins frustration in both compounds being the disruption strongest in orthorhombic ErMnO$_{3}$ where the THz band and short range magnetic onset may be traced even at 150 K, probably, due to the increment in the Jahn-Teller distortion. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L6.00012: Phase Separation and Percolative Insulator-Metal Transition in LaMnO$_3$ Under Pressure: A Gutzwiller Variational Study Mohammad Sherafati, Sashi Satpathy We study the pressure-induced insulator-metal transition and phase separation in LaMnO$_3$ (LMO) using the Gutzwiller variational method. Being an insulator at ambient pressure, a long-debated question is whether LMO is a correlation-driven Mott insulator or a band insulator driven by the Jahn-Teller (JT) mechanism. Recent Raman measurements of LMO (Ref. 1) reveal a coexistence of domains of JT-distorted and undistorted octahedra between 3 and 34 GPa and indicate a critical threshold for the volume fraction of the latter domains as a herald of the metallic state. To explain these findings, we solve an extended Hubbard model including the JT distortions and cohesive energy as a function of volume for spinless $e_g$ electrons of Mn$^{3+}$ in paramagnetic LMO at room temperature. Our results clearly show a phase separation on both sides of the transition pressure ($P_c=32$ GPa) where domains of distorted (insulating) and undistorted (metallic) octahedra coexist supporting the percolative nature of the transition. Based on our work, the ground state under pressure is determined by the interplay of both Coulomb and JT interactions and it is predicted that the mixed-phase region extends well above $P_c$ into the metallic region. Ref. : 1) M. Baldini et al., PRL 106, 066402 (2011) [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L6.00013: Local orthorhombic distortion and enhanced susceptibility in LaNiO$_{3}$ paramagnet Bing Li, Shinichiro Yano, Despina Louca, Luke Marshall, Jian-Shi Zhou, John Goodenough, Mikhail Feygenson, Jorg Neuefeind The perovskite LaNiO$_3$ is metallic, and unlike other systems in this class of materials, it remains paramagnetic where only an enhancement in the magnetic susceptibility ($\chi$) is observed below 200 K. Other rare earth nickelates are antiferromagnetic with an enhancement of $\chi$ in paramagnetic metallic state. Using neutron powder diffraction and the pair density function analysis, it is observed that the temperature dependence of the local atomic structure cannot be reproduced assuming the average crystal symmetry which is rhombohedral with the $R\bar{3}c$ space group. With rising temperature, octahedral distortions involving displacements of oxygen set in, and the symmetry is reduced to $Pbnm$. In this symmetry, the equivalent O site in the $R\bar{3}c$ splits into two and can account for all the features observed in the local lattice. The structural changes occur gradually, between 100 and 200 K. The local Ni-O-Ni bond angles are reduced from 164.5 to 163.5 $^\circ$ during this transition. Such reduction of Ni-O-Ni bond angles may facilitate antiferromagnetic coupling and responsible for the temperature dependence of $\chi$ observed in LaNiO$_3$ below 200 K. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L6.00014: Origins of bad metal conductivity and the insulator-metal transition in the rare-earth nickelates ($R$NiO$_{3}$, $R =$ rare earth) Rafael Jaramillo, Sieu Ha, Daniel Silevitch, Shriram Ramanathan For most metals increasing temperature ($T)$ or disorder quickens electron scattering. This scattering time hypothesis informs the Drude model of electronic conductivity. However, for so-called bad metals with very low conductivity this hypothesis predicts scattering times so short as to conflict with Heisenberg's uncertainty principle. Bad metal conductivity has remained a puzzle since its discovery in the 1980s in high T superconductors. Here we introduce the rare-earth nickelates ($R$NiO$_{3}$, $R =$ rare earth) as a class of bad metals. We study SmNiO$_{3}$ thin films using infrared (IR) spectroscopy while varying $T$ and disorder. We show that the interaction between lattice distortions and Ni-O bond covalence explains both the bad metal conductivity and the insulator-metal transition (IMT) in the nickelates. It does so by shifting spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low conductivity while preserving the Drude model and without violating the uncertainty principle. [Preview Abstract] |
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