Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N3: Insulating Cobaltates: Mottness on a Triangular Lattice |
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Sponsoring Units: DCMP Chair: Philip Phillips, University of Illinois Room: Baltimore Convention Center Ballroom I |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N3.00001: Neutron scattering study of novel magnetic order in Na$_{0.5}$CoO$_{2}$. Invited Speaker: The layered sodium cobaltates, Na$_{x}$CoO$_{2}$, have attracted much recent attention, due to their unusual thermodynamic properties, as well as the recent discovery of superconductivity in the hydrated composition.~ These strongly correlated systems exhibit a rich electronic phase diagram as a function of sodium doping, $x$. A particularly intriguing insulating phase is realized at $x$=1/2, featuring a long range sodium order, a metal-insulator phase transition at 51 K, and a magnetic ordering transition at 88 K.~ We present polarized and unpolarized neutron scattering measurements of the magnetic order in single crystals of Na$_{0.5}$CoO$_{2}$. Our data indicate that below T$_{N}$ = 88 K the spins form a novel antiferromagnetic pattern within the CoO2 planes, consisting of alternating rows of ordered and non-ordered Co ions. The domains of magnetic order are closely coupled to the domains of Na ion order, consistent with such a two-fold symmetric spin arrangement. Magnetoresistance and anisotropic susceptibility measurements further support this model for the electronic ground state. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 9:12AM |
N3.00002: Quasiparticles, Fermi surface topology and Phase transitions in Na$_x$CoO$_2$ Invited Speaker: Recently discovered triangular cobaltate class is a novel realization of doped Mott insulators on a triangular spin- lattice. This system exhibits superconductivity, spin-density- waves, charge-order, metal-insulator phase transitions and colossal thermopower as well as Mott and Band insulation. We employ state-of-the-art ARPES to uncover the nature of electron motion in the cobaltates over the phase diagram. Quasiparticle dynamics (Fermi velocity, bandwidth, FS topology, correlation parameters, quasiparticle coherence) we extract from the data provides valueable insights into the novel phases of matter realized on this first realization of a triangular lattice Mott system. Low-T metal-insulator (order-disorder) phase transition will be discussed in this presentation. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:48AM |
N3.00003: Sodium Ion Ordering in double-layered and triple-layered Na$_{x}$CoO$_{2}$ Invited Speaker: The layered sodium cobalt oxide Na$_{x}$CoO$_{2}$ is studied by electron diffraction for a wide range of sodium contents, 0.15$<$x$<$0.75. This range in compositions is obtained by removal of Na by various methods for the starting materials Na$_{0.7}$CoO$_{2}$, and Na$_{1.0}$CoO$_{2 }$ The structure of Na$_{x}$CoO$_{2}$ is based the stacking of triangular O-Co-O layers with Na planes. The Co atoms are in edge-sharing CoO$_{6}$ octahedra. For the starting compound Na$_{0.7}$CoO$_{2}$, the Na$^{+}$ ions are in a trigonal prismatic coordination whereas for Na$_{1.0}$CoO$_{2 }$ the Na$^{+}$ coordination is octahedral. Prismatic coordination occurs when the close packed oxygen planes directly adjacent to the Na plane have the same projection into the basal plane (A-Na-A), whereas octahedral coordination of Na occurs when the directly adjacent oxygen planes have different projections (A-Na-B) into the basal plane. Due to this difference in stacking the a axis is about 1.08 nm and 1.65 nm for Na$_{0.7}$CoO$_{2}$ and Na$_{1.0}$CoO$_{2 }$respectively. For Na$_{0.7}$CoO$_{2}$ as well as Na$_{1.0}$CoO$_{2 }$a series of superstructures are observed, which can be explained with ordered Na ion-Na vacancy superlattices. The structural principle for some of the observed ordering schemes, particularly near x=0.5, is, surprisingly, the presence of lines of Na ions and vacancies rather than simply maximized Na-Na separations. With Na$_{0.7}$CoO$_{2}$ as starting material, the most strongly developed superlattice is found for the composition Na$_{0.5}$CoO$_{2}$. With Na$_{1,0}$CoO$_{2}$ as starting material, the most strongly developed superlattice is found for the compositions Na$_{0.75}$CoO$_{2 }$and Na$_{0.5}$CoO$_{2}$. In particular the superstructure Na$_{0.75}$CoO$_{2 }$of is very complicated. \newline \newline In collaboration with M.L. Foo, Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA; Q. Xu and V. Kumar, National Centre for HREM, Department of Nanoscience, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands ; L. Viciu, Department of Chemistry and Princeton Materials Institute, Princeton University; Q. Huang, NIST Center for Neutron Research, NIST, Gaithersburg, MD 20899; and R.J. Cava, Department of Chemistry and Princeton Materials Institute, Princeton University. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:24AM |
N3.00004: Magnetic-Field-Induced Suppression of the Charge Ordered State in Na$_{0.5}$CoO$_{2}$ and the observation of Shubnikov--de Haas Oscillations in Na$_{x}$CoO$_{2}$ Invited Speaker: We performed electrical transport measurements at low temperatures and high magnetic fields in Na$_{x}$CoO$_{2}$ single crystals for both $x$ = 0.5 and $x$ = 0.3. For $x$ = 0.5 Shubnikov de Haas oscillations corresponding to only 1{\%} of the area of the orthorhombic Brillouin zone (BZ) were clearly observed, indicating that most of the original Fermi surface vanishes at the charge ordering (CO) transition. While in-plane magnetic fields were found to strongly suppress the charge ordered state observed for x = 0.5 via a field-induced strongly hysteretic transition. When the external fields are rotated within the conducting planes, we observe angular magnetoresistance oscillations whose periodicity changes from two-to six-fold at the transition suggesting the reconstruction of the Fermi surface of this material. These facts indicate that the charge order is a delicate one, more akin to a charge-density-wave, and consistent with the small gap observed in the optical conductivity. While for $x$ = 0.3 we clearly observe quantum oscillatory phenomena for two frequencies $f_{1} \cong $ 480 and $f_{2}$ $\cong $ 800 T corresponding respectively to only 0.8 and 1.36{\%} of the first Brillouin zone (FBZ), with very weak indications of possible additional frequencies. These values contrast markedly with what is predicted by band structure calculations for $x$ = 0.3, i.e., 2.26{\%} and 22.3{\%} of the FBZ for the pockets resulting from the $e_{g}$$'$ and the $a_{1g}$ bands, respectively. We speculate that the Na superstructures seen for both concentrationsm re-define the Brillouin zone and thus the geometry of the Fermi surface explaining perhaps such discrepancies. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 11:00AM |
N3.00005: $^{17}$O and $^{59}$Co NMR Studies of Strongly Correlated Electrons in Na$_{x}$CoO$_{2}$ Invited Speaker: The anomalous electronic properties of triangular-lattice system Na$_{x}$CoO$_{2}$ has been attracting strong interest over the last several years since the discovery of superconductivity in hydrated Na$_{1/3}$CoO$_{2}\cdot _{4/3}$[H$_{2}$O]. The electronic phase diagram of these materials is quite rich, as the physical properties depend very strongly on Na concentration. Here we report our $^{17}$O and $^{59}$Co NMR studies of the local electronic properties and low-frequency spin dynamics in these materials for a variety of Na concentrations [1,2]. \newline \newline [1] F.L. Ning, T. Imai, B.W. Statt, and F.C. Chou, PRL \underline {93} (2004) 237201.\newline [2] F.L. Ning and T. Imai, PRL \underline {94} (2005) 227004. [Preview Abstract] |
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