Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H13: Focus Session: NaxCoO2 and AxNiO2 |
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Sponsoring Units: DMP GMAG Chair: Zenji Hiroi, University of Tokyo Room: Colorado Convention Center Korbel 4C |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H13.00001: Novel sodium ordering on a NaxCoO2 surface Woei Wu Pai, S.S. Huang, C.H. Lin, H. S. Hsue, F.C. Chou The conducting layered sodium cobaltate, NaxCoO2, has generated great research interests recently. This material exhibits surprising properties as the Na concentration x is varied. Despite intense studies, Na ordering and its subtle interplay with charge ordering in the CoO2 layer remains unclear. Here we report the first direct observation of Na ordering on a NaxCoO2 surface (x=0.84) with scanning tunneling microscopy. Three distinct Na phases, all of hexagonal symmetry, were identified. These new findings did not fit any theoretical prediction at present. Plausible structure models were proposed. In additional, an one-dimensional stripe modulation on the surface was discovered, which was found to be a bulk phenomenon as well. Our results should prompt more detailed theoretical investigations into the mechanism of Na ordering. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H13.00002: Electronic Structures of Na in $Na_xCoO_2$ Paoan Lin, D. J. Huang, Horng-Tay Jeng, Chen-Shiung Hsue Sodium cobalt oxides ($Na_xCoO_2$) have attracted renewed because of their exceptionally large thermoelectric power recent discovery of superconductivity in their hydrated counterparts. In order to investigate the dependence on th doping-concentration for the electronic structures, we hav carried out a series of LDA+U Ab initio calculation on sod cobalt oxides ($Na_xCoO_2$) of various dopings. The calcul results were compared with experimental results of polarization-dependent soft x-ray absorption spectroscopy. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H13.00003: Pressure effect on magnetic and structural phase transitions in {Na$_x$CoO$_2$} ($x$=0.75, 0.80) O.B. Korneta, S.O. Leontsev, Y.V. Sushko, R. Jin, B.C. Sales, D. Mandrus The sodium-rich metallic compounds of Na$_x$CoO$_2$ family with $x \sim 3/4$ are known to exhibit an order-disorder structural transition at $\sim 340$K and a magnetic transition at $\sim 22$K. We have performed the magnetization and resistivity measurements under hydrostatic pressure to study both phase transitions in compounds with $x=0.75$, $x=0.80$. The data established positive pressure dependence of both the structural and magnetic transitions. Positive pressure effect on the Neel temperature suggests that superexchange interactions of localized moments may play an important role in magnetic properties of these materials. Such a conjecture is further supported by the observation of the metal-insulator transition (and its pressure evolution) in interplane resistivity of the $x=0.80$ compound. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H13.00004: Weak coupling SDW ground state with strong Fermi surface gapping in Na$_{x}$CoO$_{2}$ , x $\approx $ 0.8 M. Bruehwiler, B. Batlogg, S.M. Kazakov, J. Karpinski, D. Sheptyakov In Na$_{x}$CoO$_{2}$ the electrons move on a triangular lattice and in the Na-rich composition range (x $\ge $ 0.75) form a SDW ground state below T$_{c}\approx $ 22.5 K with a small ordered moment. We have studied this Fermi surface instability with heat capacity, magnetic and transport measurements on a series of samples with various nominal Na content. The SDW phase is characterized by a jump $\Delta $C at T$_{c }$ and an associated reduction of the electronic density of states. This removal of DOS has been deduced from the high-temperature value of the Sommerfeld $\gamma $ and the extrapolation from below 1K to T $\to $ 0. Interestingly, the ratio $\Delta $C/($\delta \gamma $.T$_{c })\approx $1.5 is close to the BCS weak coupling value. Even more surprising is the observation that up to $\approx $ 80{\%} of the DOS is removed in this Fermi surface instability. In addition to the gapped electronic excitation spectrum a broad hump in the specific heat is measured above $\approx $5K, consistent with excitations of the gapped spin wave spectrum in the SDW ordered state. Crystal structure analysis reveals for the SDW an orthorhombic symmetry and thus a slight distortion of the triangular lattice. Similarities and differences to the CDW-like state, which forms at x=0.5 also in a distorted triangular lattice, will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H13.00005: Structure prediction and phase diagram calculation of NaxCoO2 by Ab Initio methods Ying Shirley Meng, Yoyo Hinuma, Osman Burak, Gerbrand Ceder The unusual electronic properties of NaxCoO2 make it a material of considerable interest. At high sodium concentration it displays a remarkable combination of high electronic conductivity and high Seebeck coefficient. Understanding these phenomena requires a detailed understanding of the local structure, since different Na-vacancy orderings are strongly coupled to the electronic structure and Co3+/Co4+ arrangement. In this study, we investigate the charge ordering and Na-vacancy ordering using first principles electronic structure methods within the GGA and GGA+U approximations. Na ordering is determined not only by a competition between Na site energies difference and Na-Na repulsion, but also by the Co-Na interlayer interaction induced by charge localization. We believe that in particular at high Na content a quantitative understanding of the coupling is essential in understanding the remarkable electronic properties. Phase diagram calculations of NaxCoO2 to understand the phase stability in the system will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H13.00006: The low-energy ARPES and heat capacity of Na$_{0.3}$CoO$_2$: A DMFT study Chris Marianetti, Olivier Parcollet, Kristjan Haule, Gabriel Kotliar We use DMFT to calculate the ARPES spectrum and heat capacity for Na$_{0.3}$CoO$_2$. Both the traditional Hirsch-Fye (HF) Quantum Monte-Carlo technique and the newly developed continuous time (CT) quantum Monte-Carlo technique are used to solve the DMFT impurity problem. We show that the e$_g$' hole pockets on the Fermi surface are destroyed as the on-site coulomb repulsion is increased. Additionally, we show that quantitative agreement with both ARPES and heat capacity can be achieved. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H13.00007: Fabrication and Control of Sodium Concentration in Cobaltate Na$_{x}$CoO$_{2}$ thin films ($x $= 0.68, 0.75) by Thermal Diffusion W.J. Chang, J.-Y. Lin, C.H. Hsu, Y.K. Kuo, K.H. Wu, T.M. Uen, H.L. Liu, Y.S. Gou, J.Y. Juang We have fabricated Na$_{x}$CoO$_{2}$ ($x\sim $0.68 {\&} 0.75) thin films on sapphire (0001) substrates via lateral diffusion of sodium into Co$_{3}$O$_{4}$ (111) epitaxial films. The environment of thermal diffusion is key to the control of the sodium concent in thin films. From the results of x-ray diffraction and in-plane resistivity \textit{$\rho $}$_{ab}$, the epitaxial growth and the sodium contents of these Na$_{x}$CoO$_{2}$ thin films were identified. The thermoelectric measurements show a large thermoelectric power in our films, similar to that of single crystals. The quasiparticle scattering rate is found to approach zero, consistent with the small residual resistivity, indicating high quality of the Na$_{x}$CoO$_{2}$ thin films. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H13.00008: Charge and spin order on the triangular lattice --- Na$_x$CoO$_2$ at $x=0.5$ Sen Zhou, Ziqiang Wang The nature of charge and spin order of strongly correlated triangular lattice fermions is investigated in connection to the unconventional insulating state of Na$_x$CoO$_2$ at $x=0.5$. We study an extended Hubbard ($t$-$U$-$V$) model of the electron doped Co $a_{1g}$ band using a spatially unrestricted Gutzwiller approximation. We find a new class of charge and spin ordered states at $x=1/3$ and $x=0.5$ where the system alleviates antiferromagnetic (AF) frustration via charge inhomogeneity. We show that the $\sqrt{3}a\times2a$ off-plane Na dopant order at $x=0.5$ plays an important but subtle role. It induces weak $\sqrt{3}a\times1a$ charge order in the Co layer without gapping the Fermi surface and allows successive $\sqrt{3}a\times1a$ AF and $2a\times2a$ charge/spin ordering transitions at low temperatures. The nesting with the $2a\times2a$ hexagonal zone boundary gaps out almost the entire Fermi surface at $x=0.5$. We study the phase structure and compare to the findings of recent experiments. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H13.00009: High Resolution Scanning Tunneling Microscopy of Na$_{x}$CoO$_{2}$ M.C. Boyer, W.D. Wise, Kamalesh Chatterjee, M.A. Zimmermann, E.W. Hudson Since the 2003 discovery of superconductivity in water doped sodium cobaltate (Na$_{x}$CoO$_{2})$, many experimental techniques have been brought to bear on not only the superconducting parent state (x $\sim $ 0.3) but on other dopings as well. Unfortunately, scanning tunneling microscopy, which has shown so much success in the study of the related cuprates, has not been as successful in the study of Na$_{x}$CoO$_{2}$. We will present results from topographic and spectroscopic measurements of Na$_{x}$CoO$_{2}$ made using our variable temperature scanning tunneling microscope, with a focus on changes observed between 130 K and 4 K. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H13.00010: Charge and Spin Order in Na$_0.5$CoO$_2$ Ting-pong Choy, Philip Phillips Several experimental puzzles surround the insulating state of Na$_0.5$CoO$_2$: 1) antiferromagnetic order is observed but with a reduced moment $\mu_B=0.25$, 2) the insulating state occurs at a temperature below which N\'eel order obtains, and 3) static charge ordering is not seen in all NMR experiments. To address these questions, we focus on controlled calculations of the spin-wave spectrum and the magnitude of the local moment in two of the models proposed for the insulating state: 1) a charge-ordered state with 4-fold symmetry and 2) charge-ordering state with only 2-fold symmetry. We present a detailed iso-spin/spin coupling model which demonstrates how the charge and spin order are coupled. The phase diagram suggests that the ground state of Na$_0.5$CoO$_2$ should be both charge and spin ordered. Serveral candidates with different ordering are studied under a generalized spin-wave theory. By comparing the Neutron results with the low energy excitaion and the calculated structure factor, we conclude that the ground state of Na$_0.5$CoO$_2$ is charge with 4-fold symmetry and long-range spin order. In this state, we find that a spin moment of $\mu_B=0.25$ is well described by the experimentally relevant parameters for the exchange couplings. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H13.00011: Unusual valency and magnetic order in silver nickelates Sergey Streltsov, M.D. Johannes, I.I. Mazin, D.I. Khomskii Ag$_2$NiO$_2$ forms as a triangular based layered nickelate, with a structure identical to the well-studied alkali nickelates LiNiO$_2$ or NaNiO$_2$, but with a double layer of Ag between the oxide planes. The metallic intercalant ions give rise to highly unusual valence state for silver: Ag$^{1/2+}$. We show that the reason for the underoxidation is that the two silver ions form extremely strong bonding-antibonding bands, pushing the lowest Ag-$s$ derived band beneath the (filled) O $p$ complex. This additionally preserves metallicity down to the lowest measured temperatures and gives rise to complex, competing magnetic interactions. The resulting spin fluctuations may explain the large discrepancy (too large for phonon renormalization) between calculated and measured linear specific heat coefficients. Our calculations do not support a controversial cooperative Jahn-Teller distortion, but a comparison with calculations and experiments for single-layer AgNiO2 suggests that magnetically driven charge disproportionation may instead explain the observed structural transition. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H13.00012: Structural disorder and magnetic properties of NaNi$_{0.5}$Mn$_{0.5}$O$_{2}$ and LiNi$_{0.5}$Mn$_{0.5}$O$_{2}$. Natasha Chernova, Miaomiao Ma, Jie Xiao, M. Stanley Whittingham, Julien Breger, Jordi Cabana, Clare Grey Magnetic properties of layered O(3) compounds LiNi$_{0.5}$Mn$_{0.5}$O$_{2}$ and NaNi$_{0.5}$Mn$_{0.5}$O$_{2}$ are studied using AC susceptibility and DC magnetization techniques in order to elucidate magnetic interactions within transition metal (TM) layers and between them in compounds with various TM distributions. In ideal layered NaNi$_{0.5}$Mn$_{0.5}$O$_{2}$, antiferromagnetic (AF) ordering transition at 60 K and a spin-flop transition at 5 K in the magnetic field of 2.2 T are found. Upon lost of Na, AF order changes with ferrimagnetic, which may be caused by Ni$^{2+}$ migration to the Na layer. LiNi$_{0.5}$Mn$_{0.5}$O$_{2}$ with flower or zigzag TM order show ferrimagnetic ordering at around 100 K, and significant magnetization hysteresis below this temperature, indicating presence of Ni$^{2+}$ in the Li layer. Magnetic interactions in all compounds are analyzed and models of spin order at low temperatures are proposed. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H13.00013: Curie-Weiss metallic state in sodium cobaltates Ilya Vekhter, Christopher Hooley One of the most intriguing properties of sodium cobaltates, Na$_x$CoO$_2$ is the so-called Curie-Weiss metallic phase appearing at relatively high doping, $x\sim 0.7$. It exhibits Curie-Weiss magnetic susceptibility in a metal not far from the onset of antiferromagnetic order. Surprisingly for a layered quasi-two-dimensional structure, the neutron scattering experiments in the ordered state yield comparable in-plane and interplane magnetic exchange constants. We consider a model layered system on the verge of transition to a type-A antiferromagnet. We investigate whether in such a system fluctuations of the in-plane magnetization may give the apparent Curie-Weiss behavior in analogy with the spin-fluctuation theory for itinerant ferromagnets. We consider the effect of the crossover from incoherent to coherent interplane transport on the magnetic susceptibility and discuss the effect of sodium doping. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H13.00014: Ferromagnetism, paramagnetism and a Curie-Weiss metal in NaxCoO2 Jaime Merino, Ben Powell, Ross McKenzie Motivated by the unconventional properties and rich phase diagram of Na$_x$CoO$_2$ we consider the electronic and magnetic properties of a two-dimensional Hubbard model on an isotropic triangular lattice doped with electrons away from half-filling. Dynamical mean-field theory (DMFT) calculations predict that for negative inter-site hopping amplitudes ($t<0$) and an on-site Coulomb repulsion, $U$, comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for $t>0$ a large enhancement of the effective mass, itinerant ferromagnetism and a metallic phase with a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. The transport and magnetic properties measured in Na$_x$CoO$_2$ are consistent with DMFT predictions of a metal close to the Mott insulator and we discuss the role of Na ordering in driving the system towards the Mott transition. We propose that the Curie-Weiss metal phase observed in Na$_x$CoO$_2$ is a consequence of the crossover from ``bad metal'' with incoherent quasiparticles at temperatures T$>$T$^*$ and Fermi liquid behavior with enhanced parameters below T$^*$, where T$^*$ is a low energy coherence scale induced by strong local Coulomb electron correlations. Our analysis shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of Na$_x$CoO$_2$. [Preview Abstract] |
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