Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H25: Focus Session: Novel and Complex Oxides: NaxCoO2 Experiment and Theory |
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Sponsoring Units: DMP Chair: Claude Ederer, UC-Santa Barbara Room: LACC 501A |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H25.00001: Electronic Structure, Magnetism, and Superconductivity in Na$_x$CoO$_2$ Invited Speaker: Electronic structure results for sodium cobaltate, Na$_x$CoO$_2$, and its superconducting derivative are presented. First principles calculations are used in conjunction with the tight-binding model to understand and interpret, on a microscopic level, the behaviors seen by experiment. A comparison of LDA-generated and observed optical spectra at different electron levels reveals that correlation in this system, while likely to be strong, does not manifest itself in the traditional Mott-Hubbard way. Six small Fermi surface sheets are predicted by calculation to exist at nearly all Na levels, but have not yet been seen directly by experiment. To explain the discrepancy, the calculated effects of structure change, Na level, hydration and magnetism on these pockets are presented and compared with existing experimental data. Finally, the implications of the small hole surfaces for superconductivity are explored and the presence of an exotic superconducting state is postulated. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H25.00002: Novel Aspects of Correlated Electron Motion in (K/Na)xCoO2 systems Revealed by Advanced Photoelectron Spectroscopy M. Zahid Hasan, D. Qian, M. Foo, Robert Cava Recent discovery of superconductivity, large thermopower and other unusual transport behavior in cobalt oxides (Na$_{0.3}$CoO$_{2}$.nH$_{2}$O) have generated much interests in understanding the many-body character of the electron liquid in this system. We employ advanced photoelectron spectroscopic techniques to reveal the microscopic electron dynamics in Na$_{x}$CoO$_{2}$ and K$_{x}$CoO$_{2}$ system classes. Results directly unveil an unusual and novel nature of many-body electron motion manifested through strong bandwidth suppression, heavy-fermion-like carrier mass and unusually slow movement of low-lying electrons despite the presence of a large Fermi surface indicating a two orders of magnitude departure from the conventional BCS paradigm. Temperature dependence of quasiparticles in the over-doped cobaltates shows that spectral weight remains well defined in the T-linear resistivity regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics have direct implications for understanding the novel phase of matter realized in this new class of strongly interacting complex system. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H25.00003: Orbital Symmetry and Electron Correlation in Na$_{x}$CoO$_2$ W. B. Wu, D. J. Huang, J. Okamoto, H.-J. Lin, C.T. Chen, A. Tanaka, F.C. Chou, A. Fujimori Measurements of polarization-dependent soft x-ray absorption reveal that the electronic states determining the low-energy excitations of Na$_{x}$CoO$_2$ have predominantly $a_{1g}$ symmetry with significant O $2p$ character. In contrast to the prediction of band theory, doping-dependent O $1s$ x-ray absorption shows a large transfer of spectral weight, providing spectral evidence for strong electron correlations of the layered cobaltates. We also found that Na$_{x}$CoO$_2$ exhibits a charge-transfer electronic character rather than a Mott-Hubbard character. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H25.00004: Theory of Sodium Ordering in Na$_x$CoO$_2$ Steven G. Louie, Peihong Zhang, Marvin L. Cohen, Rodrigo B. Capaz The ordering of Na ions in Na$_x$CoO$_2$ is investigated systematically over the full range of doping by combining detailed density functional theory (DFT) studies with model calculations. Various ground state ordering patterns are identified and are in excellent agreement with available experimental results. Our results suggest that the primary driving force for the Na ordering is the screened electrostatic interaction among Na ions. Possible effects of the Na ordering on the electronic structure of the CoO$_2$ layer are discussed. We propose that the nonexistence of a charge ordered insulating state at $x = 2/3$ is due to the lack of a commensurate Na ordering pattern, whereas an extremely stable Na ordering at $x = 0.5$ enhances the charge ordering tendency, which results in an insulating state as observed experimentally. RBC acknowledges financial support from the John Simon Guggenheim Memorial Foundation and Brazilian funding agencies CNPq, CAPES FAPERJ, Instituto de Nanoci{\^e}ncias, FUJB-UFRJ and PRONEX-MCT. Work partially supported by NSF Grant No. DMR00-87088 and DOE Contract No. DE-AC03-76SF00098. Computational resources were provided by NPACI and NERSC. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H25.00005: A resonant ultrasound spectroscopy study of the phase transitions in Na$_{0.75}$CoO$_{2}$ Veerle Keppens, Rapha\"el P. Hermann, Zhiying Zhang, Kathleen Affholter, Ivan Sergienko, Rongying Jin, Brian C. Sales, David G. Mandrus The layered transition metal oxides Na$_{x}$CoO$_{2}$ have attracted much interest in the past few years. Crystals with the $x\simeq0.75$ composition undergo an order-disorder transition near 340 K, a spin-density-wave transition near 22 K and other subtle transitions at intermediate temperatures. These phase transitions, likely related to a rearrangement of the Na atoms among the available sites, have been mapped out using resonant ultrasound spectroscopy. The results are modeled within the Landau theory for second order phase transitions. [Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725] [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H25.00006: A DMFT study of Na$_{x}$CoO$_{2}$ Chris Marianetti, Gabriel Kotliar Various recent experiments have roughly mapped out the phase diagram of Na$_{x}$CoO$_{2}$ (versus $x$) in an effort to understand the basic electronic structure. Interestingly, the magnetic susceptibility has been observed to be Curie-Weiss- like near NaCoO$_{2}$, and Pauli-like near CoO$_{2}$. This is counterintuitive given that NaCoO$_{2}$ is believed to be a band insulator, and CoO$_{2}$ is in the vicinity of the hydrated superconductor. LDA+DMFT calculations are presented in order to shed light on why signatures of correlations (ie. Curie-Weiss behavior) are observed near what is believed to be a band insulator (ie. NaCoO$_{2}$). [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H25.00007: Temperature dependence of DOS at the surface of Na0.75CoO2 Lei Cai, Chenxi Lu, Jiandi Zhang, R. Jin, B. C. Sales, D. G. Mandrus Although many interesting bulk phenomena of layered compound Na$_{x}$CoO$_{2}$ are unveiled, the lack attention is paid to the surface properties. The structure and the composition in the proximity of the surface can make the surface properties to be drastically different from the bulk in spite of its quasi-2D layered character. We have studied the density of states (DOS) as a function of the temperature for in-situ cleaved Na$_{x}$CoO$_{2}$ with x $\sim $ 0.75 by Variable Temperature Scanning Tunneling Microscopy (VT-STM). While it shows strong temperature dependence, the dI/dV curve indicates the presence of a finite DOS at the Fermi level (E$_{F})$ at all measured temperatures between 91 K to 294 K. This is consistent with previous report that Na$_{0.75}$CoO$_{2}$ is metallic in bulk. However, an unusual shift of the minimum DOS occurs below 230 K. Combining with the observation of a hump on the curve of DOS \textit{vs.} T at the energy levels between 0.11 eV and 0.51 eV, we attribute the shift to the local charging effect on the surface which indicates an unconventional metal-insulator transition near 230 K. The work was supported by NSF-DMR0346826 and DOE DE-FG02-04ER46125. ORNL is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725 [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H25.00008: A Study of Magnetoresistance of Na0.75CoO2 Rongying Jin, Zhixian Zhou, Alexei Souslov, Brian Sales, Qinghong Kou, David Mandrus The transverse (I$\bot $H) and longitudinal (I//H) magnetoresistance (MR) of Na$_{0.75}$CoO$_{2 }$single crystals have been measured above and below the spin-density-wave (SDW) transition temperature T$_{SDW}$ = 22 K. While there is little effect at T $>$ T$_{SDW}$, the MR(H=constant) is positive and increases with decreasing temperature along both ab plane and c direction at T $<$ T$_ {SDW}$. Strikingly, MR(T=constant$<$ T$_{SDW})$ increases monotonically with applied field, showing no sign of saturation up to 32 tesla in all cases. Quantitative analysis and implications of the data will be reported. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H25.00009: Optical Properties of Na$_x$CoO$_2$ Single Crystals (0.20$\leq$x$\leq$0.85) N.L. Wang, D. Wu, G. Li, J.L. Luo, X.H. Chen, C.H. Wang, R. Jin, D. Mandrus, T. Xiang We present in-plane optical measurements over broad frequencies on NaxCoO2 single crystals with 0.20$\leq$x$\leq$0.85.[1] We found that the spectral weight associated with the conducting carriers increases systematically with decreasing Na contents. The evolution suggests against a picture of doped Mott insulator for the NaxCoO2 system. At high energy, two broad interband transition peaks near 1.5$\sim$1.6 eV and 3$\sim$3.1 eV were observed. For metallic samples, anomalous charge dynamics was revealed at low frequencies. A peak at finite frequency is commonly observed, which shifts to higher frequencies with increasing temperature, irrespective of a positive slope of the dc resistivity. In addition, the optical spectra reveal a combination of the coupling effect of electrons with a Boson mode and pseudogap-like phenomenon. For the charge ordering sample of x=0.5, a broad hump near 800 cm$^ {-1}$ develops below around 100 K. Upon entering the charge- ordering state, a sharp suppression of the spectral weight is seen near 230 cm$^{-1}$, indicating the opening of a charge gap. [1] N. L. Wang, et al., Phys. Rev. Lett. 93, 147403 (2004); ibid, 93 (issue of Dec.3, in-press) (2004). [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H25.00010: Formation of the one-electron structure of CoO$_{2}$ layers Igor Mazin, Ole Andersen, Ove Jepsen, Michelle Johannes We use the N-order muffin tin orbital method to reduce the LDA band structure of a CoO$_{2}$ layer, the principal element of the novel superconductor Na$_{x}$CoO$_{2}\cdot y$H$_2$O, to a three-band model and to understand it in terms of simple tight binding. We show how to introduce correctly the trigonal distortion on top of the approximately cubic local symmetry. Some of the conclusions are: (i) no adequate description of the electronic structure is possible in a one-band model and (ii) third nearest neighbor interaction appears to be important, which can be understood in terms of a superexchange via intermediate Co $e_g$ states. We also present a set of parameters for what we believe is the minimal tight-binding model. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H25.00011: ARPES studies of Na0.7CoO2 system D. Qian, M.Z. Hasan, Y.-D. Chuang, A. Kuprin, A.V. Fedorov, R. Kimmerling, E. Rotenberg, Z. Hussain, H. Koh, N.S. Rogado, M.L. Foo, R.J. Cava We have carried out a detailed (and first) angle-resolved photoemission study of Na0.7CoO2, the host material of the superconducting cobaltate series. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band, a small Fermi velocity, and a large Hubbard U. The quasiparticle band crosses the Fermi level from M toward suggesting a negative sign of effective single-particle hopping teff (about 10 meV) which is on the order of magnetic exchange coupling J in this system. Temperature dependence of quasiparticles shows that spectral weight is well defined in the T-linear resistivity (non-Fermi-liquid) regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics may have implications for understanding the phase of matter realized in this new class of strongly interacting quantum system. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H25.00012: Ferromagnetic spin fluctuation and possible triplet superconductivity due to inter-orbital Hund's-rule coupling in Na$_x$CoO$_2 \cdot y$H$_2$O Masahito Mochizuki, Youichi Yanase, Masao Ogata Electronic structure and superconductivity in Na$_x$CoO$_2 \cdot y$H$_2$O are studied theoretically by using a fluctuation- exchange approximation. This material has partially- filled Co $t_{2g}$ orbitals and LDA calculation shows that its Fermi surface consists of more than two bands. Thus, we expect that the multi-band or multi-orbital contributes to the low- energy electronic state in this material. We employ a multi-orbital Hubbard model which includes the Co $t_{2g}$ orbitals. Tight-binding parameters are determined to reproduce the LDA band dispersions. To analyse this model, we extend the fluctuation-exchange approximation to a triply- degenerate orbital case. We will discuss that several important and interesting aspects appear which are not expected in a single-band model. One of them is a ferromagnetic (FM) spin fluctuation which is enhanced by the inter-orbital Hund's-rule coupling. This FM spin fluctuation leads to triplet pairing mainly on the disconnected hole-pocket Fermi surfaces, in contrast to the naive expectation of RVB superconductivity in a single-band $t$-$J$ model. We will also discuss the obtained results in the light of available experimental findings. On the basis of these analyses, we will point out that Na$_{0.35}$CoO$_2 \cdot$1.3H$_2$O can provide a key material for clarification of roles of orbitals on the superconductivity in the strongly correlated electron systems. [Preview Abstract] |
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H25.00013: Optical investigations on Na$_{0.7}$CoO$_{2}$ Leonardo Degiorgi, Giulio Caimi, Helmut Berger The discovery of superconductivity at 5 K in hydrated sodium cobaltate has attracted considerable attention. The investigation of non-hydrated sample is of relevance for the understanding of superconductivity and a considerable research effort has been devoted to Na$_x$CoO$_2$ specimens with $x$ ranging between 0.3 and 0.85. We present the optical properties of Na$_{0.7}$CoO$_2$ single crystals, measured over a broad spectral range as a function of temperature ($T$). The capability to cover the energy range from the far-infrared up to the ultraviolet allows us to perform reliable Kramers-Kronig transformation, in order to obtain the absorption spectrum (i.e., the complex optical conductivity). To the complex optical conductivity we apply the generalized Drude model, extracting the frequency dependence of the scattering rate ($\Gamma$) and effective mass ($m^*$) of the itinerant charge carriers. We find that $\Gamma(\omega)\sim \omega$ at low temperatures and for $\omega > T$. This suggests that Na$_{0.7}$CoO$_2$ is at the verge of a spin-density-wave metallic phase. [Preview Abstract] |
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