Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R39: Superconductivity-Sodium Cobaltate |
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Sponsoring Units: DCMP Chair: Igor Mazin, Naval Research Laboratory Room: Baltimore Convention Center 342 |
Wednesday, March 15, 2006 2:30PM - 2:42PM |
R39.00001: Charge-fluctuation excitations in Na$_{1/3}$CoO$_{2}$$\cdot$yH$_{2}$O(*) O.D. Restrepo(1), A.G. Eguiluz(1), K.-W. Lee, J. Kunes, W.E. Pickett(3) We report a theoretical study of the electron-hole excitation spectrum of the hydrated superconductor Na$_{1/3}$CoO$_{2}$$\cdot$yH$_{2}$O. We use a paramagnetic LDA ground state as our starting point, and calculate the dynamical density-response function using time- dependent density-functional theory. Our results are compared with calculations for the parent compound Na$_{1/3}$CoO$_{2}$ and with recent electron-energy loss spectroscopy (EELS) experiments reported by H. X. Yang et al. (PRB 72, 075106 (2005)). Interestingly, the dielectric function obtained from our ab-initio calculations differ appreciably, in the key spectral region, from the one extracted from the experimental loss function by H. X. Yang et al. In fact, our dielectric function shows better qualitative correspondence with the loss data; this finding highlights the relevance of ab-initio calculations for the study of the experimental manifestation of the electronic structure. The physics of the prominent peaks in the loss spectra is discussed; we identify signatures of the underlying electronic structure in the hydrated compounds. (*) DOE-CMSN PCSCS collaboration. (1) Supported by NSF ITR-DMR 0219332 (2) Managed by UT-Battelle for the U.S. DOE under contract DE- AC05-00OR22725. (3) Supported by DOE Grant DE-FG03-01ER45876 [Preview Abstract] |
Wednesday, March 15, 2006 2:42PM - 2:54PM |
R39.00002: Charge and Spin Ordering in Insulator Na$_{0.5}$CoO$_2$: Effects of Correlation and Symmetry Kwan-Woo Lee, Warren Pickett The discovery by Takada and coworkers of superconductivity in Na$_{0.3}$CoO$_2 \bullet$1.3 H$_2$O near 5K has led to extensive studies of the rich variation of properties in the Na$_x$CoO$_2$ system ($0.2 \le x \le 1$), which has a triangular lattice of Co sites and a layered structure. In addition, specifically at $x$=0.5, the system has been observed to undergo a charge disproportionation (2Co$^{3.5+} \rightarrow$ Co$^{3+}$+Co$^{4+}$) and metal-insulator transition at 50 K, while the rest of the phase diagram is metallic. We will present results of studies of charge disproportionation and charge- and spin-ordering in insulating in Na$_{0.5}$CoO$_2$, applying ab initio band theory including correlations due to intra-atomic repulsion. Various ordering patterns (zigzag and two striped) for four-Co supercells are analyzed before focusing on the observed ``out-of-phase stripe'' pattern of antiferromagnetic Co$^{4+}$ spins along charge-ordered stripes. This pattern relieves frustration and shows distinct analogies with the cuprate layers: a bipartite lattice of antialigned spins, with axes at 90$^{\circ}$ angles. Substantial distinctions with cuprates are also discussed, including the tiny gap of a new variant of ``charge transfer'' type within the Co $3d$ system. [References] [1] K. Takada et al., Nature {\bf 422}, 53 (2003). [2] M. L. Foo et al., Phys. Rev. Lett. {\bf 92},247001 (2004). [3] K.-W. Lee, J. Kunes, P. Novak, and W. E. Pickett, Phys. Rev. Lett. {\bf 94}, 026403 (2005). [4] K.-W. Lee and W. E. Pickett, cond-mat/0510555. [Preview Abstract] |
Wednesday, March 15, 2006 2:54PM - 3:06PM |
R39.00003: Ab Initio Study on Na Ordering and its Relation to the Electronic/Magnetic Properties of P2-NaCoO2 Thermoelectrics Ying Shirley Meng, Anton Van der Ven, Maria Chan, Gerbrand Ceder The unusual electronic properties of NaxCoO2 make it a material of considerable interest. The system displays an unusual combination of high Seebeck coefficient and low metallic resistance. NaxCoO2 is also an interesting material on which to test our fundamental knowledge of mixed valence transition metals. NaxCoO2 is a mixed valence system with a fraction of x Co4+ and (1- x) Co3+ ions. Because of the high mobility of Na and large vacancy concentration, Na-vacancy ordering is likely in NaxCoO2. This ordering breaks the symmetry on the Co sublattice and may assist in charge ordering of Co3+ and Co4+. Mixed Co3+/Co4+ systems tend to display rich physics as they are often close to spin transitions and metal insulator transitions. In this study, we use both standard Density Functional Theory (DFT) in the Generalized Gradient Approximation (GGA) as well as GGA+U calculations to investigate the possible Na-vacancy and charge-ordered structure of P2-NaxCoO2 throughout the sodium composition range. We have identified new ground state structures at Na concentrations 0.11, 0.75 and 0.80. We have also found a strong coupling between the Na-vacancy ordering and Co3+/Co4+ charge ordering in the system. Such knowledge is crucial for understanding the thermoelectric properties of this material and similar mixed valence oxides. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R39.00004: Electron Correlation and Fermi Surface Topology of Na$_x$CoO$_2$ Sen Zhou, Meng Gao, Hong Ding, Patrick Lee, Ziqiang Wang The electronic structure of Na$_x$CoO$_2$ revealed by recent photoemission experiments shows important deviations from band theory predictions. The six small Fermi surface pockets predicted by LDA calculations have not been observed as the associated $e_g^\prime$ band fails to cross the Fermi level for a wide range of sodium doping concentration $x$. In addition, significant bandwidth renormalizations of the $t_{2g}$ complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multi-orbital Hubbard model in the Hartree-Fock and strong-coupling Gutzwiller approximation. The quasiparticle dispersion and the Fermi surface topology obtained in the presence of strong local Coulomb repulsion are in good agreement with experiments. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R39.00005: Systematic doping evolution of quasiparticle mode and Fermi surface topology in NaxCoO$_2$ Dong Qian, A.V. Fedorov, Zahid Hussain, R.J. Cava, N.L. Wang, M.Z. Hasan We have carried out a detailed and systematic state-of-the-art photoemission study of NaxCoO$_2$ over a wide range of doping. The Fermi velocity is found to be weakly anisotropic and small leading to a strongly suppressed quasiparticle bandwidth as a universal behavior in all dopings. No surface state or Ruthanate-type reconstruction is observed. Systematic studies show that the 2-D Fermi-surface area evolves according to the Na doping count only below x=0.5. The measured Fermi surface topology near superconducting Na doping kinematically allows for commensurate fluctuations (charge or lattice) as potential competing instabilities. The temperature evolution of quasiparticles will be discussed in addition. [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R39.00006: Correlation Effects and Magnetism in Na$_x$CoO$_2$ Meng Gao, Sen Zhou, Hong Ding, Ziqiang Wang Correlation effects are known to be important for the electronic structure and the Fermi surface topology of Na$_x$CoO$_2$ [ref to prl of Zhou et al.]. Here we study magnetic properties of Na$_x$CoO$_2$ based on the multi-orbital Hubbard model for the cobalt $t_{2g}$ electrons in weak and strong coupling limits using Hartree-Fock and Gutzwiller approximations. Both commensurate and incommensurate magnetic order is considered. We present the phase diagram in the parameter space of local Coulomb repulsion and Hund’s rule coupling for different sodium doping concentrations. The interplay between the correlation effects and Fermi surface nesting on the magnetic properties will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R39.00007: Thermal Expansion Measurements of the Antiferromagnetic Transition in Na$_x$CoO$_2$ Single Crystals (x=0.75 and 0.80) C. A. M. dos Santos, J. J. Neumeier, R. K. Bollinger, Y.-K. Yu, R. Jin, D. Mandrus Thermal expansion, heat capacity and magnetic susceptibility measurements of Na$_x$CoO$_2$ (x=0.75 and 0.80) single crystals are reported. The results show second-order antiferromagnetic transition near $T_N$ = 22 K. The high-resolution thermal expansion measurements, performed with a fused quartz dilatometer cell in both basal plane and c-axis directions, reveal strong anisotropy that is attributed to the hexagonal structure. Above and below $T_N$, the thermal expansion coefficients and specific heat measurements exhibit $\lambda$- like peaks which are similar to those reported for other antiferromagnetic compounds. Analysis of the critical behavior for both samples will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R39.00008: Specific-heat study for the coupling between two superconducting order parameters in Na$_{x}$Mn$_{z}$Co$_{1-z}$O$_{2}$·yH$_{2}$O. C.P. Sun, J. -Y. Lin, C.L. Huang, C.-J. Liu, J.S. Wang, H.D. Yang The substitution of Mn doping into the Co site, has been confirmed by the Mn $L$-edge XANES and EXAFS spectra, is performed by the specific-heat measurement with and without magnetic field. Due to the possible coexistence of isotropic and nodal-line gap, the two superconducting order parameters should exhibit different $T_{c}$ suppression rate by the doping impurity. Without field, two anomalies are seen below $T_{c}$, $\sim $ 4 K determined from magnetization, may be caused by different superconducting contributions due to the impurity effect and then merge into one under higher magnetic field ($\sim $1T). Schottky effect needs to be taken into account in the normal state. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R39.00009: Transport Properties and X-ray Absorption Spectroscopy in Layered Cobaltate Na$_{x}$CoO$_{2}$ thin films W. J. Chang, J. Y. Juang, J.-Y. Lin, C. M. Lee, C.-M. Huang, J. M. Chen, C.-H. Hsu Na$_{x}$CoO$_{2}$ ($x\sim $0.7) thin films ($\sim $200 nm) were fabricated on sapphire (0001) substrates via lateral diffusion of sodium into Co$_{3}$O$_{4}$ (111) epitaxial films. From the results of x-ray diffraction and in-plane resistivity \textit{$\rho $}$_{ab}$, the single phase and the metallic behaviors of these Na$_{x}$CoO$_{2}$ films were identified, and consisting with the results of single crystals [1] and Ohta's films [2]. The anomalous Hall effect and the magnetoresistance were also measured in Na$_{x}$CoO$_{2}$ thin films. In addition, the O 1$s$ x-ray absorption spectra of thin films show different anisotropy to those of single crystals. The electronic structures of Na$_{x}$CoO$_{2}$ single crystals and thin films will be discussed to investigate the splitting of $e_{g}$ and $t_{2g}$ states in Co 3$d$ bands of Na$_{x}$CoO$_{2}$ thin films. *This work was supported by the National Science Council of Taiwan, under Grant Nos. NSC-94-2112-M-009-006. [1] Maw Lin Foo \textit{et al}., Phys. Rev. Lett. \textbf{92}, 247001 (2004). [2] Hiromichi Ohta \textit{et al}., Crystal Growth {\&} Design \textbf{5}, 25 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R39.00010: Variational Monte Carlo study of pairing symmetries of Na0.3CoO2.1.3H2O Rui Fan, Yong-Qiang Wang, Hai-Qing Lin To investigate possible pairing symmetries of Na0.3CoO2.1.3H2O, we study the single band extended Hubbard model on triangular lattice with variational Monte Carlo method. For the on site s-wave, the extended s-wave, the d-wave, the f-wave and the extended-f wave parings, we use the corresponding BCS type mean field wave functions with Jastrow factors as our trial wave functions. We found that in a suitable parameters region, the extended f-wave pairing is stabilized by the nearest-neighbor Coulomb interaction V. In order to further investigate the true ground state of this system we use the constrained path Monte Carlo (CPMC) method to calculate different correlation function in the ground state. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R39.00011: Band Structures, Fermi Surface Topology and Superconductivity in Na$_x$CoO$_2 \cdot y$H$_2$O: Effects of the trigonal CoO$_6$ distortion Masahito Mochizuki, Masao Ogata In the recently discovered Co-oxide superconductor Na$_x$CoO$_2 \cdot y$H$_2$O, the edge-shared CoO$_6$ octahedra are trigonally contracted along the $c$-axis in the CoO$_2$-plane. We study how this CoO$_6$ distortion affects the magnetic properties and superconductivity in this compound by analyzing the multiorbital Hubbard model using the fluctuation-exchange approximation. It is shown that through generating the trigonal crystal field, the distortion pushes the Co ${e'}_g$ bands up and consequently gives rise to the hole-pocket Fermi surfaces, which have been predicted in the band calculations. As the distortion increases, the hole pockets are enlarged and the ferromagnetic fluctuation as well as the pairing instability increases, which is in good agreement with recent NQR results. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R39.00012: Photoemission study on core levels of NaxCoO2 Jihua Ma, Hongbo Yang, Zhihui Pan, A.V. Fedorov, R. Jin, B.C. Sales, D. Mandrus, Hong Ding A Comprehensive study on Na 2p and Co 3p core levels of NaxCoO2 is carried out by photoemission. A surface component of Na 2p core level is found. By comparing the surface state and bulk state at different emission angles, different photon energies and different doping levels, we not only extract the electron escape length in this system, but also discover other interesting features. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R39.00013: Determination of $t_{2g}$ Complex in Na$_{x}$CoO$_{2}$ by Angle-Resolved Photoemission Spectroscopy Hongbo Yang, Jihua Ma, Zhihui Pan, Alexei Fedorov, Jongyin Jin, Brian Sales, David Mandrus, Ziqiang Wang, Hong Ding A systematic investigation on $t_{2g}$ complex in Na$_x$CoO$_2$ is carried by ARPES. By taking advantage of the matrix element effect, we recover and trace the dispersion of all three $t_{2g} $ bands for different Na concentration ($x=0.3\sim0.7$). The dispersion ``kink'' we observed before is found to be cause by the hybridization between $a_{1g}$ and $e'_{g}$ bands. This band hybridization forms a narrow band near the Fermi level, which causes a large mass renormalization. The width of the non- hybridized bands is about half of the one predicted by LDA calculations, which is possibly due to the correlation effect. [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R39.00014: Possible coexistence of $s$-wave and unconventional pairing in hydrated Na$_{x}$CoO$_{2}$: a new insight from impurity effects Jiunn-Yuan Lin, Y.-J. Chen, C.-J. Liu, J.-S. Wang, C. P. Sun, H. D. Yang, S. W. Huang, J. M. Chen, J. M. Lee, J. F. Lee, D. G. Liu Impurity effects on $T_{c}$ were known to provide indispensable information in many novel superconductors. To shed light on the symmetry of the superconducting order parameter in hydrated Na$_ {x}$CoO$_{2}$, the Mn doping effects have been studied. X-ray absorption spectroscopy reveals that the doped Mn impurities occupy the Co sites and have a valance close to 4+. Impurity scattering by Mn is in the unitary limit that, however, does not lead to strong $T_{c}$ suppression. This absence of the strong impurity effects on $T_{c}$ is not consistent with the simple picture of a sign-changing order parameter. For reconciliation among all existing experiments, the model of coexistence of $s$-wave and unconventional pairing is proposed. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R39.00015: Effect of heavy water intercalation in sodium cobaltate . Cinzia Metallo, Takeshi Egami, Thomas Proffen, David Mandrus, Brian Sales, Rongying Jin Superconductivity in the hydrated compound Na$_{x}$CoO$_{2 }$yH$_{2}$O (x=0.35, y=1.3) is directly related to water intercalation. Nevertheless, so far, no clear explanation of the effect of water on the superconducting mechanism has been given. Here we present our study on deuterated sodium cobaltate Na$_{x}$CoO$_{2 }$yD$_{2}$O (x=0.35, y=1.4) at two different temperatures, T=15K, 100K. Neutron diffraction data were analyzed using the PDF technique. The PDF of Na$_{x}$CoO$_{2 }$yD$_{2}$O was compared with the PDF of heavy water. The diffraction spectra at both temperatures show a weak D-D correlation, and a pronounced D-O correlation. The weakened D-D correlations imply that the dynamics of the hydrogen bond is modified. This increases the dielectric response, and may enhance the electron-phonon coupling. Our results suggest a modification of the geometry of the heavy water molecules inserted in Na $_{x}$CoO$_{2}$, and subsequently a possible active role of water in setting the superconducting state. [Preview Abstract] |
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