Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W2: Electronic Structure, Magnetism and Superconductivity of Sodium Cobaltate |
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Sponsoring Units: DCMP Chair: Michelle Johannes, Naval Research Laboratory Room: Morial Convention Center LaLouisiane C |
Thursday, March 13, 2008 2:30PM - 3:06PM |
W2.00001: What we do and do not understand about electronic structure and superconductivity in sodium cobaltate? Invited Speaker: I will pose several questions and will attempt to answer at least some of them. (1) Why details of the electronic structure, including the Fermi surface topology, are particularly important for understanding superconductivity in this material? (2) What do standard band structure calculations get right, where they surely fail and where are they questionable? (3) How well do we understand the ARPES results? (4) What are possible effects of electron-electron correlations beyond LDA? Why LDA+U is worse than LDA for Na$_x$CoO$_2$ and to what extent can we believe DMFT? (5) What is the role of Na in the formation of the electronic structure and Fermi surface? To what extent the surface bands are the same as bulk? [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W2.00002: Pairing symmetry of the hydrated cobaltate superconductor Invited Speaker: We report NMR/NQR measurements on the hydrated cobaltate superconductor Na$_{x}$CoO$_{2}$*1.3H$_{2}$O at elevated pressures. The spin-lattice relaxation rate (1/$T_{1})$ decreases below $T_{c}$ with no coherence peak [1], and is in proportion to $T^{3 }$down to $T\sim T_{c}$/10, which provides compelling evidence for the existence of line nodes in the gap function [2,3]. The spin susceptibility obtained from the Knight shift measurement in a single crystal decreases below $T_{c}$ along all crystal-axis directions [4]. These results indicate anisotropic, spin-singlet pairing, and are most consistent with a $d$-wave gap. The electron correlations in the normal state are antiferromagnetic-like, which increases with decreasing Na-content [1,2]. The phase diagrams of $T_{c}$ and various physical properties as functions of Na-content [2], and pressure [3] will be presented, and the inter-relation between the superconductivity and the spin correlations will be discussed. \newline \newline References: \newline [1] T. Fujimoto, G. - q. Zheng, Y. Kitaoka, R.L. Meng, J. Cmaidalka, and C.W. Chu, Phys. Rev. Lett. \textbf{92}, 047004 (2004). \newline [2] G. - q. Zheng, K. Matano, R.L. Meng, J. Cmaidalka, and C.W. Chu, J. Phys.: Condens. Matter \textbf{18}, L63 (2006). \newline [3] E. Kusano, S. Kawasaki, K. Matano, G. - q. Zheng, R.L. Meng, J. Cmaidalka, and C.W. Chu, Phys. Rev. B \textbf{76}, 100506 (R) (2007). \newline [4] G. - q. Zheng, K. Matano, D.P. Chen and C.T. Lin, Phys. Rev. \textbf{B73}, 180503 (R) (2006). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W2.00003: NMR studies of Na$_{x}$CoO$_{2}$ Invited Speaker: Sodium cobaltate Na$_{x}$CoO$_{2}$ is the parent phase of triangular-lattice superconductor Na$_{1/3}$CoO$_{2}$-[H$_{2}$O]$_{4/3}$. Co ions take a mixed-valence state of +4-x in this system. Since Co$^{4+}$ and Co$^{3+}$ possess spin S=1/2 and S=0, respectively, one may view the CoO$_{2}$ layers as charge doped S=1/2 triangular-lattice. CoO$_{2}$ layers exhibit a rich variety of strongly correlated electron behavior as a function of sodium concentration x, ranging from itinerant antiferromagnet (x$\sim $0.84 and 0.5), ``Curie-Weiss metal'' (x$\sim $0.71) to Fermi liquid (x$\sim $1/3). In this talk, I will discuss our NMR studies of Na$_{x}$CoO$_{2}$ for various Na concentration x, with particular emphasis on $^{59}$Co evidence for charge ordering in the insulating ground state of Na$_{0.5}$CoO$_{2}$ [1-4]. \newline \newline [1] F.L. Ning et al., arXiv : 0711.4023 \newline [2] F.C. Chou et al., arXiv : 0709.0085 \newline [3] F.L. Ning et al., PRL 94, 227004 (2005) \newline [4] F.L. Ning et al., PRL 93, 237201 (2004) [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W2.00004: Novel electronic states in Na$_x$CoO$_2$: Role of strong correlation and Na dopant order Invited Speaker: We argue that the strong Co intra-atomic Coulomb repulsion renormalizes the crystal field splitting and the bandwidths of the $t_{2g}$ complex in Na$_x$CoO$_2$, resulting in a single band crossing the Fermi level at all doping levels $x$ explored by ARPES experiments [1]. On this basis, we study the electronic states using a minimal electron-doped, one-band Hubbard model with large $U$ on the triangular lattice. The important role played by the off-plane Na dopants is taken into account by including the ionic electrostatic potential. We find a class of charge and spin density ordered states where the system alleviates antiferromagnetic (AF) frustration via charge inhomogeneity [2]. We show that the $\sqrt{3}\times2$ Na order at $x=0.5$ causes weak $\sqrt{3} \times1$ charge order in the Co layer and the emergence of AF order with small electron and hole Fermi surface pockets [2]. This theory of the ``0.5 phase'' is consistent with neutron scattering, NMR, Shubnikov-de Haas oscillations, and transport experiments. In the sodium rich phases, the high density of off-plane Na dopants (or dilute Na vacancies), in their ordered or disordered form, increases the tendency toward carrier localization in the Co plane [3], which competes with in-plane ferromagnetic (FM) correlations described by a renormalized Stoner theory [4]. We argue that the newly discovered electronic phases associated with Na vacancy order [5,6] can be described by a useful notion of ``super-Mottness'', where strong correlation effects on the superlattice structure give rise to the competition and possible coexistence of localized magnetic moments and itinerant FM carriers. \newline \newline [1] S. Zhou, M. Gao, H. Ding, P.A. Lee, and Z. Wang, Phys. Rev. Lett. 94, 206401 (2005). \newline [2] S. Zhou and Z. Wang, Phys. Rev. Lett. 98, 076401 (2007). \newline [3] C.A. Marianetti and G. Kotliar, Phys. Rev. Lett. 98, 176405 (2007). \newline [4] M. Gao, S. Zhou, and Z. Wang, Phys. Rev. B 76, 180402 (2007). \newline [5] M. Roger, et al., Nautre, 445, 631 (2007). \newline [6] F.C. Chou, et al., arXiv:0709.0085. [Preview Abstract] |
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