Acknowledgements. (aperçu de) la physique des oxydes de cobalt et de sodium Na x CoO 2. Objectifs de la présentation

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1 Acknowledgements (aperçu de) la physique des oxydes de cobalt et de sodium x CoO 2 Marc-Henri Julien Laboratoire de Spectrométrie Physique UMR 5588 Université J. Fourier Grenoble I Cédric de Vaulx (PhD ) NMR : H. Mayaffre, C. Berthier, M. Horvatic (Grenoble) Material synthesis & crystal growth P. Lejay, P. Strobel, H. Muguerra (Grenoble) C.T. Lin & D.P. Chen (MPI Stuttgart), J. Wooldridge, G. Balakrishnan & M.R. Lees (Coventry) J.M. Tarascon, M. Armand (Amiens) V. Pralong (Caen) XRD & SQUID: P. Bordet, V. Simonet (Grenoble), S. Hébert (Caen) Objectifs de la présentation Search for triangular analogs of high T C s 1) Aperçu global du diagramme de phase de x CoO 2 et de ses propriétés électroniques principales (corrélations électroniques) Sr 2 RuO 4 - metallic, multibands - p-wave superconductor Nickelates La2-xSrxNiO4 - insulating - Spin and charge stripes 2) Particularité majeure de ces composés : mise en ordre des + et impact sur les propriétés électroniques Change Cu for Ru Cu-based HTc cuprates Change Cu for Ni Cut CuO2 planes into slices Change for Cu-based spin-ladders - spin gap - d-wave superconductor Cu-based delafossites (LaCuO 2.66, etc.) - No superconductivity - Insulating - Magnetic LRO xcoo2

2 A x MO 2 genealogy Cobaltate genealogy AxCoO2: A =, Li, K, Sr, Ca, etc. Intercalation materials, ionic conductors «Li-ion» LiNiO2, NiO2, AgNiO2, magnetism LiVO2, LiMn02, cathode materials «misfits» 1 CoO 2 a band insulator? + (O 2- ) x 2 Co 3+ e g t 2g Octahedral CF splitting ~ 2 ev (e g & a 1g bands) K c [%] T 1-1 [s -1 ] 1 CoO 2 is a band insulator (Co 3+, S=0) NMR de Vaulx et al., PRL 2005 Lang et al., PRB Co K orb de Vaulx et al., PRL 95, (2005) 59 Co T[K] No spin shift (susceptibility) Extremely slow nuclear Relaxation (> 10 s) S = 0 N(E F ) = 0

3 A 1 CoO 2 are all band insulators (A=Li,, K, Ag, etc.) Hole-doping the x=1 band insulator By removing x( + ) from the layers Although obtaining pure x=1 compounds can be challenging LiCoO2 : Ménétrier et al., Electrochem. Solid-State Lett., 11, A179 (2008) AgCoO2 : Muguerra et al., J. Solid State Chem. 181, 2883 (2008) + Co 3+ (S=0) Co 4+ (S=1/2) e g χ (10-5 emu mol -1 Oe -1 ) AgCoO 2 χ = χ orb + χ imp = const + 1/T 0.5 χ orb T (K) H. Muguerra et al. e - t 2g (e g & a 1g bands) Orbital and band structure issues Not discussed here Co 3+ intermediate spin state? (local symmetry breaking) No spin-state transition ever observed in x CoO 2 Co 4+ Co 3+ S=1 2 Two kinds of sites Co lattice 1 E F predominant a1g character D.J. Singh, PRB 2000 Khaliullin and coworkers a 1g & e g bands, fermiology Hybridization with O 2p orbitals Orbital ordering, spin-orbit coupling? M. Roger et al., LT25 proceedings & ture 445, 631 (2007)

4 Observed phases depend on synthesis method & conditions electrochemical, chemical, direct, ion-exchange, pulsed laser deposition, etc. Accurate measure of x() is challenging (while physical properties can change with variation of x ~ 1%) Surface ordering different from bulk Sample aging ( losses) Lots of material issues Good samples are clean! (sharp NMR lines, Shubnikov de Haas oscillations) T M (K) 30 (b) 15 Samples Finite number of stable phases Ubiquitous phase separation for 0.67 < x < 0.9 MH Julien et al., unpublished 0 (c) (1+3) 0 (2) (1) 0 (?) (5) 22 K x() (1) 27 K (2) 20 K (some of the) stable phases 0 x CoO 2 Delmas, Solid State Ionics (1981) de Vaulx, PRL (2005) Lee, ture Materials (2006) Li x CoO 2 Van der Ven, PRB (1998) Ménétrier, J. Mater. Chem. (1999) Marianetti, ture Materials (2004) T and x dependent + mobility Blangero et al., PRB 2008 x=0.63 ρ (mω cm) High T anomalies in transport measurements Impact of + ordering on magnetism Apparition of a magnetic transition at 8 K, depending on cooling protocol above ~200 K Schulze et al., PRL 100, (2008) 0.8 CoO T (K) Freezing 180 K x=0.75 Slow dynamics detected in NMR down to ~180 K Julien et al., PRL 2008 Lang et al., PRB 2008 T(K)

5 Neither 1 nor 2 59 Co NMR sites 3 different sites (at long time scale, ~ 10-6 s) 30% (not 75%) of localized Co 3+ no Co4+ Finite number of 23 NMR lines Demonstrates spatial ordering of the different Co states Julien et al., PRL (2008) x= CoO 2 Mukhameshin, PRL (2005) Alloul et al., EPL (2008) Julien et al., PRL (2008) 1 Hyperfine field transferred from Co to «charge disproportionation», «charge ordering», «electronic texture», electron localization But itinerant character remains! 23 spectrum if Co states were at random positions (simulation) NMR summary 59 Co NMR 23 NMR + ordering is observed e.g. vacancy clustering for x ~ 0.8 Co 3+ Co 3.3+ Co 3.4+ (possibly) One example of possible ordered pattern M. Roger et al., ture 445, 631 (2007) 1-2 stripe correlations for x < 0.7 (G. Collin) Details are controversial (accuracy of x values?)

6 Electrostatic potential from + ions Modulates charge density in cobalt planes Relevance to high Tc cuprates Dopant induced electronic inhomogeneity 1 should dislike Co 4+ more than Co 3+ (Coulomb repulsion) Co M. Roger et al., ture 2007 «Texture» in cobalt planes depends on 3D stacking of + ordering pattern High values of the thermoeletric power Magnetic properties Magnetic field dependent Strongest at the highest x() Curie-Weiss bulk magnetization Magnetic transitions Ex: T M = 22 K for x = 0.75 None for x = 0.67 and x = 0.71 A-type antiferromagnet Bayrakci PRL (2005), Helme PRL (2005) FM planes no frustration too bad Lee et al, ture Materials (2006) Wang et al., ture (2003) Magnetic properties / electron correlations are involved

7 Signature of electron correlations ARPES HB Yang, PRL (2005) U ~ 2 4 ev As well as in transport measurements (T-linear resistivity, m*/m ~ 3-10) 0.5 CoO 2 Fermi surface instability at x=0.5 Very small charge differentiation Huang, PRB 2004 G. Lang et al., PRB 2008 J. Bobroff et al., PRL 2006 Transition Magnétique 88 K Transition métal-isolant 53 K Co3+/Co4+ mixture order FS reconstruction Nesting Transition

8 Weak electron correlation at x = 0.3 Water-induced superconductivity in 0.3 CoO 2 yh2 O Schaak et al., ture (2003) Pauli-like χ spin H 2 O CoO 2 ρ T 2 below ~ 30 K Takada et al., ture (2003) Fermi liquid? T c max = 4.8 K Foo et al., PRL (2004) Unconventional superconductivity? Type II Short coherence length (~100 Angstrom) Singlet pairing Pairing symmetry? AFM spin fluctuations and proximity to magnetic ordering Ihara et al., JPSJ (2006)

9 The x = 0 limit CoO 2 is not a Mott insulator But a correlated metal LiCoO 2 CoO 2 20 T* 7 K Curie-Weiss Full electrochemical extraction of Li + Amatucci, Klein & Tarascon, J. Electrochem. (1996) 59 Co NMR : de Vaulx et al., PRL 2007 (T 1 T) -1 [s -1 K -1 ] Korringa Fermi liquid AF (q 0) spin fluctuations T [K] (Metal-insulator) Mott transition occurs for U/t on the triangular lattice x CoO 2 summary Carrier concentration can be varied over a wide range (1 elect./cell) Electron correlations throughout the phase diagram Unexpectedly, correlations are strongest at high concentrations Some correlations AFM Correlations ++ Impact of ions Thermopower Magnetism FM Correlations can be enhanced by + potential G. Lang et al. PRB (2008) Limited analogy with high Tc s