Evidence of a single nonmagnetic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mi>Co</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>state in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>Na</mml:mi><mml:mn>1</mml:mn></mml:msub><mml:msub><mml:mi>CoO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>cobaltate

Lang, Guillaume; Bobroff, J.; Alloul, H.; Mendels, P.; Blanchard, N.; Collin, G.

doi:10.1103/physrevb.72.094404

Cited by 73 publications

(69 citation statements)

References 20 publications

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“…In the x=1 limit, the material is a band insulator with only nonmagnetic S=0 Co 3+ ions. 5 As sodium is removed, a Curie-Weiss susceptibility behavior appears 6,7 down to supposedly x=0.67, together with long-range ordered magnetic states (T N =19-27 K) 8,9,10,11,12 for x=0.75-0.9. In the x≈0.7 region, intralayer correlations of spin fluctuations have been shown to be those of a two-dimensionnal quasi-ferromagnetic metal.…”

Section: Introductionmentioning

confidence: 99%

Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

et al. 2008

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Using 23 Na NMR measurements on sodium cobaltates at intermediate dopings (0.44≤x≤0.62), we establish the qualitative change of behavior of the local magnetic susceptibility at x * =0.63-0.65, from a low x Pauli-like regime to the high x Curie-Weiss regime. For 0.5≤x≤0.62, the presence of a maximum T * in the temperature dependence of the susceptibility shows the existence of an x-dependent energy scale. T1 relaxation measurements establish the predominantly antiferromagnetic character of spin correlations for xx * . It is suggested that at a given x the ferromagnetic correlations might dominate the antiferromagnetic ones above T * . From 59 Co NMR data, it is shown that moving towards higher x away from x=0.5 results in the progressive appearance of nonmagnetic Co 3+ sites, breaking the homogeneity of Co states encountered for x≤0.5. The main features of the NMR-detected 59 Co quadrupolar effects, together with indications from the powder x-ray diffraction data, lead us to sketch a possible structural origin for the Co 3+ sites. In light of this ensemble of new experimental observations, a new phase diagram is proposed, taking into account the systematic presence of correlations and their x-dependence.

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Section: Introductionmentioning

confidence: 99%

Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

et al. 2008

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“…The Co 3+ systems where the cobalt ions have full t 6 2g shell are naturally referred to as band insulators, 18), 19) whereas in Co 4+ S = 1/2 rich systems strongly correlated Mott behavior is expected. Thus by reducing sodium content in Na x CoO 2 one should be able to observe an evolution from the weakly correlated band-insulator regime to a doped Mott limit.…”

Section: §1 Introductionmentioning

confidence: 99%

Unusual Electron Correlations in Na_xCoO₂Due to the Spin-State Quasidegeneracy of Cobalt Ions

Chaloupka¹,

Khaliullin²

2008

Prog. Theor. Phys. Suppl.

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Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S = 1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides -the spin-state quasidegeneracy of CoO6 octahedral complex -which has been known, e.g., in the context of an unusual physics of LaCoO3 compound. Another important ingredient of the model is the 90• Co-O-Co bonding in NaxCoO2 which allows nearest-neighbor t2g − eg hopping. This hopping introduces a dynamical mixture of electronic configurations t 6 2g , S = 0 and t 5 2g e 1 g , S = 1 of neighboring cobalt ions. We show that scattering of charge carriers on spin-state fluctuations suppresses their coherent motion and leads to the spin-polaron physics at x ∼ 1. At larger doping when coherent fermionic bands are formed, the model predicts singlet superconductivity of extended s-wave symmetry. The presence of low-lying spin states of Co 3+ is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed. §1. IntroductionThe physics of transition metal oxides offers a wealth of interesting phenomena related to their strongly correlated nature. This is because the bandwidth is relatively small compared to the intraionic Coulomb repulsion between the 3d electrons. The understanding of many unique properties of oxides, such as high-T c superconductivity and colossal magnetoresistivity is therefore based on the well-known Mott physics. 1)Among the various families of transition metal oxides, attention has recently focused on layered cobaltates, in particular on the sodium cobaltate Na x CoO 2 . The interest was initiated by the large thermoelectric power observed in this compound, 2)-4) i.e. the capability of an efficient conversion of heat energy to electricity. The research on these systems was further boosted by the unexpected discovery of superconductivity (SC) in water-intercalated Na x CoO 2 . 5) Soon after, many remarkable properties were found 6) such as spin-sensitive thermopower, 3) unusual charge and spin orderings, 7)-11) very narrow quasiparticle bands 12)-16) and especially a very unusual phase diagram. 7) While the strongly correlated nature of Na x CoO 2 is no longer at doubt, the mechanisms by which the correlated electrons design such an exotic phase diagram are not fully understood even on a qualitative level.Layered cobaltates consist of CoO 2 planes with a triangular lattice of Co ions

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“…Describing the Co1 and Co2 valences of Na 0.5 CoO 2 as 3.5+δ and 3.5-δ, respectively, we represent the magnitude of δ by the difference of ν Q values, ∆ν Q (= ν Q Co1 -ν Q Co2 ), between these two Co sites: Adopting a linear relationship between δ and ν Q , ν Q = 3.43 + 5.51δ (ν Q = 3.43 -5.51δ) for Co1 (Co2), we roughly estimate the δ value to be ~ 0.1 for Na 0.5 CoO 2 . (Note that ν Q of NaCoO 2 , in which all Co ions have the valence of +3, is ~ 0.67 MHz, 18) and that the ν Q values of Co1 and Co2 of Na 0.5 CoO 2 are 4.1 MHz and 2.8 MHz, respectively. )…”

Section: Discussionmentioning

confidence: 99%

NMR Studies of Successive Phase Transitions in Na_0.5CoO₂and K_0.5CoO₂

et al. 2008

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(Received ) 59 Co-and 23 Na-NMR measurements have been carried out on polycrystalline and c-axis aligned samples of Na 0.5 CoO 2 , which exhibits successive transitions at temperatures T = 87 K (= T c1 ) and T = 53 K (= T c2 ). 59 Co-NMR has also been carried out on c-axis aligned crystallites of K 0.5 CoO 2 with similar successive transitions at T c1 ~ 60 K and T c2 ~ 20 K. For Na 0.5 CoO 2 , two sets of three NMR lines of 23 Na nuclei explained by considering the quadrupolar frequencies ν Q ∼ 1.32 and 1.40 MHz have been observed above T c1 , as is expected from the crystalline structure. Rather complicated but characteristic variation of the 23 Na-NMR spectra has been observed with varying T through the transition temperatures, and the internal fields at two crystallographically distinct Na sites are discussed on the basis of the magnetic structures reported previously. The internal fields at two distinct Co sites observed below T c1 and the 59 1/T 1 -T curves of Na 0.5 CoO 2 and K 0.5 CoO 2 are also discussed in a comparative way.

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Evidence of a single nonmagneticCo3+state in theNa1CoO2cobaltate

Cited by 73 publications

References 20 publications

Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

Unusual Electron Correlations in Na_xCoO₂Due to the Spin-State Quasidegeneracy of Cobalt Ions

NMR Studies of Successive Phase Transitions in Na_0.5CoO₂and K_0.5CoO₂

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Evidence of a single nonmagneticCo3+state in theNa1CoO2cobaltate

Cited by 73 publications

References 20 publications

Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

Unusual Electron Correlations in NaxCoO2Due to the Spin-State Quasidegeneracy of Cobalt Ions

NMR Studies of Successive Phase Transitions in Na0.5CoO2and K0.5CoO2

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Product

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Unusual Electron Correlations in Na_xCoO₂Due to the Spin-State Quasidegeneracy of Cobalt Ions

NMR Studies of Successive Phase Transitions in Na_0.5CoO₂and K_0.5CoO₂