Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

Laverock, J.; Dugdale, Stephen B; Duffy, J. A.; Wooldridge, Jenny; Balakrishnan, G.; Lees, Martin R.; Zheng, Guojun; Chen, D.; Lin, C. T.; Andrejczuk, A.; Itou, M.; Sakurai, Y.

doi:10.1103/physrevb.76.052509

Cited by 32 publications

(24 citation statements)

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“…Moreover, insight in the (x, q)-dependent spin excitations at finite frequency is provided. Since we are mainly interested in the x>0.5 part of the phase diagram, the low-energy band dispersion of sodium cobaltate is described within an a 1g -like singleband approach, justified from photoemission [25] and Compton scattering [26] experiments. We primarily focus on the in-plane processes on the effective triangular Co lattice with tight-binding parameters up to 3rd nearest-neighbor (NN) hopping, i.e., (t, t , t )=(-202, 35, 29)meV [27] for the 2D dispersion.…”

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confidence: 99%

Competing orders in NaxCoO2from strong correlations on a two-particle level

Boehnke

Lechermann

2012

Phys. Rev. B

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Based on dynamical mean-field theory with a continuous-time quantum Monte-Carlo impurity solver, static as well as dynamic spin and charge susceptibilites for the phase diagram of the sodium cobaltate system NaxCoO2 are discussed. The approach includes important vertex contributions to the q-dependent two-particle response functions by means of a local approximation to the irreducible vertex function in the particle-hole channel. A single-band Hubbard model suffices to reveal several charge-and spin-instability tendencies in accordance with experiment, including the stabilization of an effective kagomé sublattice close to x=0.67, without invoking the doping-dependent Na-potential landscape. The in-plane antiferromagnetic-to-ferromagnetic crossover is additionally verified by means of the computed Korringa ratio. Moreover an intricate high-energy mode in the transverse spin susceptiblity is revealed, pointing towards a strong energy dependence of the effective intersite exchange.PACS numbers: 71.27.+a, 71.30.+h, 71.10.Fd, 75.30.Cr The investigation of finite-temperature phase diagrams of realistic strongly correlated systems is a quite formidable task due to the often tight competition between various low-energy ordering instabilities. In this respect the quasi-twodimensional (2D) sodium cobaltate system Na x CoO 2 serves as a notably challenging case [1,2]. Here x∈[0,1] nominally mediates between the Co 4+ (3d 5 , S=1/2) and Co 3+ (3d 6 , S=0) low-spin states. Thus the Na ions provide the electron doping for the nearly filled t 2g states of the triangular CoO 2 layers up to the band-insulating limit x=1. Coulomb correlations with a Hubbard U up to 5 eV for the t 2g manifold of bandwidth W ∼1.5 eV [3] are revealed from photoemission [4]. Hence with U /W 1 the frustrated metallic system is definitely placed in the strongly correlated regime.Various different electronic phases and regions for temperature T vs. doping x are displayed in the experimental sodium cobaltate phase diagram (see Fig. 1). For instance a superconducting dome (T c ∼4.5K) stabilized by intercalation with water close to x=0. Several theoretical works have dealt with the influence of the sodium arrangements on the electronic properties of Na x CoO 2 , both from the viewpoint of disordered sodium ions [12] as well as from orderings for certain dopings. [14][15][16] However, wether such sodium patterns are due to sole (effective) single-particle potentials or mainly originating from many-body effects within the CoO 2 planes is still a matter of debate [17,18].In this letter, we report the fact that a large part of the electronic (spin and charge) phase diagram of sodium cobaltate may be well described within a Hubbard model using realistic dispersions, and without invoking the details of the sodium arrangement. Thereby most of the observed crossovers and instabilities are truly driven by strong correlation effects and cannot be described within weak-coupling scenarios. The theoretical study is elucidating the two-particle correlations in the pa...

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confidence: 99%

Competing orders in NaxCoO2from strong correlations on a two-particle level

Boehnke

Lechermann

2012

Phys. Rev. B

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“…In turn, transport measurements, employing the Shubnikov-de Haas effect, corroborated the presence of the FS pockets in Na x CoO 2 at x = 0.71 and 0.84 [31]. Another evidences in favor of an existence of these features are provided by bulk measurements with the x-ray Compton scattering technique, performed in the unhydrated and hydrated sodium cobaltates [32]. On the theoretical ground, a more accurate treatment of the 3d correlations beyond the mean-field GGA+U approximation could verify a robustness of the FS pockets within the present approach.…”

Section: Discussionmentioning

confidence: 60%

Ab-initioinvestigation of spin states of sodium cobaltate Na_2/3CoO₂

Lysogorskiy¹,

Nedopekin²,

Krivenko³

et al. 2012

J. Phys.: Conf. Ser.

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Na ordering imprints a metallic kagomé lattice onto the Co planes of Na2/3CoO2 H. Alloul, I. R. Mukhamedshin, T. A. Platova et al. Abstract. Resent experiments in the lamellar system NaxCoO2 detected a transition of Co planes into a puzzling metallic state at x ≥ 2/3, which co-exists with a robust arrangement of the 3d cobalt electrons: The triangular Co lattices are disproportionated in the spinless Co 3+ sites (Co1), and Co 3.44+ sites (Co2) with enhanced magnetism forming conducting sublattices. This textures concur with a tightening of the ferromagnetic (FM) interaction in planes, and emerge when the sodium ions become arranged in layers in between the CoO2 slabs. In the present research we have investigated ab-initio the appearance of such state in Na 2/3 CoO2. Towards this end in view we studied an interplay between the electronic coupling to the superstructure of the Na + ions and local correlations of the itinerant d electrons treated within the GGA+U approximation. Employing the exact crystallographic supercell, the electronic organization has been analyzed upon increasing the energy U of the Coulomb repulsion within the 3d shells at T = 0. The metallic ground state, being a spin density wave with the inplane FM and antiferromagnetic interplane correlations, has been obtained and established to posses two regimes. When U > 2 eV, a crossover develops from a uniform state of the d-lattice to the regular phase with the spin/charge disproportionation between the sites. In particular at the representative value U = 5 eV, the Co1 3+ sites with suppressed magnetism appears, while the spin-active Co 4+ holes are accumulated by the Co2 sites. A related formation of an isolated, narrow conduction band at the Fermi level implies a considerable enhancement of the electron correlations in the crystal field imposed by the Na + patterns.

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“…A second explanation would involve the e ′ g character hole pockets that were intensely sought for in numerous ARPES studies. In a rigid band picture these should rather disappear with increasing Fermi level which has also been predicted by numerous theoretical studies 14,37 and recently been claimed to be observed experimentally 38 . Even if the hole pockets were stabilized also for higher x as predicted by some models 13,39 , the DOS behaviour observed here could hardly be explained.…”

Section: Comparing Cp Arpes and Band Structure Calculationsmentioning

confidence: 86%

Spin fluctuations, magnetic long-range order, and Fermi surface gapping inNaxCoO2

et al. 2008

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In this study an extended low energy phase diagram for NaxCoO2 is experimentally established with emphasis on the high x range. It is based on systematic heat capacity studies on both polycrystalline and single crystalline samples and on µSR measurements. Main features are the existence of mass enhancement, spin fluctuations without long-range order, and magnetic order with associated Fermi surface gapping. The latter is seen in the electronic density of states (DOS) and suppression of nuclear specific heat. While there is agreement between the band structure and the low energy DOS in the low x range, in the high x range (x ≥ 0.6) the thermodynamically determined DOS is approximately three times that deduced from the angle-resolved photoemission spectroscopy (ARPES)-measured band dispersion or local-density approximation (LDA) calculations.

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Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

Cited by 32 publications

References 45 publications

Competing orders in NaxCoO2from strong correlations on a two-particle level

Competing orders in NaxCoO2from strong correlations on a two-particle level

Ab-initioinvestigation of spin states of sodium cobaltate Na_2/3CoO₂

Spin fluctuations, magnetic long-range order, and Fermi surface gapping inNaxCoO2

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Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

Cited by 32 publications

References 45 publications

Competing orders in NaxCoO2from strong correlations on a two-particle level

Competing orders in NaxCoO2from strong correlations on a two-particle level

Ab-initioinvestigation of spin states of sodium cobaltate Na2/3CoO2

Spin fluctuations, magnetic long-range order, and Fermi surface gapping inNaxCoO2

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Ab-initioinvestigation of spin states of sodium cobaltate Na_2/3CoO₂