Disproportionation and Spin Ordering Tendencies in NaxCoO2 AT x = 1/3

Kuneš, J.; Lee, K.-W.; Pickett, Warren E.

doi:10.1007/1-4020-3085-1_34

Cited by 4 publications

(9 citation statements)

References 20 publications

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“…(The intra-atomic exchange integral J=1 eV was left unchanged.) As found in previous studies [9,11,14], this cobaltate system seems to lie in an intermediate regime where the two forms [35,36] of LDA+U functional give similar results; the difference in the double-counting corrections is small when the spin polarization is small. The Brillouin zone was sampled with 162 (545 for the fixed spin moment calculations) k-points in the irreducible wedge and the basis sets were 3s3p4s4p3d for Co and 2s2p3s3p3d for O.…”

Section: Crystal Structure and Methods Of Calculationsupporting

confidence: 87%

“…This first order transition was obtained also at x =1/3, 1/2 and 2/3. [9,11,14] For z 0 ≥ 0.21, the amount of jump in the Co magnetic moment is decreased with increasing the O height. As a result, z 0 =0.235 case shows a slightly smoothed transition, but the slope at U c is still very steep.…”

Section: Crystal Structure and Methods Of Calculationmentioning

confidence: 96%

“…The t 2g -e g crystal-field splitting is 2.5 eV, the same amount as x=1/3, 1/2, and 2/3. [11,14,31] The t 2g manifold is in the range of -1 eV to 0.5 eV relative to the Fermi energy E F , but is hybridized with O 2p bands which are mainly below -1 eV, as can be seen by the Co 3d character in the O 2p band region, and vice versa.…”

Section: Crystal Structure and Methods Of Calculationmentioning

confidence: 98%

“…[9,11,14,22,23,24,31,32] The variation with x is provided for comparison in the enlarged band structures around the t 2g manifold at x =0, 1/3, and 2/3 given in Fig. 3.…”

Section: Crystal Structure and Methods Of Calculationmentioning

confidence: 99%

“…Possibilities of such behavior for x ≈ 0.3 have been discussed [11,12] because strong on-site repulsion, geometrical effects, and commensurability all should conspire to encourage ordering at x=1/3. Nevertheless, there has been no experimental indication of any such tendencies, [13] with the most direct conclusion being that correlation effects seem really minor in this regime.…”

Section: Introductionmentioning

confidence: 99%

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NaxCoO2in thex→0regime: Coupling of structure and correlat

Lee

Pickett

2005

Phys. Rev. B

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The study of the strength of correlations in NaxCoO2 is extended to the x = 0 end of the phase diagram where Mott insulating behavior has been widely anticipated. Inclusion of correlation as modeled by the LDA+U approach leads to a Mott transition in the ag subband if U≥Uc=2.5 eV. Thus U smaller than Uc is required to model the metallic, nonmagnetic CoO2 compound reported by Tarascon and coworkers. The orbital-selective Mott transition of the ag state, which is essentially degenerate with the e ′ g states, occurs because of the slightly wider bandwidth of the ag bands. The metal-insulator transition is found to be strongly coupled to the Co-O bond length, due to associated changes in the t2g bandwidth, but the largest effects occur only at a reduced oxygen height that lies below the equilibrium position.

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Section: Crystal Structure and Methods Of Calculationsupporting

confidence: 87%

Section: Crystal Structure and Methods Of Calculationmentioning

confidence: 96%

Section: Crystal Structure and Methods Of Calculationmentioning

confidence: 98%

“…[9,11,14,22,23,24,31,32] The variation with x is provided for comparison in the enlarged band structures around the t 2g manifold at x =0, 1/3, and 2/3 given in Fig. 3.…”

Section: Crystal Structure and Methods Of Calculationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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NaxCoO2in thex→0regime: Coupling of structure and correlat

Lee

Pickett

2005

Phys. Rev. B

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Band structure renormalization and weak pseudogap behavior inNa0.33CoO2: Fluctuation exchange study based on a single-band model

Yao

Wang

2007

Phys. Rev. B

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Based on a single band Hubbard model and the fluctuation exchange approximation, the effective mass and the energy band renormalization in Na0.33CoO2 is elaborated. The renormalization is observed to exhibit certain kind of anisotropy, which agrees qualitatively with the angle-resolved photoemission spectroscopy (ARPES) measurements. Moreover, the spectral function and density of states (DOS) in the normal state are calculated, with a weak pseudogap behavior being seen, which is explained as a result of the strong Coulomb correlations. Our results suggest that the large Fermi surface (FS) associated with the a1g band plays likely a central role in the charge dynamics.PACS numbers: 71.18.+y, 71.27.+a, 74.25.Jb The layered oxide Na x CoO 2 has attracted much attention due to its possible connection to the high-T c superconductivity since the discovery of superconductivity in it.1,2 This material consists of two-dimensional CoO 2 layers, where Co's form a triangular lattice, making the Na x CoO 2 a possible realization of Anderson's resonating valence bond (RVB) state.3 By hydration, it becomes a superconductor with T c ≈ 5K in Na 0.337 CoO 2 ·1.3H 2 O 1 . The unhydrated compound Na x CoO 2 exhibits a rich phase diagram: a paramagnetic metal (x < 0.5), a charge-ordered insulator (x = 0.5), a "Curie-Weiss metal" (x ∼ 0.7), and a magnetic order state (x ≧ 0.75). 4Experimentally, it is indicated that Na x CoO 2 seems to be strongly electronic correlated.2,5,6 Angle-resolved photoemission spectroscopy (ARPES) measurements show a strong mass renormalization (The effective mass is about 5-10 times larger than the bare mass.) for x = 0.6.7 Decreasing the Na concentration to x = 0.3, the cobaltates appear to be less electron-correlated with a weaker but still apparent energy band renormalization factor (∼ 2). 8For the hydrated compounds, the effective mass is estimated to be ∼ 2 − 3.9 In addition, recent experiments reported that Na 0.33 CoO 2 ·1.3H 2 O display certain pseudogap behaviors such as the decreasing of the Knight shift 10 and the density of states (DOS) at the Fermi level below 300K.11 Optical spectroscopy measurements for Na 0.2 CoO 2 and Na 0.5 CoO 2 also suggest the incipient formation of pseudogap.12 However, unlike the pseudogap effect observed in the high-T c cuprates, the pseudogap behavior is rather weak. The renormalization of the energy band and the pseudogap formation are directly related to the electronic structure, such as the quasiparticle spectral function, the quasiparticle dispersion, and the Fermi surface (FS) topology. Therefore it is of importance and significance to study the normal state quasiparticle dynamics.It is believed that the topology of the FS plays an important role in the unconventional superconductivity. The local density approximation (LDA) calculations 13,14 predict a large FS associated with the a 1g band and six pockets associated with the e ′ g band. However, intriguingly, the small pockets have not been observed in the ARPES measurements. 8,9,15 The unexpected inconsi...

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Measurement of electron correlations inLixCoO2(x=0.0–
Kawasaki
¹
,
Motohashi
²
,
Shimada
³

et al. 2009
Phys. Rev. B
27
5
15
1
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CoO2 is the parent compound for the superconductor NaxCoO2·1.3H2O and was widely believed to be a Mott insulator. We performed 59 Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x = 0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical de-intercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x →0 for AxCoO2 (A = Li, Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two dimensional structure of AxCoO2.PACS numbers: 74.25.Jb, 74.25.Nf Electronic correlations and superconductivity in transition-metal oxides have been a main focus in condensed matter physics since the discovery of high transition-temperature (T c ) superconductivity in copper oxides. The hydrated cobalt-oxide superconductor Na x CoO 2 ·1.3H 2 O has intensified the research interest in the past few years [1]. This compound bears similarities to the high-T c copper oxides in that it has a quasi-two dimensional crystal structure and contains a transition-metal element that carries a spin of 1 2 . Indeed, nuclear quadrupole resonance (NQR) measurements on Na x CoO 2 ·1.3H 2 O have found T 3 variation below T c in the spin-lattice relaxation rate 1/T 1 , which is a strong indication of existence of line nodes in the superconducting gap function [2,3,4]. Precise measurements of the Knight shift in a high quality single crystal reveals that the spin susceptibility decreases below T c along both aand c-axis directions, which indicates that the Cooper pairs are in the spin-singlet state [5]. Thus, the superconductivity in Na x CoO 2 ·1.3H 2 O appears to be of d-wave symmetry, as in the case of high-T c copper oxides. It has also been found that antiferromagnetic spin correlations are present in the superconducting cobaltates, though being much weaker than those in the cuprates [2,3]. The correlations are anisotropic in the spin space [6], which is different from the cuprate case.Then, a natural question is how to model the cobalt oxides. Many authors applied the so-called t−J model that had been widely used to describe the cuprates [7,8,9]. In these theories, one virtually starts from CoO 2 in which Co is in the Co 4+ state and there is one electron (s = 1/2) in the lowest level (a 1g orbital). Upon adding Na, one dopes electrons into the a 1g orbital, and creates a Co 3+ (s = 0) state. In such a scenario, one may be in a situation of dealing with a doped Mott insulator, as in the cuprates case [7,8,10]. Therefore, it is important to synthesize the CoO 2 phase and reveal its electronic ground state. Unfortunately, it has been chemically diff...

show abstract

Disproportionation and Spin Ordering Tendencies in NaxCoO2 AT x = 1/3

Cited by 4 publications

References 20 publications

NaxCoO2in thex→0regime: Coupling of structure and correlat

NaxCoO2in thex→0regime: Coupling of structure and correlat

Band structure renormalization and weak pseudogap behavior inNa0.33CoO2: Fluctuation exchange study based on a single-band model

Measurement of electron correlations inLixCoO2(x=0.0–
Kawasaki
¹
,
Motohashi
²
,
Shimada
³

et al. 2009
Phys. Rev. B
27
5
15
1
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Disproportionation and Spin Ordering Tendencies in NaxCoO2 AT x = 1/3

Cited by 4 publications

References 20 publications

NaxCoO2in thex→0regime: Coupling of structure and correlat

NaxCoO2in thex→0regime: Coupling of structure and correlat

Band structure renormalization and weak pseudogap behavior inNa0.33CoO2: Fluctuation exchange study based on a single-band model

Measurement of electron correlations inLixCoO2(x=0.0–Kawasaki1, Motohashi2, Shimada3 et al. 2009Phys. Rev. B275151View full textAdd to dashboardCite

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Measurement of electron correlations inLixCoO2(x=0.0–
Kawasaki
¹
,
Motohashi
²
,
Shimada
³

et al. 2009
Phys. Rev. B
27
5
15
1
View full text Add to dashboard Cite