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Gunnarsson, O.; Erwin, Steven C.; Koch, Erik; Martin, Richard M.

doi:10.1103/physrevb.57.2159

Cited by 30 publications

(40 citation statements)

References 20 publications

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“…However, later direct optical measurements [13] have shown that the actual band gap is close to 0.5 eV, about twice the value of the highest energy level found here. Early band structure calculations by both LDA-DFT [14] and tight-binding [15] methods indicated that K 4 C 60 should be metallic, but at present there is a consensus that a gap opens at the Fermi surface due to a Jahn-Teller distortion of the C 60 molecules [2,13]. Because no ionic conductivity is expected from the large, heavy K + ions and because the band gap is quite large, both energy levels observed in this work probably correspond to localized states in the band gap.…”

Section: Results For K 4 C 60mentioning

confidence: 99%

High pressure studies of alkali metal doped fullerides A4C60

Sundqvist

Wågberg

Yao

2011

Diamond and Related Materials

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Abstract:The electrical resistance R of K 4 C 60 and Li 4 C 60 has been measured between 100 and 300 K under pressures up to 1 and 2 GPa, respectively. For both materials, the temperature coefficient of R is negative at all pressures and temperatures in this range. The transport properties of Li 4 C 60 at low pressure are compatible with a recent observation of high ionic conductivity, but surprisingly this conductivity term increases strongly with increasing pressure, contradicting an intuitive model. For K 4 C 60 we see no sign of a recently suggested structural phase transformation at low temperature, nor can we find any convincing evidence for a lattice disproportionation as recently observed for Rb 4 C 60 .

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Section: Results For K 4 C 60mentioning

confidence: 99%

High pressure studies of alkali metal doped fullerides A4C60

Sundqvist

Wågberg

Yao

2011

Diamond and Related Materials

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“…Even though the non-negligible temperature dependence of the relative intensities prevented us from a detailed analysis, we may still attribute the more shifted peak to the disordered lithium ions in the 32f sites, as a higher electronic density is expected at these asymmetric positions, due to a theoretically predicted enhancement of the fullerene-alkali metal electronic hopping [18].…”

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confidence: 99%

Recovering metallicity inA4C60: The case of monomericLi4C6

et al. 2007

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The restoration of metallicity in the high-temperature, cubic phase of Li4C60 represents a remarkable feature for a member of the A4C60 family (A = alkali metal), invariably found to be insulators. Structural and resonance technique investigations on Li4C60 at T ¿ 600 K, show that its fcc structure is associated with a complete (4e − ) charge transfer to C60 and a sparsely populated Fermi level. These findings not only emphasize the crucial role played by lattice symmetry in fulleride transport properties, but also re-dimension the role of Jahn-Teller effects in band structure determination.PACS numbers: 61.48.+c, 76.30.Pk, 76.60.Cq, 78.30.Na In recent years the competition between Coulomb repulsion, kinetic energy, Jahn-Teller effect, and Hund's coupling rules has been subject of intensive research in the domain of alkali-doped fullerenes. Apparently, most of the phenomena, including high-T c superconductivity in A 3 C 60 , seem to be relatively well understood. Nevertheless, the reason why A 4 C 60 are in general nonmagnetic insulators [1, 2] is still puzzling and remains subject of strong controversy. The situation is made even more complex as, differently from fcc A 3 C 60 , the A 4 C 60 arising from large-size alkali metals (A = K, Rb, Cs) adopt a bct structure [3]. Hence, the natural suggestion that the difference in lattice structure favors the metallicity in the former and an insulating behavior in the latter, with the coupling to H g Jahn-Teller phonons accounting for the lack of magnetic ordering in A 4 C 60 [4]. Doping the C 60 with the significantly smaller Li + and Na + alkali ions leads to interesting deviations from the standard behavior. Indeed, Li x C 60 can easily form compounds with x > 6 [5], whereas in Na 4 C 60 the fullerene molecules form a 2D planar polymer, with the molecular units linked by four single C-C bonds [6]. As commonly observed in singly bonded C 60 polymers [7], the latter system too was reported to be metallic [6,8]. Li 4 C 60 , though, represents a different situation. At normal conditions, it is a 2D polymer displaying a novel architecture based on the coexistence of single and double bonds oriented along two perpendicular directions within the polymer plane [9]. Although the presence of single bonds could suggest a metallic behavior, NMR, Raman and ESR spectroscopy show beyond doubt the diamagnetic and insulating character of this polymer [10]. Important questions hence arise both regarding the role of structure, as well as of polymerization in depressing the electron delocalization and destroying the expected metallicity.In this article we try to answer these questions by a thorough investigation of the high temperature monomeric Li 4 C 60 phase, obtained by thermally induced depolymerization which takes place above 590K. First we show that, in spite of its stoichiometry, the Li 4 C 60 monomeric phase retains a closed packed (fcc) cubic structure, and therefore can be directly compared to other, well known A 3 C 60 fcc systems. Subsequently, as revealed from the ESR l...

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“…12 We have shown, however, that at half-filling the ratio of U/W where the transition takes place grows with the orbital degeneracy N of the t 1u orbital as √ N or somewhat faster. 10,13 In the large U limit it was shown that an extra electron in a system with otherwise integer occupancy, e.g., half-filled, can move more efficiently if the orbital degeneracy N is large. The reason is that the extra occupancy can move through a hop of any of the electrons on the site with the extra occupancy.…”

Section: Introductionmentioning

confidence: 99%

“…The reason is that the extra occupancy can move through a hop of any of the electrons on the site with the extra occupancy. 10,13 In the half-filled case, of the order N electrons can hop to a neighboring site. For a different integer filling than half-filling, this effect is reduced.…”

Section: Introductionmentioning

confidence: 99%

Hubbard model with orbital degeneracy and integer or noninteger filling

Gunnarsson

1997

Zeitschrift für Physik B Condensed Matter

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The photoemission spectrum and the specific heat are studied in the Hubbard model of small clusters including orbital degeneracy. We focus on the degeneracy and valence dependence in the limit of a large Coulomb interaction. For integer valence, it is found that the degeneracy increases the width of the photoemission spectrum and it reduces the contribution from the charge degrees of freedom to the specific heat. A deviation from integer valence reduces the width of the photoemission spectrum. 71.10.Fd,71.20.+h

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Role of alkali atoms inA4C60

Cited by 30 publications

References 20 publications

High pressure studies of alkali metal doped fullerides A4C60

High pressure studies of alkali metal doped fullerides A4C60

Recovering metallicity inA4C60: The case of monomericLi4C6

Hubbard model with orbital degeneracy and integer or noninteger filling

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