Ab initio MO Studies of the Hole Delocalization in Copper Oxides and Related Species: Necessity of the Extended Hubbard Model

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A previous theoretical model, the J-model, for superconductivity of triangular, Kagome, and related spin lattice systems, has been reinvestigated in relation to the recent discovery of superconductivity in Na x CoO 2 ⅐ yH 2 O. To elucidate a possible interrelationship between magnetism and superconductivity, effective exchange integrals ( J ab ) for magnetic clusters with triangular lattices are calculated by ab initio generalized Hartree-Fock (GHF) and generalized density functional theory (GDFT) methods through the use of general spin orbitals (GSO). From the computational results, several model Hamiltonians such as J-and t-J-models are derived for elucidation of the electronic properties of molecule-based materials with triangular, Kagome, and related spin lattice systems. The theoretical possibilities of spin-mediated superconductivity are examined on the basis of the J-model in the intermediate region for metal-insulator transitions. The interplay of spin exchange or spin fluctuation and other effects for superconductivity of strong correlation systems is also feasible both on theoretical and on experimental grounds, supporting the cooperative mechanism toward high-T c superconductivity. The implications of the calculated results are discussed in relation to theoretical possibilities of non-BardeenCooper-Schrieffer (BCS) superconductivity on the basis of cooperative mechanisms of electron correlation (or spin fluctuation) and multi-band effects in magnetic clusters with icosidodecahedral and related symmetries. Noncoplanarity of spins in molecule-based magnets is finally discussed to elucidate spin chirality, which is related to a persistent

Section: Introductionsupporting

confidence: 86%

J‐model for magnetism and superconductivity of triangular, kagome, and related spin lattice systems

Yamanaka

Yamaki

Takeda

et al. 2004

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“…The discovery of high‐ T c copper oxide suggested the idea that doping in an antiferro‐ or ferromagnetic system may provide several exotic electronic phases 1–5, which are: 1. ferromagnetic metal or insulator; 2. spin glass; 3. paramagnetic metal; 4. antiferromagnetic metal; 5. ferrimagnetic metal or insulator; and 6. charge‐ or spin‐mediated superconducttor. Recently, we have therefore started our theoretical attempts to search possible models for charge‐ or spin‐mediated superconductivity from these works and have presented the hypotheses about the possiblities of the superconductivity in the intermediated region of metal–insulator transition in the previous papers 6–11.…”

Section: Introductionmentioning

confidence: 99%

Theoretical studies on field‐induced superconductivity in molecular crystals

Nagao

Kitagawa

Kawakami

et al. 2001

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ABSTRACT:We investigate superconductivity of molecular crystals induced by charge injection by using a two-band model based on the Green function techniques. Expressions of transition temperature for several phases derived from the model are summarized. The model is applied to a model of CuO 2 plane for copper oxides, anthracene, oligothiophene, and C 60 crystals and phase diagrams of the crystals are presented. It is important that the band structure of crystals is described by a two-or multiband model for realization of high-T c superconductivity.

“…As shown previously 13, double helical DNA is formally regarded as double chains of so‐called copper oxides, namely ladder, which has been accepted great interest in relation to high‐ T C superconductivity 41–46. Similarly, triple 47 and quadruple 48–49 helical DNA chains may be considered to be biomolecular alternatives of triple and quadruple CuO chains 13.…”

Section: Introductionmentioning

confidence: 92%

“…Previously, we have examined electronic structures of high‐ T C copper oxides using ab initio MO methods 45, 83, showing the necessity of N ‐band models for systematic descriptions of magnetism, conductivity, and superconductivity of the species. Some organometallic and organic systems isoelectronic to copper oxides have been also proposed on the basis of the ab initio results 45, 46 as shown in Figure 1. N ‐band models often reduce to the two‐band model, which has been used for elucidation of the Kubo–Inagaki 84, 85 mechanism of ferromagnetism and other magnetic states, and Suhl–Kondo mechanism 86, 87 and related multi‐band models 13, 45, 46 for superconductivity.…”

Section: Hubbard Heisenberg and Rkky Models For Strong Correlationmentioning

confidence: 99%

Possibilities of molecule‐based spintoronics of DNA wires, sheets, and related materials

Kawakami

Taniguchi

Hamamoto

et al. 2005

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ABSTRACT:Our theoretical efforts toward molecule-based magnetic conductors and superconductors on the basis of ab initio Hamiltonians and effective model Hamiltonians are summarized in relation to recently developed DNA-based molecular materials. Guanine and adenine derivatives coupling with organic radicals (R) are investigated as possible -R components. To elucidate electronic and magnetic properties of these species, effective exchange integrals ( J ab ) for magnetic clusters are calculated by ab initio hybrid density functional (HDFT) methods. The ab initio J ab values are numerically reproduced by using model Hamiltonians such as the t-J, Kondo, Anderson, and RKKY models. The theoretical possibilities of organic magnetic conductors are elucidated on the basis of these models for self-assembled DNA wires, sheets, and related materials. The use of these materials for nanoscale molecular electronic devices is also elucidated theoretically in relation to an important role of electron-electron repulsion effect for quantum electron transport, together with the electron-exchange interaction in the Kondo effect. The implications of the calculated results are finally discussed to obtain a unified picture of many p-d, -d, and -R molecule-based systems with strong electron correlations.