Chemical model for superconductivity

Larsson, Sven

doi:10.1002/qua.10423

Cited by 8 publications

(5 citation statements)

References 112 publications

(142 reference statements)

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“…Lattice distortion versus coupling is. As has been shown in previous articles 19, 25, 26, 32–34, the coupling may very well “cancel out” the trapping forces of lattice distortion, thereby implying a free bipolaron. Contrary to the conclusions of Chakraverty et al 53, delocalization of the bipolaron is possible under special circumstances.…”

Section: Discussionsupporting

confidence: 52%

“…We will use as a fundamental condition for itinerancy that the pair moves without activation energy. This problem is part of the well understood MV models 20–26. The same theory that applies to cuprates applies to a number of other systems as well 26, 32–34.…”

Section: Introduction: Historical Notesmentioning

confidence: 97%

“…After disproportionation two electrons may be transferred between equivalent sites 19. To study this problem in more detail we will resort to the well‐established Mixed‐Valence (MV) theory 20–26.…”

Section: Introduction: Historical Notesmentioning

confidence: 99%

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Superconductivity in cuprates: Details of electron phonon coupling

Larsson

2009

Int J of Quantum Chemistry

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ABSTRACT:The Bardeen-Cooper-Schrieffer (BCS) model explains superconductivity (SC) as due to correlation between electronic momentum and nuclear momentum (phonons) in a free electron gas. The BCS model lacks chemical specificity, however, since the coupling mechanism is left unspecified. After the discovery of high T C superconductivity in 1986 it was concluded that electron-phonon interactions are insufficient to explain electron pairing. A large part of theoretical research has since been aiming at finding another mechanism that would allow us to consider the superconducting system as a gas of charged free bosons. However, there appears to be no reason to assume free electrons in oxides. In this article the free-electron criterion is therefore replaced by the criterion that a pair of electrons can move freely between sites without resistance, i.e., without activation energy. Electron pair transfer is treated in a many-electron real space approach using standard mixed-valence theories. MottHubbard-U is strictly defined, its dependence on breathing mode coordinates analyzed, and the connection between U and the energy gap for superconductivity clarified. dwave gap anisotropy is found to be consistent with the general atomic level model presented here. Softening of phonon half-breathing modes in inelastic neutron scattering (INS) is connected to mixed-valency. The fundamental vibronic interaction between spin density wave (SDW) and charge density wave (CDW) states leads to a new phase with energy gap and electron pair carriers that can only be the superconducting phase.

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Section: Discussionsupporting

confidence: 52%

Section: Introduction: Historical Notesmentioning

confidence: 97%

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Superconductivity in cuprates: Details of electron phonon coupling

Larsson

2009

Int J of Quantum Chemistry

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“…The anisotropic carrier-phonon coupling, reaching its maximum along the CuOO bond, and the distortion of the CuOO planes, suggest a direct role for structural dynamics and considerations (41)(42)(43) beyond simple 2D models. Recent theoretical work has incorporated lattice phonons in the t-J model to account for the observed optical conductivity (44), whereas new band structure calculations suggested that large and directional electron-phonon coupling can favor spin ordering (45).…”

Section: Resultsmentioning

confidence: 99%

Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

Carbone

Yang

Giannini

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

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The mechanism of electron pairing in high-temperature superconductors is still the subject of intense debate. Here, we provide direct evidence of the role of structural dynamics, with selective atomic motions (buckling of copper-oxygen planes), in the anisotropic electron-lattice coupling. The transient structures were determined using time-resolved electron diffraction, following carrier excitation with polarized femtosecond heating pulses, and examined for different dopings and temperatures. The deformation amplitude reaches 0.5% of the c axis value of 30 Å when the light polarization is in the direction of the copper-oxygen bond, but its decay slows down at 45°. These findings suggest a selective dynamical lattice involvement with the anisotropic electronphonon coupling being on a time scale (1-3.5 ps depending on direction) of the same order of magnitude as that of the spin exchange of electron pairing in the high-temperature superconducting phase.electron diffraction ͉ electron-phonon coupling ͉ superconductivity ͉ ultrafast T he pairing of electrons is now accepted as being essential in the formation of the superconducting condensate in hightemperature superconductors. What is debatable is the nature of forces (''glue'') holding the pairs (1). The mechanism is different from that of conventional superconductors; for them, loss of the electric resistance is due to phonon-mediated electron pairing [Bardeen-Cooper-Schrieffer (BCS)] (2). Ceramic cuprates become superconductors when extra holes or electrons are doped into their magnetically ordered charge-transfer insulator (ground) state (3, 4); the highest transition temperature (T c ) occurs at a doping of 0.15 extra hole per copper ion and it increases with the number (n) of CuOO planes per unit cell, reaching a maximum at n ϭ 3 (5). Because of the d-wave symmetry of the superconducting gap (6), the relatively small isotope effect (7,8), and the magnitude of electron repulsion (U) and exchange (J) (appropriate for the antiferromagnetic phase), magnetic interactions have been considered as the source of binding (1, 9). The role of phonons in pairs formation has also been discussed, from both experimental and theoretical perspectives (10, 11).Angle-resolved photoemission spectroscopy (ARPES) experiments revealed the presence of kinks in the band dispersion at energies corresponding to specific (optical) phonon modes (7,(11)(12)(13). In some samples, inelastic neutron scattering data at similar energies supported a magnetic resonance mode below the transition temperature (14). The issue was raised over whether the low-energy features observed in the ARPES spectra are induced by magnetic or structural bosonic coupling. Based on energetics, the out-of-plane motion of the oxygen ions in the CuOO plane, referred to as the out-of-plane buckling mode, has been assigned as responsible for the kink in the band dispersion observed along the direction of CuOO bonds (11,12). However, the electron-phonon coupling strength obtained by means of angle-integrated probes is no...

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“…Cooper's discussion [10] of the motion of electron pairs in interaction with phonons led to the development of the BCS theory, which has introduced great clarification in the field of superconductivity [9]. Electron pairs of the BCS theory (Cooper pairs) are appealing to chemists because there is no reason to believe electron pairs in superconductors should differ greatly from electron pairs in other systems, particularly the mixed valence systems where difference between oxidation states must be two units-given that at least two electrons are involved in each elementary charge transfer step [10,11]. In RVB theory, the electronic structure of the superconductor is described in successive steps that involve integrals of decreasing magnitude.…”

mentioning

confidence: 99%

Resonating valence‐bond mechanism for the superconductivity in K₃C₆₀

Bastos

Costa

Pavão

2010

Int J of Quantum Chemistry

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Density functional theory calculations in different cluster models, the largest one K 38 (C 60 ) 23 with 1,418 atoms, combined with the resonating valence-bond theory show that superconductivity in K 3 C 60 involves interaction of C 60 with K þ , K 0 , and K À in octahedral interstices as well as immobilization of negative charges on positive tetrahedral K atoms. We found that the orbitals involved in the highest occupied molecular orbital-lowest unoccupied molecular orbital gap are mainly from C 60 and octahedral potassium atoms. We suggest that a K 3 C 60 superconductor improvement can be achieved through potassium atom vacancy in octahedral sites.

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Chemical model for superconductivity

Cited by 8 publications

References 112 publications

Superconductivity in cuprates: Details of electron phonon coupling

Superconductivity in cuprates: Details of electron phonon coupling

Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

Resonating valence‐bond mechanism for the superconductivity in K₃C₆₀

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Chemical model for superconductivity

Cited by 8 publications

References 112 publications

Superconductivity in cuprates: Details of electron phonon coupling

Superconductivity in cuprates: Details of electron phonon coupling

Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

Resonating valence‐bond mechanism for the superconductivity in K3C60

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Resonating valence‐bond mechanism for the superconductivity in K₃C₆₀