I. Molecular simulations of buckyball fullerenes. II. Quantum chemistry studies on high-T_c superconductors.

Citation

Guo, Yuejin (1992) I. Molecular simulations of buckyball fullerenes. II. Quantum chemistry studies on high-T_c superconductors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jvvc-ry41. https://resolver.caltech.edu/CaltechTHESIS:10282009-111805575

Abstract

Part I. In order to interpret and predict the unusual chemical and physical properties of the C_(60) and related fullerenes, fullerites, and molecular/solid state derivatives, we started with the graphite force field (GraFF) developed for sp^2 carbon centers (based on fitting experimental lattice parameters, elastic constants, phonon frequencies for graphite and alkali-intercalated graphite), and successfully predicted vibrational frequencies, fullerite and alkali-doped fullerite crystal structure, density, heat of sublimation, and compressibility, etc., for C_(60), C_(70) and their derivatives. We also developed a highly accurate force field for C_(60) in excellent agreement with all 14 experimental frequencies within abs error 3.0 cm^(-1). Part II. We have applied ab initio electronic methods (GVB and CI) to various clusters representing La_2CUO_4, Nd_2CuO_4, YBa_2Cu_3O_7, and Bi and Tl containing high-Tc materials to study their basic electronic structure and magnetic interaction. Particularly, we develop the GVB superexchange CI (GVB - X - CI) method to study the superexchange coupling interaction. Using this method, we can calculate the J_(dd) from the first principle at about the same accuracy as experiment. Our results indicate that the superconductivity in Cu-0 plane of these cuprates arise from a essentially magnetically induced interaction, that is, (i) all Cu have a Cu^(II) d^9 oxidation state with one unpaired spin that is coupled antiferromagnetically to the spins of adjacent Cu^(II) sites; (ii) reduction below the cupric Cu^(II) state leads to Cu^I d^(10) sites with a highly mobile Cu(3d) electron, and these extra electrons hop from site to site (while the oxygen remains in the O^(2-) state). The hopping of these extra electrons causes the flipping of the local spin moment of the antiferromagnetic background; (iii) oxidation beyond the cupric Cu^(II) state leads not to Cu^(III) but rather to oxidized oxygen atoms with an highly mobile Op hole, which is ferromagnetically coupled to the adjacent Cu^(II) d electrons despite the fact that this is opposed by the direct dd exchange. This coupling induces an attractive interaction between conduction electrons that is responsible for the superconductivity

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