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Electronic Structure and Phonon Calculations for Unconventionaly358, Lbco and 1111-Iron Pnictide Superconductors

Tavana, Ali | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41904 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Akhavan, Mohammad
  7. Abstract:
  8. Experiments indicate the important role of the electron-phonon interaction and crystal lattice effects on the electronic and superconductivity properties of the unconventional superconductors. Thus, in the theoretical studies of the mechanism of unconventional superconductivity, role of phonons and crystal lattice distortions can not be ignored. Based on this fact, in this dissertation, it has been tried to investigate the effect of phonons and lattice distortions in the electronic properties of some unconventional superconductors. For this purpose, we have used the ab-initio, density functional theory of the ground state and perturbed states, which is the most suitable approach when it is aimed to study the differences between different compounds. We have studied three groups of superconductors:
    1) new Y3Ba5Cu8O18cuprate compound and the RE BaCuO family,
    2) La2xBaxCuO4 cuprate compound and
    3) 1111- family of iron-pnictide superconductors.
    Electronic structure, hole concentration in the planes, van Hove singularity, phonons and electron-phonon interactions, the Hubbard-U parameter, displasive structural phase transitions and some other important properties are the studied subjects in this dissertation.
    Calculations on the Y3Ba5Cu8O18 shows that the hole concentratin in the some planes increases and in some other planes decreases. These change enhances Tc by putting one or more planes closer to the optimal doping concentration. Also, it seems that a van Hove singularity is strongly correlating with the oxygen buckling phonon mode. In the La superconductor, our calculations show that the results of previous electron-phonon coupling calculations are not too trustable and they must be done in the proper lattice structure. Based on the obtained soft phonon modes and displacive structural phase transitions theory, the structural phase transitions with doping and pressure can be explained. In the 1111-ironpnictide family, results shows that weaker electron correlations and larger density of states at the Fermi level lead to larget Tcs. Calculated soft phonon modes, also indicate the instable lattice structure in these compounds
  9. Keywords:
  10. Electron Structure ; Density Functional Theory (DFT) ; Electron-Phonon Interaction ; Unconventional Superconductivity ; Structural Phases

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