The Origin of the Charge Ordering and Its Relevance to Superconductivity in θ-(BEDT-TTF)_2X : The Effect of the Fermi Surface Nesting and the Distant Electron-Electron Interactions(Condensed matter: electronic structure and electrical, magnetic, and optic
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The origin of the charge ordering in organic compounds θ-(BEDT-TTF)_2X (X=MM'(SCN)_4, M=Tl, Rb, Co, M'=Cs, Zn) is studied using an extended Hubbard model. Calculating the charge susceptibility within random phase approximation (RPA), we find that the (3×3)-(3×4) charge ordering observed at relatively high temperatures can be considered as a consequence of a cooperation between the Fermi surface nesting, controlled by the hopping integral in the c direction, and the electron-electron interactions, where the distant (next nearest neighbor) interactions that have not been taken into account in most of the previous studies play an important role. Mean field analysis at T=0 also supports the RPA results, and further shows that in the 3×3 charge ordered state, some portions of the Fermi surface remain ungapped and are nested with a nesting vector close to the modulation wave vector of the horizontal stripe ordering observed at low temperatures in X=MM'(SCN)_4. We further study the possibility of superconductivity by taking into account the distant off-site repulsions and the band structure corresponding to X=I_3, in which superconductivity is experimentally observed. We find that there is a close competition between d_<xy>-wave-like singlet pairing and p_<x+2y>-wave-like triplet pairing due to a cooperation between the charge and the spin fluctuations. The present analysis provides a possible unified understanding of the experimental phase diagram of the θ-(BEDT-TTF)_2X family, ranging from a charge ordered insulator to a superconductor.
- 2006-11-15
著者
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Kuroki Kazuhiko
Department Of Applied Physics And Chemistry The University Of Electro-communications
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