The Origin of the Charge Ordering and Its Relevance to Superconductivity in $\theta$-(BEDT-TTF)${}_2$X: The Effect of the Fermi Surface Nesting and the Distant Electron-Electron Interactions

Abstract

The origin of the charge ordering in organic compounds $\theta$-(BEDT-TTF)${}_{2}X$ ($X=M{M}^{\prime }$(SCN)${}_4$, $M=$Tl,Rb,Co, $M^{\prime }=$Cs,Zn) is studied using an extended Hubbard model. Calculating the charge susceptibility within random phase approximation (RPA), we find that the $(3\times 3)\sim (3\times 4)$ 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 $3\times 3$ 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 $X=M{M}^{\prime }$(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 $\theta$-(BEDT-TTF)${}_{2}X$ family, ranging from a charge ordered insulator to a superconductor.

arXiv comment13 pages, 18 figures (Figs.5,6,7,14,15,18 compressed using jpeg2ps)

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• Crystallization and vitrification of electrons in a glass-forming charge liquid
• Growth Dynamics of Photoinduced Domains in Two-Dimensional Charge-Ordered Conductors Depending on Stabilization Mechanisms
• Charge Order with Structural Distortion in Organic Conductors: Comparison between θ-(ET)2RbZn(SCN)4 and α-(ET)2I3
• Non-Perturbative Treatment of Charge and Spin Fluctuations in the Two-Dimensional Extended Hubbard Model: Extended Two-Particle Self-Consistent Approach
• Charge ordering and coexistence of charge fluctuations in quasi-two-dimensional organic conductors $\theta$-(BEDT-TTF)${}_{2}X$
• Coexistence of distinct charge fluctuations in $\theta$-(BEDT-TTF)${}_2$X
• Charge, Lattice, and Spin Dynamics in Photoinduced Phase Transitions from Charge-Order-Insulator to Metal in Quasi-Two-Dimensional Organic Conductors
• Effects of Electron-Lattice Coupling on Charge Order in θ-(ET)2X
• Geometrical frustration effects on charge-driven quantum phase transitions
• Strong magnetic field enhancement of spin triplet pairing arising from coexisting $2k_F$ spin and $2k_F$ charge fluctuations
• Theoretical Aspects of Charge Correlations in $\theta$-(BEDT-TTF)${}_{2}X$
• Enhancement of Spin Susceptibility near Charge-Ordering Transition in a Two-Dimensional Extended Hubbard Model
• Long-Period Charge Correlations in Charge-Frustrated Molecular theta-(BEDT-TTF)2X
• Coulomb Frustrated Phase Separation in Quasi-Two-Dimensional Organic Conductors on the Verge of Charge Ordering
• Superconducting pairing symmetry on the extended Hubbard model in the presence of the Rashba-type spin-orbit coupling
• Collective excitation of a short-range charge ordering in $\theta$-$\mbox{(BEDT-TTF)}_2\mbox{CsZn(SCN)}_4$
• Superconductivity in the Three-Fold Charge-Ordered Metal of the Triangular-Lattice Extended Hubbard Model
• Fermi Surface Deformation near Charge-Ordering Transition
• Ferrimagnetism Induced by Off-Site Coulomb Interaction in an Itinerant Electron System
• Density Matrix Renormalization Group study of superconducting pairing near the quarter-filled Wigner crystal