Charge Disproportionation without Charge Ordering in Organic Conductors

Based on a study of a $3/4$-filled extended Hubbard model with an inter-site Coulomb interaction, we show that charge disproportionation is self-generated in the presence of strong charge fluctuations even if there is no charge ordering. In the absence of a frustrating Coulomb interaction, the extended Hubbard model is found to be unstable to phase separation into electron-rich and electron-poor metallic phases near its charge-ordering transition point. By switching on the frustrating Coulomb interaction, this phase separation is transformed into exotic disproportionation such as a stripe glass or a mosaic studed with mesoscale charged droplets. Such disproportionation is a possible origin of extremely slow charge dynamics observed in $\theta $-type BEDT-TTF organic conductors [1,2]. [1] R. Chiba, \textit{et al}. Phys. Rev. Lett \textbf{93}, 216405 (2004). [2] R. Chiba, \textit{et al}. Phys. Rev. B \textbf{77}, 115113 (2008).