Charge disproportionation in α-(BEDT-TTF)2x

We performed scanning tunneling microscopy (STM) at room temperature on a−ba−b surface of α-(BETD-TTF)2I3 which undergoes a charge ordering transition at TCO=135K. BEDT-TTF molecular arrangement with α  -type was clearly observed. Charge modulation with two-fold molecular periodicity was observed along both columns I and II. The auto correlation of the charge modulation along column II has extended in the area observed by STM. On the other hand, that along the column I is much weaker than column II although the charge modulation locally develop. The charge modulations along columns I and II form horizontal stripe structure with some inhomogeneity, whose structure is similar to the charge order below TCO135K. Our result suggests that horizontal charges stripe at room temperature is a precursor of the charge ordered state.