SURFACE SUPERSTRUCTURES OF QUASI-ONE-DIMENSIONAL ORGANIC CONDUCTOR BETA-(BEDT-TTF)(2)PF6 CRYSTAL STUDIED BY SCANNING-TUNNELING-MICROSCOPY

Two kinds of superstructures were found at similar to 280 K by scannin g tunneling microscopy on the crystal surface of beta-(BEDT-TFF)(2)PF6 , a quasi-one-dimensional organic conductor with a metal-insulator pha se transition at similar to 297 K. One has a structural modulation whi ch is perpendicular to the one-dimensional conductive axis, similar to that previously observed on a TTF-TCNQ crystal surface. The other has a twofold periodicity along the one-dimensional conductive axis, as e xpected from the theory. Under the assumption that the former is an in termediate structure caused by the interaction between neighboring one -dimensional molecular chains, superstructures which have been observe d in BEDT-TTF compounds could be explained well.