SURFACE RECONSTRUCTION FORMED BY ORDERED MISSING MOLECULAR ROWS OBSERVED ON THE QUASI-ONE-DIMENSIONAL ORGANIC CONDUCTOR BETA-(BEDT-TTF)(2)PF6

The surface structure of a quasi-one-dimensional organic conductor bet a-(BEDT-TTF)(2)PF6 crystal [BEDT-TTF is bis(ethylenedithio)-tetrathiaf ulvalene] was studied by scanning tunneling microscopy (STM). Twofold periodic modulation along the crystal a axis was observed on the cryst al a-c plane with a PF6 top layer, similarly as the previous result ob tained for the surface with a BEDT-TTF top layer. With the analysis us ing high-resolution STM images including the second BEDT-TTF molecular layer, the surface superstructure was found to be formed by alternate missing rows of PF6 molecules. To our knowledge, this is the first ob servation of an organic surface reconstruction involving such a drasti c change in the molecular arrangement, which was comprehensively expla ined using a proposed model based on the charge balance in the surface layers.