Surface reconstruction formed by ordered missing molecular rows observedon the quasi-one-dimensional organic conductor Β-(BEDT-TTF${)}_2$ ${\mathrm{PF}}_6$s

The surface structure of a quasi-one-dimensional organic conductor Β-(BEDT-TTF${)}_2$ ${\mathrm{PF}}_6$ crystal [BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene] was studied by scanning tunneling microscopy (STM). Twofold periodic modulation along the crystal a axis was observed on the crystal a-c plane with a ${\mathrm{PF}}_6$ top layer, similarly as the previous result obtained for the surface with a BEDT-TTF top layer. With the analysis using high-resolution STM images including the second BEDT-TTF molecular layer, the surface superstructure was found to be formed by alternate missing rows of ${\mathrm{PF}}_6$ molecules. To our knowledge, this is the first observation of an organic surface reconstruction involving such a drastic change in the molecular arrangement, which was comprehensively explained using a proposed model based on the charge balance in the surface layers.