Anomalous structural transition of the β-(BEDT-TTF)2PF6 surface observed by scanning tunneling microscopy

In a unit cell of the crystal a-c plane of the β-(BEDT-TTF)2PF6 crystal surface, four BEDT-TTF molecules form two dimers. Since the alternate molecular rows consisting of the dimers are observed to be bright by scanning tunneling microscopy (STM) along the crystal a-axis, the surface molecular structure is considered to have a periodic modulation. From the analysis of the STM images, the modulated surface structure was found to fluctuate in an anomalous manner. The two dimers are known to have a similar conformation in the crystal, and one of the BEDT-TTF molecules in each dimer is located about 0.2 nm to the surface. Despite the difference in the distance from the surface, molecules that are farther from the surface were observed to be brighter by STM.