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

In a unit cell of the crystal a-c plane of the beta-(BEDT-TTF)(2)PF6 crysta l surface, four BEDT-TTF molecules form two dimers. Since the alternate mol ecular rows consisting of the dimers are observed to be bright by scanning tunneling microscopy (STM) along the crystal alpha-axis, the surface molecu lar structure is considered to have a periodic modulation. From the analysi s 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 conform ation in the crystal, and one of the BEDT-TTF molecules in each dimer is lo cated about 0.2 nm to the surface. Despite the difference in the distance f rom the surface, molecules that are farther from the surface were observed to be brighter by STM.