Low-temperature Fermi surface of the organic conductor β″ -(BEDT-TTF) (TCNQ)(1-x) (F1-TCNQ)x (x=0,0.05) from magnetooptical measurements

Magnetooptical measurements have been performed on the organic conductor β″ -(BEDT-TTF)(TCNQ) (x=0 salt), and a related compound, β″ -(BEDT-TTF)(TCNQ) 0.95 (F1-TCNQ) 0.05 (x=0.05 salt), to investigate their low-temperature Fermi surfaces (FSs). Although the room-temperature FS of both salts can be considered the same, our results indicate that the low-temperature FSs are completely different from each other. In the x=0 salt, the low-temperature FS consists of two very anisotropic quasi-two-dimensional (Q2D) FSs. Cross-sectional areas of anisotropic Q2D-FSs are very small, and correspond to 1-2 % of the first Brillouin zone at room temperature. On the other hand, in the x=0.05 salt, only a pair of quasi-one-dimensional FSs exist at low temperature. Compared with the band calculation of x=0 salt, this Q1D-FS originates from the BEDT-TTF conducting layers. This result indicates that the two hump-like anomalies in the temperature dependence of resistivity at 80 and 20 K, which are observed only in the x=0 salt, are related to the nesting of BEDT-TTF Q1D-FS. Hence, it is considered that anisotropic Q2D-FS pockets observed in the x=0 salt are generated by the imperfect nesting of Q1D-FS, and the density wave state may be formed in the x=0 salt at low temperatures.