Charge Order in (TMTTF)2PF6 Investigated by Infrared Spectroscopy

To explore novel physical phenomena related to strong π-d interaction, we measured the resistivity (ρ) and magnetoresistance of the first organic ferrimagnetic π-d system, (EDT-TTFVO)₂FeBr₄ under high pressures up to 8 GPa, where EDT-TTFVO denotes ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide. At ambient pressure, ρ(T) exhibits resistivity minimum near T_min ∼ 170 K followed by a gradual increase below it. With increasing pressure, T_min abruptly decreases till 4 GPa, beyond which it slightly increases. The increase ofT _min above 4 GPa is discussed in terms of the enhancement of the π-d interaction by applying pressure.