Evidence for the π-d interaction comparing magnetoresistance in (EDT-DSDTFVO)2X, X=FeCl4, GaCl4

We have measured the electrical resistivities and magnetoresistances (MR) of (EDT-DSDTFVO)(2)X (X = FeCl4, GaCl4), where EDT-DSDTFVO stands for ethylenedithiodiselenadithiafulvalenothioquinone-1,3-dithiolemethide. These materials undergo gradual metal-insulator transitions at T-min = 52 K for FeCl4-salt and T-min = 30 K for GaCl4-salt, respectively. In spite of the similarity of the temperature dependence of the resistivity and its pressure effect, MR of both salts exhibit a clear contrast, i.e. FeCl4-salt shows negative and GaCl4, positive. Origin of the difference in the sign of MR between these salts are discussed in terms of the existence of pi-d interaction.