A new electronic state in the variably doped two-dimensional charge transfer salt, tau-(EDO-S,S-DMEDT-TTF)2(I-3)(1) (I-3)(y) and tau-(EDO-S,S-DMEDT-TTF)(2) (AuBr2)(1) (AuBr2)(y) with y similar to 0.75

Two topics are presented which are remarkable in the organic charge transfer salts. First, in this two-dimensional conductor, we could achieve varying the effective doping concentration so that we could change the nature from semiconducting (d rho/dT<0) to a metallic behavior (d rho/dT>0) by reducing y in the titled formula. This drastic change can be interpreted as an increase in the carrier number by expanding the Fermi surface by reducing y in the I-3-salt. Second, a strong negative magnetoresistance state is observed suddenly below similar to 60 K. Since the magnetic response;changes across this temperature, an electronic phase transition might occur. In this low temperature phase, below about 30 K, the angular dependence of the magnetoresistance becomes hysteretic, implying that this system is related to magnetism.