A NEW ELECTRONIC-STATE IN THE VARIABLY DOPED 2-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 tr ansfer salts. First, in this two-dimensional conductor, we could achie ve 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. S ince the magnetic response;changes across this temperature, an electro nic phase transition might occur. In this low temperature phase, below about 30 K, the angular dependence of the magnetoresistance becomes h ysteretic, implying that this system is related to magnetism.