A NOVEL ELECTRONIC-STATE IN (BEDT-TTF)2XHG(SCN)4 - X=K, NH4

Magnetotransport measurements are made in the title compounds under hi gh pressures. An anomalous state is found in the low-temperature-low-f ield-low-pressure regime in the K-compound while the NH4-one behaves a s expected from the current model for quasi two-dimensional electronic systems. The anomalies are found in the so-called angle-dependent osc illation of magnetoresistance and the field and temperature dependence of the resistance. A phase diagram for the K-compound is made experim entally and electronic structures are discussed in relation to the pos sible spin-density wave found already in the anomalous state.