Abstract :
Summary form only given. Since the discovery of a characteristic magnetoresistance[l], for example, the kink at 23T and 1.5K, of the organic conductor /spl alpha/-(BEDT-TTF)/sub 2/XH/sub g/(SCN)/sub 4/ , a series of /spl alpha/-(BEDT-TTF)/sub 2/MH/sub g/(SCN)/sub 4/ with M=K, Rb, Tl and NH/sub 4/ has become one of the most attractive materials in the research of the BEDT-TTF based organic conductors. In the mysterious low temperature ground state of /spl alpha/-(BEDT-TTF)/sub 2/XH/sub g/(SCN)/sub 4/ below 10K, 23T and 3-5 kbar, the magnetoresistance, Hall effect, magnetization, Shubnikov-de Haas and de Haas-van Alphen effects, angular dependent magnetoresistance oscillations and so on show the anomalous behavior. The interplay between the open and closed sections of the Fermi surface has been considered to be of great importance. We have presented the systematic investigations of the magnetic field and electrical current orientation dependence of the magnetoresistance in order to clarify what the role of the open Fermi surface in the phase transition is[2]. Based on the experimental results, a possibility of the mixed state of spin- and charge-density-waves is proposed as the origin of the mysterious low temperature ground state.
