Density wave v.s. magnetic breakdown: mechanism of high-field phase transition in /spl alpha/-(BEDT-TTF)/sub 2/XH/sub g/(SCN)/sub 4/

Summary form only given. We discuss the magnetic phase diagram of the unidentified low-temperature electronic phase of /spl alpha/-(BEDT-TTF)/sub 2/XH/sub g/(SCN)/sub 4/ (X=K, Rb, and Tl). These compounds are the quasi-2D conductors with two conduction bands; one is a 1D-like band with two parallel sheetlike Fermi surfaces (FS´s), and the other is a 2D-like band with a cylindrical FS. Recent experiments suggest that in the low-temperature phase the SDW (or CDW) is formed resulting from the nesting of the sheetlike FS´s. However, the mechanism of the high-field transition returning to the metallic phase has still been unknown. We consider the SDW/CDW instability of this two-band system under magnetic fields taking into account the interband coupling due to the magnetic breakdown. The magnetic breakdown opens the energy gaps on the magnetic energy spectrum of the 1D-like electron, which has the ID dispersion easy to occur the SDW/CDW nesting, by mixing with the Landau levels of the 2D-like electron. When the Fermi level locates around one of these gaps, the SDW/CDW instability is strongly suppressed. As the magnetic breakdown gap increases at high magnetic fields, the SDW/CDW state becomes unstable and undergoes the phase transition to the metallic state.