Fermi-surface studies in the two-dimensional organic conductors (BEDT-TTF${)}_2$MHg(SCN${)}_4$ (M=Tl,K,Rb,${\mathrm{NH}}_4$)

The Shubnikov–de Haas (SdH) measurements of the two-dimensional organic conductors (BEDT–TTF${)}_2$MHg(SCN${)}_4$ for M=Tl, K, Rb, and ${\mathrm{NH}}_4$ have been performed in order to investigate the structure of the Fermi surface. In addition to the closed orbit and the magnetic breakdown orbit in the original Fermi surface, many different SdH oscillations are found for the salts (M=Tl, K, and Rb) which have the spin-density-wave (SDW) ground state whereas only one frequency is observed for the ${\mathrm{NH}}_4$ salt. All the frequencies follow the same angular dependence. On the basis of the relation between the SdH frequencies, the Fermi surface in the SDW phase, which has no open sheets, is reconstructed. The origin of the magnetic breakdown orbit and the large hysteresis in the resistance observed below the transition field for M=Tl, K, and Rb is explained in terms of the coexistence of the SDW phase and the normal metallic phase. © 1996 The American Physical Society.