Fermi-surface nesting in the organic conductor (BEDT-TTF)2TlHg(SCN)4

The Shubnikov–de Haas (SdH) measurements in the quasi-two-dimensional (2D) conductor (BEDT-TTF${)}_2$TlHg(SCN${)}_4$ have been performed to clarify the ground state. The SdH oscillations with frequencies of 663 and 4270 T are observed for H?${\mathit{b}}^{\mathrm{*}}$ above 10 T, which come from the closed orbit of the 2D Fermi surface and the magnetic-breakdown orbit, respectively. Below 3 T, three SdH oscillations with frequencies lower than 13 T are found. These low-frequency oscillations are interpreted to correspond to the small closed orbits caused by the nearly perfect nesting of the 1D Fermi surface. The results indicate that the ground state is a spin-density-wave state at low magnetic fields, but the nesting is removed by high magnetic fields. Two of the three low frequencies depend on temperature below 0.6 K. The observation shows that the Fermi surfaces change as temperature varies.