AMRO study of Fermi surface nesting of (BEDT-TTF)2MHg(SCN)4, M=K and NH4

The title compound (BEDT-TTF)2KHg(SCN)4 shows the anomalous behavior in the low-temperature-low-magnetic-field-low-pressure region, while the isostructural compound (BEDT-TTF)2NH4Hg(SCN)4 shows the normal metallic one. In order to clarify the origin of the difference of the ground states in these salts, we measured the angular dependence of magnetoresistance of NH4 compound under pressure. In NH4 salt, we found angular dependent magnetoresistance oscillations due to a quasi-one-dimensional Fermi surface. A detailed analysis reveals the higher harmonic components of the warping is comparatively large in NH4 salt. The resistance peak is observed for the magnetic fields nearly parallel to the conducting plane. It can be ascribed to the small closed orbits on the side of the warping cylindrical Fermi surface. These results suggest a poor nesting of the Fermi surface in NH4 salt.