Fermi surfaces and crystal structure of a new organic conductor alpha-(BEDT-TTF)(2)KHg(SeCN)(4)

We have synthesized a new organic conductor alpha-(BEDT-TTF)(2)KHg(SeCN)(4) which is found to be isostructural with a family of alpha-(BEDT-TTF)(2)MHg(SCN)(4) with M=K, Tl, Rb and NH4, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. In contrast to superconducting (M=NH4) or antiferromagnetic (M=K, Tl, Rb) salts, the title compound remains a normal metal in the same way as alpha-(BEDT-TTF)(2)TlHg(SeCN)(4). The magnetoresistance and the Shubnikov-de Haas (SdH) effect have been measured at temperatures down to 0.5K and magnetic fields up to 23T. The SdH frequency of 670T, the effective mass of 2.0m(0), and the Dingle temperature of 1.6K are obtained in fields perpendicular to the conductive a-c plane. The magnetic field direction dependence of the frequency shows that the observed Fermi surface has a cylindrical shape. The temperature and the field dependence of the SdH oscillation amplitude are well explained by the standard Lifshitz-Kosevich formulation, indicating that the effective mass is essentially constant up to 23T.