Evidence for a metallic but unusual ground state in τ-conductors

The band calculation for the τ-conductors predicted the existence of a Fermi surface, which has a two-dimensional star-shaped 4-fold symmetry. Actually, both τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+y and τ-(P-S,S-DMEDT-TTF)2(AuBr2)1+y (y∼0.75) show metallic temperature dependence of resistivity (dR/dT>0) below 300 K with a resistance upturn at 30–50 K. Below the temperature of resistance minimum, a large negative magnetoresistance (MR) suddenly appears in temperature, accompanied by a remarkable features such as the switching of the periodicity of the angular dependence of MR, large electronic specific heat coefficient γ (0.06 J/mol K2). Lots of works have been devoted to clarify whether or not the low temperature electronic state is metal. Recently we could demonstrate a giant Shubnikov de Haas oscillation in both materials, which is a direct evidence for a metallic state. We note that the observed FSs were not consistent with the previous calculation. How this metallic state explains those remarkable features is still unsolved.