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

The band calculation for the tau-conductors predicted the existence of a Fermi surface, which has a two-dimensional star-shaped 4-fold symmetry. Actually, both tau-(EDO-S,S-DNMDT-TTF)(2)(AuBr2)(1+y) and tau-(P-S,S-DMEDT-TTF)(2)(AuBr2)(1+y) (y similar to 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 gamma (0.06 J/mol K-2). 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. (C) 2002 Elsevier Science B.V. All rights reserved.