Shubnikov-de Haas oscillations in the two-dimensional organic conductor (formula presented)

The quasi-two-dimensional organic (formula presented)-type conductor, (formula presented) shows metallic temperature dependence of the resistivity down to (formula presented) below which resistance upturn is observed. There has been no evidence of the phase transition through (formula presented) In this work, in pursuit of direct evidence for metal at low temperature by transport measurement, we show an observation of the Shubnikov-de Haas (SdH) oscillations for this salt. The observed frequencies of the oscillations are inconsistent with the previous band calculation. Further, it is remarkable that the amplitude of the oscillation was so giant (formula presented) and the oscillation above 20 T corresponds to a (formula presented) Landau state, i.e., next to a quantum limit. We also observed the SdH oscillations in other (formula presented)-type conductor, (formula presented) which almost reproduced the previous report by Storr et al. [Phys. Rev. B 64, 045107 (2001)].