Direct Observation of Reconstructed Fermi Surfaces of (TMTSF)₂ClO₄ Utilizing the Third Angular Effect of Magnetoresistance

The organic superconductor, (TMTSF)₂ClO₄, is believed to have two kinds of quasi-one-dimensional (Q1D) Fermi surfaces in its anion ordered (R-) state below 24 K at ambient pressure. We succeeded for the first time in detecting each of the Fermi surfaces independently by utilizing the third angular effect of magnetoresistance of quasi-one-dimensional metals at 0.5 K and 20 T. From the width of the third angular effect anomaly (Δφ = 24 °), the gap induced by the ordered anions is estimated as E_g = 0.083t_a, where t_a is the transfer integral along the most conducting a-axis. The small Δφ and the rather large E_g. the latter of which is comparable to the transverse transfer integral, t_b, suggest that the Q1D Fermi surfaces of (TMTSF)₂ClO₄ in the R-state are anomalously flat compared with that ol its analogs.