Role of the dimerized gap due to anion ordering in spin-density wave   phase of (TMTSF)$_2$ClO$_4$ at high magnetic fields
  Magnetoresistance measurements have been carried out along the highly conducting a axis in the FISDW phase of hydrogened and deuterated (TMTSF)$_2$ClO$_4$ for various cooling rates through the anion ordering temperature. With increasing the cooling rate, a) the high field phase boundary $\beta_{\rm {HI}}$, observed at 27 T in hydrogened samples for slowly cooled, is shifted towards a lower field, b) the last semimetallic SDW phase below $\beta_{\rm {HI}}$ is suppressed, and c) the FISDW insulating phase above $\beta_{\rm {HI}}$ is enhanced in both salts. The cooling rate dependence of the FISDW transition and of $\beta_{\rm {HI}}$ in both salts can be explained by taking into account the peculiar SDW nesting vector stabilized by the dimerized gap due to anion ordering. 
N.Matsunaga, A.Ayari, P.Monceau, A.Ishikawa, K.Nomura, M.Watanabe,   J.Yamada, S.Nakatsuji