Unconventional spin density wave in (TMTSF)2PF6 below T=4.2 K

It is well documented that SDW in (TMTSF)(2)PF6 undergoes another phase transition at T* approximate to 4 K, though the nature of the new phase has not been identified. In particular the angular dependence of the magnetoresistance for Bparallel to(a, b') plane below 4.2 K is dramatically different from the one above 4.2 K. We propose that below T* the new phase unconventional SDW (USDW) appears which modifies drastically the quasiparticle spectrum. The present model describes the observed angular dependence of the magnetoresistance reasonably well, which cannot be interpreted in terms of the conventional SDW.