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We report a systematic study of longitudinal and transverse magnetoresistance in the quasi-one-dimensional organic conductor (TMTSF) 2 ClO 4 . We found two distinctly different quantum ͑rapid͒ oscillations ͑RO's͒ at different temperature regions in the field-induced spin-density-wave ͑FISDW͒ phase. For temperature sweeps in a fixed field, a resistance peak is observed at T* which depends on the field. Surprisingly, the type of RO is correlated with T*(H). The difference between two oscillations leads us to establish subphases in the main FISDW phase of (TMTSF) 2 ClO 4 . We also found the low-field boundary of McKernan's 3.5 K phase from a sudden increase of resistance. Our results conclude that the separate SDW transition on each pair of the Fermi surfaces is possibly responsible for subphases in the FISDW phase.

Stereoscopic angular dependence of the interlayer magnetoresistance of the Bechgaard salt ͑TMTSF͒ 2 NO 3 is investigated under pressure. This compound is believed to be a semimetal having the quasi-two-dimensional Fermi surface ͑FS͒ at low temperature. Previously, a field-induced spin-density-wave ͑FISDW͒ transition was reported at 8.5 kbar above ϳ20 T, which is atypical with a closed FS. We present strong evidence that the FS of this compound remains quasi-one-dimensional under moderate pressure even in the presence of anion ordering. The occurrence of the FISDW is therefore unsurprising. In addition, the possibility of an anion ordering along the b axis will be discussed. Bechgaardsalts ͑TMTSF͒ 2 X ͓TMTSF = tetramethyltetraselenafulvalene and X is an inorganic anion͔ offer unique physics of the quasi-one-dimensional ͑Q1D͒ electron system. According to the temperature, pressure, and magnetic field to which a sample is subjected, various grounds states such as metallic, superconducting, spindensity wave ͑SDW͒, and field-induced spin-density-wave ͑FISDW͒ states can be realized in a sample. The anion ordering in noncentrosymmetric anion compounds puts additional effects on the common Q1D behaviors. 1 The magnetic orbital effect reduces the effective dimensionality of electrons and favors reappearance of the SDW state, FISDW. 2 However, there are two compounds whose properties substantially depart from the typical Q1D behavior. One is ͑TMTSF͒ 2 NO 3 when below the anion ordering temperature ͑T AO ͒ and the other is ͑TMTSF͒ 2 FSO 3 when under moderate pressure and below the given anion ordering temperature. The angular magnetoresistance oscillations ͑AMROs͒ of ͑TMTSF͒ 2 FSO 3 observed between 6 and 10 kbar can be explained in terms of Kartsovnik-Kajita-Yamaji ͑KKY͒ oscillations as evidence of the cylindrical quasi-two-dimensional ͑Q2D͒ Fermi surface ͑FS͒. [3][4][5][6] In this pressure range, this compound shows superconductivity below around 3 K but no FISDW up to 33 T. 7 The FS of ͑TMTSF͒ 2 NO 3 below T AO has been regarded as small hole and electron pockets created by an anion ordering transition with a wave vector ͑1/2,0,0͒ ͑Refs. 8 and 9͒ that renders it semimetallic at low temperature. 10,11 At lower temperatures, an SDW state with incommensurate wave vector and imperfect nesting is formed ͑T SDW Ӎ 9 K at ambient pressure͒. The SDW state is suppressed by hydrostatic pressure, but the anion ordering remains. While the superconductivity has never been observed in ͑TMTSF͒ 2 NO 3 , an FISDW was recently observed above ϳ20 T at 8.5 kbar. 12 FISDW without superconductivity was also reported in Q1D ͑DMET-TSF͒ 2 AuCl 2 , which remains metallic down to 42 mK. 13 However, FISDW in a Q2D metal is unprecedented.So, establishing the dimensionality of the title material, whether Q1D or Q2D, for the FISDW phenomena is pivotal to further understand the underlying physics of Bechgaard salts. A traditional way to determine the anisotropy of FS is by measuring angular dependence of the magnetic quantum oscillations, such as de Haa...