SLOW QUANTUM OSCILLATIONS IN THE SEMIMETALLIC SPIN-DENSITY-WAVE STATEOF TETRAMETHYLTETRASELENAFULVALINIUM NITRATE (TMTSF)(2)NO3

We have observed a clear series of fine structures superposed on the a ngular dependence of the transverse magnetoresistance in the semimetal lic spin-density-wave state of the organic conductor (TMTSF)(2)NO3. Th ey appear at the positions defined by the strength of the field compon ent parallel to the least-conduction direction (B cos theta similar or equal to const). An observed asymmetry in the magnitude is due to the striking angular dependence of the oscillations amplitude. The freque ncy of the oscillations (B-S=63 T) is close to the frequency of the fi eld-induced spin-density-wave transitions found in the other Bechgaard salts. This oscillatory phenomenon is considered to result from the o rbit quantization inside the closed parts of the Fermi surface which a re due to the 2k(F) anion ordering together with the imperfect spin-de nsity-wave nesting.