Fast oscillations in (TMTSF)(2)X

We report on the temperature dependence of the ''fast oscillation'' phenomenon in quasi-one dimensional molecular conductors. Magnetoresistance and magnetization measurements to 30T in the ambient pressure spin density wave state have been taken for the X = NO3, PF6 and AsF6 members of the (TMTSF)(2)X family. Complementary data on X=AsF6 are presented by Ulmet et al. in these proceedings. All three compounds exhibit a peak in the transverse magnetoresistances rho(xx) (B//c*(,) I//a) and rho(yy), (B//c(*), I//a) and their oscillatory components around 3K, with the oscillations vanishing at higher and lower temperatures. No magnetic moment oscillations are detected in PF6 or AsF6, while only extremely weak oscillations (delta m similar to 10(-10) emu) are seen in the NO3 salt above 25T. Also, no oscillations were observed in longitudinal magnetoresistance rho(zz) (B//I//c*) measurements performed on AsF6 and PF6, Possible scenarios which have been used to explain the data include imperfect nesting at T-SDW leaving behind quantizable closed orbits, and quantum interference effects arising from magnetic breakdown orbits. Another may involve a new phase transition inside the SDW state.