FAST OSCILLATIONS IN (TMTSF)(2)X

We report on the temperature dependence of the ''fast oscillation'' ph enomenon in quasi-one dimensional molecular conductors. Magnetoresista nce and magnetization measurements to 30T in the ambient pressure spin density wave state have been taken for the X = NO3, PF6 and AsF6 memb ers of the (TMTSF)(2)X family. Complementary data on X=AsF6 are presen ted by Ulmet et al. in these proceedings. All three compounds exhibit a peak in the transverse magnetoresistances rho(xx) (B//c(,) I//a) an d 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 e xtremely weak oscillations (delta m similar to 10(-10) emu) are seen i n the NO3 salt above 25T. Also, no oscillations were observed in longi tudinal 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 clo sed orbits, and quantum interference effects arising from magnetic bre akdown orbits. Another may involve a new phase transition inside the S DW state.