Fast oscillations in (TMTSF)₂X

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)₂X 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 ρ_xx (B//c*, I//a) and ρ_yy (B//c*, I//b) 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 (δm∼10^-10 emu) are seen in the NO₃ salt above 25T. Also, no oscillations were observed in longitudinal magnetoresistance ρ_zz (B//I//c*) measurements performed on AsF₆ and PF₆. Possible scenarios which have been used to explain the data include imperfect nesting at TSDW 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.