Fast oscillations and the high-field phase boundary in (TMTSF)2ClO4 (where TMTSF represents tetramethyltetraselenafulvalene)

A recent theory proposed by Lebed and Bak suggests that the anion ordering which occurs at 24 K in (TMTSF)2ClO4 with TMTSF=tetramethyltetraselenafulvalene, leads both to the fast oscillations observed in transport and thermodynamic measurements and to the reentrant behavior of the phase diagram of the magnetic-field-induced spin-density-wave (FISDW) transitions. In their model, the gap resulting from the anion ordering also causes the fast oscillations to appear on the phase boundary. We report a detailed study of the H-T phase boundary between 13 and 16 T, where fast oscillations in magnetoresistance are strongly evident. We find no oscillations in the phase boundary within experimental uncertainty of 0.3% of Tc. This puts an upper limit on the effect of the anion gap and demonstrates that the Lebed-Bak model cannot account for the reentrance. Beyond any model, our experiment indicates that the occurrence of the FISDW is unaffected by the fast oscillations. It therefore appears that the quantum oscillations and the reentrant transition are completely different phenomena and require their own interpretations.