Magnetoacoustic oscillations in (TMTSF)2ClO4in high magnetic fields, where TMTSF is tetramethyltetraselenafulvalene

We have studied the sound velocity in the quasi-one-dimensional organic conductor (TMTSF${)}_2$ ${\mathrm{ClO}}_4$, where TMTSF is tetramethyltetraselenafulvalene. The system shows profound magnetoacoustic oscillations at low temperatures and in high magnetic fields. At the reentrant transition from the field-induced spin-density-wave (FISDW) state to the metallic state, the sound velocity shows a discontinuous jump. In the reentrant metallic phase, the amplitude of the oscillations is greatly enhanced and their phase is different by 180° from that in the FISDW state. Above ∼20 T the sound velocity shows complex, temperature-dependent structure, which may indicate sharp oscillations of the phase boundary as predicted in some recent theories.