FIELD-INDUCED SPIN-DENSITY-WAVE TRANSITIONS AND RAPID OSCILLATIONS OBSERVED IN THE MICROWAVE SURFACE IMPEDANCE OF (TMTSF)(2)CLO4

We report microwave surface impedance measurements performed in the re laxed state of the quasi-one-dimensional organic conductor (TMTSF)(2)C lO4 at 16.5 GHz in a magnetic field applied along the hard axis (c) d irection. The transition from the metallic to the field-induced spin-d ensity-wave (FISDW) state is not as sharp as it:is generally observed in the de regime, but the phase diagram is in very good agreement with ; existing data. An enhanced ac conductivity is measured in the FISDW phases, which should reflect the contribution of the collective mode. The rapid oscillations of the microwave magnetoresistivity appear in b oth the metallic state and/or in the FISDW phase. The deduced frequenc y (260 T) and the temperature dependence of the rapid oscillation ampl itude in the normal and condensed states are consistent with reported de results. A frequency dampening of the amplitude that is different i n the two phases is, however, observed compared to de data;its possibl e origin is discussed and confirms the different origin of the two mec hanisms.