Abstract :
Summary form only given. We studied the depinning of the magnetic field-induced spin-density-wave of the organic super- conductor (TMTSF)/sub 2/CIO/sub 4/, stabilized above 4T and at 300mK. Although successive quantum Hall steps are well developed in our well relaxed sample, the depinning is observed only in the negative Hall plateau region between 6.2 and 6.8T. For the sample cross-sectional area of 1.07 x 10/sup -3/cm/sup 2/, the quantized Hall resistance begins to decrease when the sample current increases more than 160/spl mu/A. One the other hand, the longitudinal resistance which is at least fifteen times as small as that of the transverse resistance in the quantized Hall region, increases when the non-linear behavior appears. This is reminiscent of that found in the conventional quantum Hall effect,sup 1/ where the phenomenon is explained in terms of localization in Landau levels without edge-state transport. However, the observation of the so-called Shapiro interference between the depinned FISDW and the externally driven radio-frequency waves strongly suggests that the breakdown of quantized Hall state of FISDW is related to the collectively moving condensate. No departure from the quantized Hall behavior is observed neither above 7.1T nor below 6.2T at least for the current two and half times more than the value given above.
