Mesoscopic behaviour of the threshold voltage in ultra-small specimens

In very small (∼10-14 cm3) specimens of o-TaS3, the threshold voltage VT for the onset of Frohlich conduction is found to vary irregularly with the temperature T. This behaviour is attributed to rearrangement of the charge-density wave (CDW) with respect to the pinning, as its wave-vector varies with T. It is consistent with weak, but not with strong, pinning by impurities. In sufficiently short (< 10 μm) specimens the variation of VT relative to its mean 〈VT〉 is substantially independent of specimen volume, as is expected when the dimensionality of the weak pinning is effectively zero. However, the value 0.15±0.05 found for δVT/〈VT〉, where δVT is the rms deviation of VT from 〈VT〉, is smaller than for a rigid CDW whose coupling to impurities varies sinusoidally with phase. Possible causes of the discrepancy are an approach to strong pinning through local elastic distortion near pinning sites, a departure from zero-dimensional behaviour due to imperfect phase coherence in one transverse direction, and the pinning of widely-separated defects (dislocations) in the CDW structure