We have studied the I-V curves of amorphous insulators at very low temperature which exhibit a crossover from variable range hopping to a low-temperature activation law. Around a critical voltage Vc, the current increases by several orders of magnitude. This is interpreted as resulting from an interplay between Coulomb interactions and disorder: below Vc charges are collectively ``pinned,'' while above Vc free charges exist. We find a critical scaling of the I-V curves near Vc, at least when hysteretic effects can be neglected. {} 1996 The American Physical Society.
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Ladieu et al. (1996) studied this question.