  The combination of strong disorder and many-body interactions in Anderson
insulators lead to a variety of intriguing non-equilibrium transport phenomena.
These include slow relaxation and a variety of memory effects characteristic of
glasses. Here we show that when such systems are driven with sufficiently high
current, and in liquid helium bath, a peculiar type of conductance noise can be
observed. This noise appears in the conductance versus time traces as
downward-going spikes. The characteristic features of the spikes (such as
typical width) and the threshold current at which they appear are controlled by
the sample parameters. We show that this phenomenon is peculiar to hopping
transport and does not exist in the diffusive regime. Observation of
conductance spikes hinges also on the sample being in direct contact with the
normal phase of liquid helium; when this is not the case, the noise exhibits
the usual 1/f characteristics independent of the current drive. A model based
on the percolative nature of hopping conductance explains why the onset of the
effect is controlled by current density. It also predicts the dependence on
disorder as confirmed by our experiments. To account for the role of the bath,
the hopping transport model is augmented by a heuristic assumption involving
nucleation of cavities in the liquid helium in which the sample is immersed.
The suggested scenario is analogous to the way high-energy particles are
detected in a Glaser's bubble chamber.
