  Ignited by the discovery of the metal-insulator transition, the behaviour of
low-disorder two-dimensional (2D) electron systems is currently the focus of a
great deal of attention. In the strongly-interacting limit, electrons are
expected to crystallize into a quantum Wigner crystal, but no definitive
evidence for this effect has been obtained despite much experimental effort
over the years. Here, studying the insulating state of a 2D electron system in
silicon, we have found two-threshold voltage-current characteristics with a
dramatic increase in noise between the two threshold voltages. This behaviour
cannot be described within existing traditional models. On the other hand, it
is strikingly similar to that observed for the collective depinning of the
vortex lattice in Type-II superconductors. Adapting the model used for vortexes
to the case of an electron solid yields good agreement with our experimental
results, favouring the quantum electron solid as the origin of the low-density
state.
