  We report the observation of two-threshold voltage-current characteristics
accompanied by a peak of broadband current noise between the two threshold
voltages in the insulating state at low densities in the 2D electron system in
ultra-high mobility SiGe/Si/SiGe heterostructures. The observed results can be
described by a phenomenological theory of the collective depinning of elastic
structures, which naturally generates a peak of a broadband current noise
between the dynamic and static thresholds and changes to sliding of the solid
over a pinning barrier above the static threshold. This gives compelling
evidence for the electron solid formation in this electron system and shows the
generality of the effect for different classes of electron systems.
