Quantum Wigner solid in two-dimensional electron systems in semiconductors

We review recent transport experiments that reveal two-threshold voltage-current characteristics, marked by a significant increase in noise between the two threshold voltages, at low electron densities in the insulating regime in two-dimensional (2D) electron systems, specifically in silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) and SiGe/Si/SiGe heterostructures. The double-threshold voltage-current characteristics closely resemble those observed in the collective depinning of the vortex lattice in type-II superconductors. By adapting the model used for vortices to the case of an electron solid, good agreement with the experimental results is achieved, which supports a quantum electron solid forming in the low electron density state. When a perpendicular magnetic field is applied, the double-threshold behavior occurs at voltages an order of magnitude lower and at significantly higher electron densities than the zero-field case. This indicates the stabilization of the quantum electron solid, aligning with theoretical predictions. Interestingly, the double-threshold voltage-current curves, indicative of electron solid formation at low densities, are not observed in the quantum Hall regime. This lack of observation does not confirm the existence of a quasi-particle quantum Hall Wigner solid and indicates that quasi-particles near integer filling do not form an independent subsystem.