  We review recent experimental results indicating the band flattening and
Landau level merging at the chemical potential in strongly-correlated
two-dimensional (2D) electron systems. In ultra-clean, strongly interacting 2D
electron system in SiGe/Si/SiGe quantum wells, the effective electron mass at
the Fermi level monotonically increases in the entire range of electron
densities, while the energy-averaged mass saturates at low densities. The
qualitatively different behavior of the two masses reveals a precursor to the
interaction-induced single-particle spectrum flattening at the chemical
potential in this electron system, in which case the fermion "condensation" at
the Fermi level occurs in a range of momenta, unlike the condensation of
bosons. In strong magnetic fields, perpendicular to the 2D electron layer, a
similar effect of different fillings of quantum levels at the chemical
potential -- the merging of the spin- and valley-split Landau levels at the
chemical potential -- is observed in Si inversion layers and bilayer 2D
electron system in GaAs. Indication of merging of the quantum levels of
composite fermions with different valley indices is also reported in
ultra-clean SiGe/Si/SiGe quantum wells.
