Merging of Landau levels in a strongly-interacting two-dimensional   electron system in silicon
  We show that the merging of the spin- and valley-split Landau levels at the chemical potential is an intrinsic property of a strongly-interacting two-dimensional electron system in silicon. Evidence for the level merging is given by available experimental data. 
A.A. Shashkin, V.T. Dolgopolov, J.W. Clark, V.R. Shaginyan, M.V.   Zverev, V.A. Khodel
cond-mat.str-el