Spin polarization of composite fermions and particle-hole symmetry breaking

We study the critical spin-polarization energy (alpha(C)) above which fractional quantum Hall states in two-dimensional electron systems confined to symmetric GaAs quantum wells become fully spin polarized. We find a significant decrease of alpha(C) as we increase the well width. In systems with comparable electron layer thickness, alpha(C) for fractional states near Landau level filling nu = 3/2 is about twice larger than those near nu = 1/2, suggesting a broken particle-hole symmetry. Theoretical calculations, which incorporate Landau level mixing through an effective three-body interaction, and finite layer thickness capture certain qualitative features of the experimental results.