Spin-Polarization of Composite Fermions and Particle-Hole Symmetry Breaking

We study the critical spin-polarization energy ($\alpha_{\rm C}$) 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 $\alpha_{\rm C}$ as we increase the well-width. In systems with comparable electron layer thickness, $\alpha_{\rm C}$ 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.