Fractional Quantum Hall Effect at $\nu=1/2$ in Hole Systems Confined to   GaAs Quantum Wells
  We observe fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor $\nu=1/2$ in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayer-like charge distributions. The $\nu=1/2$ FQHE is stable when the charge distribution is symmetric and only in a range of intermediate densities, qualitatively similar to what is seen in two-dimensional electron systems confined to approximately twice wider GaAs quantum wells. Despite the complexity of the hole Landau level structure, originating from the coexistence and mixing of the heavy- and light-hole states, we find the hole $\nu=1/2$ FQHE to be consistent with a two-component, Halperin-Laughlin ($\Psi_{331}$) state. 
Yang Liu, A.L. Graninger, S. Hasdemir, M. Shayegan, L.N. Pfeiffer,   K.W. West, K.W. Baldwin, R. Winkler
cond-mat.mes-hall cond-mat.str-el