Evolution of 7/2 fractional quantum Hall state in two subband systems*

We report the evolution of the fractional quantum Hall state (FQHS) at total Landau level (LL) filling factor $\nu=7/2$ in wide GaAs quantum wells in which electrons occupy two electric subbands. The data reveal subtle and distinct evolutions as a function of density, magnetic field tilt-angle, or symmetry of the charge distribution. At intermediate tilt angles, for example, we observe a strengthening of the $\nu=7/2$ FQHS. Moreover, in a well with asymmetric change distribution, there is a developing FQHS when the LL filling factor of the symmetric subband $\nu_S$ equals 5/2 while the antisymmetric subband has filling $1<\nu_A<2$.