Evolution of the 7/2 fractional quantum hall state in two-subband systems

We report the evolution of the fractional quantum Hall state (FQHS) at a total Landau level (LL) filling factor of ν=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 ν=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 ν _S equals 5/2 while the antisymmetric subband has a filling factor of 1<ν _A<2.