Quantum Hall effect in a triple-layer electron system

We report magnetotransport measurements in a low-disorder three-layer electron system in a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As triple quantum-well structure. Fine adjustment of densities in different layers is achieved with the use of front and back gates and by monitoring the capacitance-voltage and low-field Shubnikov–de Haas data. At intermediate fields the diagonal resistance (${\mathit{R}}_{\mathit{x}\mathit{x}}$) shows strong minima at every third integer filling factor, ν=3, 6, 9, and 12, consistent with the subband structure. At higher fields, we observe deep ${\mathit{R}}_{\mathit{x}\mathit{x}}$ minima at ν=7/5 and 5/7, suggesting anomalously strong 7/5 and 5/7 fractional quantum Hall states in this triple-layer system.