Quantum Hall effect in a triple-layer electron system

We report magnetotransport measurements in a low-disorder three-layer electron system in a GaAs/AlxGa1-xAs 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 (Rxx) 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 Rxx 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.