High-quality two-dimensional electron system confined in an AlAs quantum well

We report the fabrication and characterization of a high-quality two-dimensional electron system in the X-point valley of an AlAs quantum well. The modulation doped structure has a density of n_s=2.5×10 ¹¹ cm^-2 and low-temperature mobility μ=3×10 ⁴ cm²/V s. Cyclotron resonance data reveal an effective mass m_c=0.46m₀, indicating that the X-point conduction valleys with heavy in-plane mass are occupied. In the magnetotransport data, we observe quantum Hall states at consecutive integral Landau-level fillings (ν), implying that the degeneracy of these valleys is lifted. Our data at high magnetic fields show well-developed fractional quantum Hall states at ν=1/3 and 2/3 with a gap of ^1/3Δ=1.3K for the ν=1/3 state at B≊30 T.