Developing fractional quantum Hall states at ν= 17 and ν= 211 in the presence of significant Landau-level mixing

Termination of the fractional quantum Hall states (FQHSs) and the emergence of Wigner crystal phases at very small Landau-level filling factors (ν) have been of continued interest for decades. Recently, in ultra-high-quality, dilute GaAs two-dimensional electron systems (2DESs), strong evidence was reported for FQHSs at ν=1/7,2/13, and 2/11, which fall in the ν=p/(6p±1) Jain series of FQHSs, interpreted as integer (p=1, 2) QHSs of six-flux composite fermions (CFs6). These states are surrounded by strongly insulating phases which are generally believed to be Wigner crystals. Here, we study an ultra-high-quality 2DES confined to an AlAs quantum well where the 2D electrons have a much larger effective mass (m∗≃0.45me) and a smaller dielectric constant (ϵ≃10ϵ0) compared to GaAs 2D electrons (m∗≃0.067me and ϵ≃13ϵ0). This combination of m∗ and ϵ renders the Landau-level mixing parameter κ, defined as the ratio of the Coulomb and cyclotron energies, ≃9 times larger in AlAs 2DESs (κ∝m∗/ϵ). Qualitatively similar to the GaAs 2DESs, we observe an insulating behavior reentrant around a strong ν=1/5 FQHS, and extending to ν<1/5. Additionally, we observe a clear minimum in magnetoresistance at ν=2/11, and an inflection point at ν=1/7 which is very reminiscent of the first report of an emerging FQHS at ν=1/7 in GaAs 2DESs. The data provide evidence for developing QHSs of CFs6 at very small fillings. This is very surprising because κ near ν≃1/6 in our sample is very large (≃4), and larger κ has the tendency to favor Wigner crystal states over FQHSs at small fillings. Our data should inspire calculations that accurately incorporate the role of Landau-level mixing in competing many-body phases of CFs6 at extremely small fillings near ν=1/6.