Developing fractional quantum Hall states at ν=1/7 and ν=2/11 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 () 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 =1/7,2/13, and 2/11, which fall in the =/(6 +/- 1) Jain series of FQHSs, interpreted as integer (=1, 2) QHSs of six-flux composite fermions (6CFs). 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 (*similar or equal to 0.45) and a smaller dielectric constant (similar or equal to 10(0)) compared to GaAs 2D electrons (*similar or equal to 0.067 and similar or equal to 13(0)). This combination of * and renders the Landau-level mixing parameter , defined as the ratio of the Coulomb and cyclotron energies, similar or equal to 9 times larger in AlAs 2DESs (proportional to*/). Qualitatively similar to the GaAs 2DESs, we observe an insulating behavior reentrant around a strong =1/5 FQHS, and extending to <1/5. Additionally, we observe a clear minimum in magnetoresistance at =2/11, and an inflection point at =1/7 which is very reminiscent of the first report of an emerging FQHS at =1/7 in GaAs 2DESs. The data provide evidence for developing QHSs of (6)CFs at very small fillings. This is very surprising because near similar or equal to 1/6 in our sample is very large (similar or equal to 4), and larger 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 6CFs at extremely small fillings near =1/6.