Developing fractional quantum Hall states at $ν$ = $\dfrac{1}{7}$ and $\dfrac{2}{11}$ 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 ($ν$) have been of continued interest for decades. Recently, in ultra-high-quality, dilute GaAs 2D electron systems (2DESs), strong evidence was reported for FQHSs at $ν=1/7, 2/13$ and 2/11 which fall in the $ν= p/(6p\pm1)$ Jain series of FQHSs, interpreted as integer ($p = 1$, 2) QHSs of 6-flux composite fermions ($^6$CFs). 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 ($m^*\simeq 0.45 m_e$) and a smaller dielectric constant ($ε\simeq10ε_0$) compared to GaAs 2D electrons ($m^*\simeq 0.067 m_e$ and $ε\simeq13ε_0$). This combination of $m^*$ and $ε$ renders the Landau level mixing parameter $κ$, defined as the ratio of the Coulomb and cyclotron energies, $\simeq 9$ times larger in AlAs 2DESs ($κ\propto m^*/ε$). 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 $^6$CFs at very small fillings. This is very surprising because $κ$ near $ν\simeq 1/6$ in our sample is very large ($\simeq4$), and larger $κ$ has the tendency to favor Wigner crystal states over FQHSs at small fillings. Our data should inspire calculations that accurately incorporate $κ$ in competing many-body phases of $^6$CFs at extremely small fillings near $ν=1/6$.