Competing many-body phases at small fillings in ultrahigh-quality GaAs two-dimensional hole systems: Role of Landau level mixing

The fractional quantum Hall state (FQHS), an incompressible liquid state hosting anyonic excitations with fractional charge and statistics, represents a compelling many-body phase observed in clean two-dimensional (2D) carrier systems. The expected non-Abelian nature of the FQHSs at even-denominator Landau level (LL) fillings has particularly sparked considerable recent interest. At sufficiently small fillings, another exotic phase, namely a quantum Wigner crystal (WC) state, dominates. Here, we report magnetotransport measurements in an ultrahigh-quality GaAs 2D hole system where the large hole effective mass leads to a significant LL mixing (LLM) even at very high magnetic fields and affects the many-body states at very small fillings. We observe numerous developing FQHSs at both even- and odd-denominator fillings, deep in the insulating regime at $\ensuremath{\nu}\ensuremath{\lesssim}1/3$ where WC states dominate. The FQHSs we observe at odd-denominator fillings on the flanks of $\ensuremath{\nu}=1/4$ and 1/6 are consistent with the Abelian Jain sequence of four-flux and six-flux composite fermions, while the ones at even-denominator fillings $\ensuremath{\nu}=1/4$ and 1/6 are likely non-Abelian states emerging from the pairing of these quasiparticles induced by severe LLM. Our results demonstrate that the competition between the FQHSs and WC phases is close at very small fillings even in the presence of severe LLM. We also measure activation energies of WC states near $\ensuremath{\nu}=1/6$, and find that they are substantially larger than what has been reported for ultrahigh-quality GaAs 2D electrons. A moderate LLM is believed to lower the activation energy associated to the formation of WC intrinsic defects. The surprisingly large activation energy for our 2DHS with significant LLM is therefore puzzling, and may suggest a different type of intrinsic WC defect compared to that in 2D electrons.