Fractional quantum Hall ferromagnetism and disappearing insulating phases near a Landau level crossing in the extreme quantum limit

When the two lowest Landau levels (LLs) in a two-dimensional (2D) carrier system cross, the resulting dominance of the exchange energy over the Zeeman energy can produce a ferromagnetic quantum Hall state (QHS). In the past, such crossings have been studied in 2D electrons in GaAs and AlAs by tuning the g factor or the valley splitting by applying hydrostatic pressure or uniaxial strain, respectively. GaAs 2D holes are unique thanks to their strong heavy-hole light-hole mixing and spin-orbit coupling which result in LLs that are highly nonlinear and exhibit numerous crossings as a function of magnetic field, including one between the two lowest-energy LLs with opposite pseudospins. It was previously demonstrated that such a crossing could be gate-tuned to occur at LL filling factor ν = 1 where a robust ferromagnetic QHS was observed [1]. In this work, we study the same crossing in the extreme quantum limit near ν = 1/3. When the crossing is gate-tuned to occur at ν = 1/3, we observe a clear and sharp dip in the measured energy gap for the ν = 1/3 fractional QHS. The 1/3 gap does not close at the crossing, signaling ferromagnetism of the fractional QHS. Additionally, we find that the re-entrant insulating phase between ν = 1/3 and 2/5 disappears near the crossing and re-appears when the crossing is tuned away.

[1] M.K. Ma et al., Phys. Rev. Lett. 129, 196801 (2022).