Evidence for fractional quantum Hall state of 6-flux composite fermions in a system with significant Landau level mixing

Termination of the fractional quantum Hall effect (FQHE) states and the emergence of Wigner crystal phases at very low Landau level filling factors (ν<<1) has been of continued interest for decades [1,2,3]. Recently, in ultra-high-quality, dilute GaAs 2D electron systems, Chung et al. [3] reported strong evidence for FQHE at ν=1/7, 2/13, 2/11 which fall in the ν=p/(6p±1) Jain series of FQHE states, interpreted as integer (p = 1, 2) quantum Hall effect of 6-flux composite fermions (⁶CFs). These states are surrounded by strongly-insulating phases which are generally believed to be Wigner crystals. Here, we study a record high quality 2D electron system confined to a 35-nm-wide AlAs quantum well with density 9.6×10¹⁰ cm^-2 [4]. The AlAs 2D electrons have a much larger effective mass (≈0.45m_e) and a smaller dielectric constant (≈10) compared to GaAs 2D electrons. Qualitatively, similar to the GaAs 2D electrons, we observe a strong insulating behavior setting at ν<1/5 , as well as a clear minimum in magnetoresistance at ν=2/11 and an inflection at ν=1/7 which is very reminiscent of the first report of FQHE at ν=1/7 [1]. The data provide evidence for developing FQHE states of ⁶CFs at very small fillings. This is very surprising since in our sample the Landau level mixing parameter κ, defined as the ratio of the Coulomb and cyclotron energies, is ≈4 and larger Landau level mixing has the tendency to favour Wigner crystal over FQHE in the lowest Landau level [5,6]. Our data should inspire theory calculations that accurately incorporate the role of Landau level mixing in competing many-body phases of ⁶CFs at extremely low fillings around ν=1/6.

References –

1.V. J. Goldman et al., Phys. Rev. Lett. 61, 881 (1988).

2.W. Pan et al., Phys. Rev. Lett. 88, 176802 (2002).

3.Y. J. Chung et al., Phys. Rev. Lett. 128, 026802 (2022).

4.Y. J. Chung et al., Phys. Rev. Materials 2, 071001(R) (2018).

5.J. Zhao et al., Phys. Rev. Lett. 121, 116802 (2018).

6.K. A. Villegas Rosales et al., Phys. Rev. Research 3, 013181 (2021).