Correlation, pairing, and crystallization of 2D electron-flux composite fermions near ν = 1/6 filling

The fate of two-dimensional electron systems (2DESs) at very low Landau level filling factors (ν << 1) has been of fundamental interest and under extensive debate for decades. The dominant electron-electron Coulomb interaction can lead to numerous, exotic, many-body phenomena, e.g., fractional quantum Hall states (FQHSs), Wigner crystal, and composite fermion (CF) Fermi sea. Our experiments on ultrahigh-quality, dilute GaAs 2DESs reveal very rich physics in the extreme quantum limit. Remarkably, we observe a pronounced, sharp, minimum in magnetoresistance at the even-denominator filling ν = 1/6 superimposed on an insulating background. The activation energies determined from the temperature dependence of the longitudinal resistance also reveal a clear minimum at ν = 1/6, qualitatively similar to what we observe for the ν = 1/7 FQHS. Our data signal a developing even-denominator FQHS in close competition with Wigner crystal states near ν = 1/6. On the flanks of ν = 1/6, a sequence of odd-denominator FQHSs are observed at ν = p/(6p±1) (p = 1, 2, 3). These are the Jain-sequence FQHSs of 6-flux CFs emanating from ν = 1/6, analogous to the standard Jain-sequence FQHSs of 2-flux and 4-flux CFs observed on the flanks of CF Fermi seas. Our results reveal a close competition between three fundamentally different ground states of 6-flux CFs in the extreme quantum limit: a BCS-type, paired state (incompressible liquid), a pinned Wigner crystal (insulating, solid), and a correlated Fermi sea (compressible, liquid).