Correlated States of 2D Electrons near the Landau Level Filling v=1/7

The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors (v less than or similar to 1/6) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultrahigh-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density n = 6.1 x 10(10)/cm(2) and mobility is mu = 25 x 10(6) cm(2)/V s, we find a deep minimum in the longitudinal magnetoresistance (R-xx) at v = 1/7 when T similar or equal to 104 mK. There is also a clear sign of a developing minimum in R-xx at v = 2/13. While insulating phases are still predominant when v less than or similar to 1/6, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence v = p/(2mp +/- 1) even in the very low Landau level filling limit. The magnetic-field-dependent activation energies deduced from the relation R-xx proportional to e(EA/2kT) corroborate this view and imply the presence of pinned Wigner solid states when v not equal p/(2mp +/- 1). Similar results are seen in another sample with a lower density, further generalizing our observations.