The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors (ν1/6) 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=6.1×1010/cm2 and mobility μ=25×106 cm2/V s, we find a deep minimum in the longitudinal magnetoresistance (Rxx) at ν=1/7 when T≃104 mK. There is also a clear sign of a developing minimum in Rxx at ν=2/13. While insulating phases are still predominant when ν1/6, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence ν=p/(2mp±1) even in the very low Landau level filling limit. The magnetic-field-dependent activation energies deduced from the relation Rxx eEA/2kT corroborate this view and imply the presence of pinned Wigner solid states when ν≠p/(2mp±1). Similar results are seen in another sample with a lower density, further generalizing our observations.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)