Termination of the series of fractional quantum hall states at small filling factors

We report magnetotransport measurements on low-density, high-mobility two-dimensional (2D) electron systems in (Al,Ga)As/GaAs heterostructures down to Landau-level filling factors of v118. We observe a striking activated temperature dependence with onset near 15 and continuing to smallest v. The activation energy increase is approximately linear with decreasing v and its slope depends monotonically on 2D density. While magnetic-field-induced localization cannot be ruled out, several features of the results are consistent with the formation of a pinned 2D Wigner crystal.