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 $v\sim \frac{1}{18}$. We observe a striking activated temperature dependence with onset near $v\sim \frac{1}{5}$ 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.