Termination of the Series of Fractional Quantum Hall States at Small Filling Factors

We report magneto-transport measurements on low-density, high- mobility two-dimensional electron systems in (AlGa)As/GaAs heterostructures down to Landau level filling factors of nu ` 1/18. We observe a striking activated temperature dependence in Hall and longitudinal resistivities with an abrupt onset at nu - 1/5 and continuing to the smallest filling factors. The transport coefficients are activated over this entire filling factor range. The activation energy increases approximately linear with decreasing nu and its slope depends monotonically on the two-dimensional carrier concentration. While magnetic field induced localization cannot be ruled out as the origin, several features of the results are remarkably consistent with the formation of a pinned two-dimensional Wigner crystal.