Wigner crystal versus Hall insulator

In a very-low-disorder two-dimensional hole system we observe a ul direct transition from a well developed ul high-order fractional quantum Hall state at ν=2/5 to an insulating state at 1/3<ν<2/5. As T→0, ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$→∞ for the insulating phase, while ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$→0 for the fractional quantum Hall states at ν=2/5 and 1/3, yielding quasiparticle excitation gaps of 0.60 and 3.6 K, respectively. The topology of the theoretical disorder versus magnetic field global phase diagram for this system expressly forbids the direct quantum liquid-to-insulator transition we observe, ruling out the disorder-induced ‘‘Hall insulator’’ as the explanation.