Scaling in spin-degenerate Landau levels in the integer quantum Hall effect

In a low-mobility In_xGa_1-xAs/InP heterostructure, we find that the quantized Hall resistance plateau, ρ_xy=h/ie² with i=3, starts developing only when T is below 50 mK. For higher T, the plateau and the associated minimum in the diagonal resistivity ρ_xx can be made to appear by tilting the sample with respect to the B field. We have studied the T dependence of the ρ_xx minimum and deduced the thermal-activation energy (Δ). We find that Δ is 67 times smaller than the Landau-level broadening deduced from the electron mobility. This result implies that there is no gap in the density of states and that, for T>50 mK, the Landau level is spin degenerate. In this higher T range, we observe a T^-0.21 power law for the maximum value of dρ_xy/dB between the i=2 and 4 plateaus, and T^-0.42 for the minimum value of d²ρ_xx/dB². These exponents are a factor 2 smaller than those for spin-polarized levels.