Fractional quantum Hall effect at low temperatures

We report a systematic study of the 2/3 fractional quantum Hall effect at low temperatures (65-770 mK) for a GaAs-AlₓGa_1-xAs sample of very high mobility (10⁶ cm²/V sec). We find the 2/3 Hall plateau to be accurately quantized. The diagonal and Hall resistivities are observed to be activated at each given filling factor ν=nh/eB around 2/3. The activation energy has a maximum value, Δₘₐₓ, at ν=2/3 and decreases to each side as ν moves away. By varying the sample mobility and density simultaneously with a backgate bias, we find Δₘₐₓ strongly mobility and magnetic field dependent.