Semicircle: An exact relation in the Integer and Fractional Quantum Hall   Effect
  We present experimental results on the quantized Hall insulator in two dimensions. This insulator, with vanishing conductivities, is characterized by the quantization (within experimental accuracy) of the Hall resistance in units of the quantum unit of resistance, h/e^2. The measurements were performed in a two dimensional hole system, confined in a Ge/SiGe quantum well, when the magnetic field is increased above the nu=1 quantum Hall state. This quantization leads to a nearly perfect semi-circle relation for the diagonal and Hall conductivities. Similar results are obtained with a higher mobility n-type modulation doped GaAs/AlGaAs sample, when the magnetic field is increased above the nu=1/3 fractional quantum Hall state. 
M. Hilke, D. Shahar, S.H. Song, D.C. Tsui, Y.H. Xie and M. Shayegan   (Princeton University)