Exact quantization of the even-denominator fractional quantum Hall state at v=5/2 Landau level filling factor

We report ultralow temperature experiments on the obscure fractional quantu m Hall effect at Landau level filling factor nu = 5/2 in a very high-mobili ty specimen of mu = 1.7 X 10(7) cm(2)/V s. We achieve an electron temperatu re as low as similar to 4 mK, where we observe vanishing R-xx and. for the first time, a quantized Hall resistance, R-xy = h/(5/2)e(2) to within 2 ppm . R-xv at the neighboring odd-denominator states nu = 7/3 and 8/3 is also q uantized. The temperature dependences of the R, minima at these fractional fillings yield activation energy gaps Delta(5/2) = 0.11, Delta(7/3) = 0.10, and Delta(8/3) = 0.055 K.