Exact Quantization of Even-Denominator Fractional Quantum Hall State at nu=5/2 Landau Level Filling Factor

We report ultra-low temperature experiments on the obscure fractional quantum Hall effect (FQHE) at Landau level filling factor mu=5/2 in a very high mobility specimen of nu=1.7x10 sup 7 cm sup 2/Vs. We achieve an electron temperature as low as ~4mK, where we observe vanishing R sub (xx) and, for the first time, a quantized Hall resistance, R sub (xy) = h/(5/2e sup 2) to within 2 ppm. R sub (xy) at the neighboring odd-denominator states nu=7/3 and 8/3 is also quantized. The temperature dependences of the R sub (xx)-minima at these fractional fillings yield activation energy gaps DELTA sub (5/2)=0.11K, DELTA sub (7/3)=0.10K, and DELTA sub (8/3)=0.055K.