Observation of an even-denominator quantum-number in the fractional quantum Hall effect.

An even-denominator rational quantum-number has been observed in the Hall resistance of a two-dimensional electron system. At partial filling of the second Landau level v=2+1/2=5/2 and at temperatures below 100mK, a fractional Hall plateau quantized to rho sub (xy)=(h/e sup 2)/(5/2) to better than 0.5%. Equivalent even-denominator quantization is absent in the lowest Landau level under comparable conditions.