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 \ensuremath{\nu}=2+(1/2= 5) / 2 and at temperatures below 100 mK, a fractional Hall plateau develops at ${\mathrm{\ensuremath{\rho}}}_{\mathrm{xy}}$=(h/${\mathrm{e}}^{2}$)/(5/2 defined to better than 0.5%. Equivalent even-denominator quantization is absent in the lowest Landau level under comparable conditions.