Even Denominator Fractions in the Fractional Quantum Hall Effect

The fractional quantum Hall effect (FQHE) had until recently been observed at exclusively odd denominator filling factors, v=p/q, q odd. Our observation of Hall resistance quantization at v=5/2 demonstrated an exception to the odd denominator rule and necessitated modification of the conventional theory of the FQHE. Further experimental studies of the FQHE state at v=5/2 have been performed, and these results suggest the even denominator state is a non-spin polarized electron condensate preferentially formed in the second Landau level.