Observation of a ν=1/2 fractional quantum Hall state in a double-layer electron system

We report the observation, for the first time, of a fractional quantum Hall state at {ν}=1/2 Landau-level filling in a low disorder, double-layer electron system realized in a 680-{}-wide GaAs/AlGaAs single quantum well. A nearly vanishing diagonal resistance and a Hall resistance quantized at 2h/e² to within 0.3% are observed at {}15 T and {}26 mK. The activated temperature dependence of the diagonal resistance minimum yields a quasiparticle excitation energy gap of 230 mK.