Observation of a =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 =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.