The longitudinal thermopower of ultrahigh mobility two-dimensional (2D) electrons has been measured at both zero magnetic field and at high fields in the compressible metallic state at filling factor ν=3/2 and the incompressible fractional quantized Hall state at ν=5/2. At zero field our results demonstrate that the thermopower is dominated by electron diffusion for temperatures below about T=150 mK. A diffusion-dominated thermopower is also observed at ν=3/2 and allows us to extract an estimate of the composite fermion effective mass. At ν=5/2 both the temperature and magnetic field dependence of the observed thermopower clearly signal the presence of the energy gap of this fractional quantized Hall state. We find that the thermopower in the vicinity of ν=5/2 exceeds that recently predicted under the assumption that the entropy of the 2D system is dominated by non-Abelian quasiparticle exchange statistics
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