Transport anomalies in the lowest Landau level of two-dimensional electrons at half-filling.

We observe deep minima in the diagonal resistivity, rho sub (xx), of a two-dimensional electon system in GaAs at Landau level filling factor upsilon=1/2 and 3/2. The anomalies saturate at nonzero values of rho sub (xx) at T - O and persist up to temperatures of ~10K. In spite of the strength of the rho sub (xx) features, no indication of plateau development is visible in the Hall resistance rho sub (xy). These structures at half- filling appear to have their origin in a novel many-particle state distinctly different from the fractional quantum Hall effect.