Experimental Demonstration of Fermi Surface Effects at Filling Factor [Formula presented]

Using small wavelength surface acoustic waves (SAW) on ultrahigh mobility heterostructures, Fermi surface properties are detected at [Formula presented] filling factor at temperatures higher than those at which the quantum Hall state forms. An enhanced conductivity is observed at [Formula presented] by employing sub-0.5-μm SAW, indicating a quasiparticle mean-free path substantially smaller than that in the lowest Landau level. These findings are consistent with the presence of a filled Fermi sea of composite fermions, which may pair at lower temperatures to form the [Formula presented] ground state.