Using small wavelength surface acoustic waves (SAW) on ultrahigh mobility heterostructures, Fermi surface properties are detected at $5/2$ filling factor at temperatures higher than those at which the quantum Hall state forms. An enhanced conductivity is observed at $5/2$ 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 $5/2$ ground state.

• Received 8 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.066801