Experimental demonstration of a Fermi surface at one-half filling of the lowest Landau level

We report the first direct observation of the cyclotron motion of new quasiparticles in a 2D electron system near 1/2 filling factor. Just as electrons in a metal geometrically resonate with a sound wave when a magnetic field is applied perpendicular to the sound propagation direction, near 1/2 filling factor we observe resonance of a surface sound wave with cyclotron orbits of charge carriers. The presence of a Fermi surface and geometric resonance of a sound wave with Chern-Simons gauge transformed fermions were explicitly predicted in a recent theory of the half filled Landau level.