Examining composite fermions with surface acoustic waves

Fermi surface effects of composite fermions (CF) in a two-dimensional electron system are exposed by using large-wavevector surface acoustic waves (SAW) to probe the system conductivity. Enhanced conductivity is observed at even-denominator filling factors for SAW wavelengths smaller than the CF mean free path. Larger-wavevector SAW demonstrate geometric resonance with the CF cyclotron orbit, allowing direct measurement of the CF Fermi wavevector. Emphasized here are recent geometric resonance observations which display an apparent inconsistency between the effective mass as measured in d.c. transport experiments and the mass necessary to resolve the geometric resonances. Using these same high-frequency SAW, a geometric resonance structure at v = 3/2 defines a composite fermion Fermi wavevector consistent with a spin-polarized CF population: this is at odds with d.c. transport measurements. Present efforts at determining composite fermion structure using large-wavevector SAW will also be discussed.