Enhanced finite-wave-vector conductivity at multiple even-denominator filling factors in two-dimensional electron systems

High-frequency surface acoustic waves (>2 GHz, λ<2 μm) have been propagated on high mobility AlxGa1-xAs/GaAs heterostructures, allowing measurement of the finite-wave-vector conductivity σxx(q,ω). We find enhanced σxx(q,ω) at multiple even-denominator filling factors ν=1/2, 3/2, 1/4, and 3/4 with σxx(q,ω) increasing linearly in q. The enhanced σxx(q,ω) at ν=1/2 persists to high temperatures (>4 K), above the temperatures at which the fractional quantum Hall effect is observed. Empirically, the effects represent a series of robust states in two-dimensional electron systems that support gapless excitations. These data are strikingly consistent with a recent theory describing Fermi-surface formation at even denominator ν.