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 {μ}m) have been propagated on high mobility Alₓ{Ga}_{1{{-}}x}$As/GaAs heterostructures, allowing measurement of the finite-wave-vector conductivity ${{{σ}}}ₓₓ(q,ω). We find enhanced{{{σ}}}ₓₓ(q,ω) at multiple even-denominator filling factors ν=1/2, 3/2, 1/4, and 3/4 with{{{σ}}}ₓₓ(q,ω) increasing linearly in q. The enhanced{{{σ}}}ₓₓ$(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}.