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High-frequency surface acoustic waves (>2 GHz, 2) have been propagated on high mobility AlₗGa₁-ₗAs/GaAs heterostructures, allowing measurement of the finite-wave-vector conductivity ₗₗ (q, ). We find enhanced ₗₗ (q, ) at multiple even-denominator filling factors =1/2, 3/2, 1/4, and 3/4 with ₗₗ (q, ) increasing linearly in q. The enhanced ₗₗ (q, ) at =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.
Willett et al. (Mon,) studied this question.