ENHANCED FINITE-WAVE-VECTOR CONDUCTIVITY AT MULTIPLE EVEN-DENOMINATOR FILLING FACTORS IN 2-DIMENSIONAL ELECTRON-SYSTEMS

High-frequency surface acoustic waves ( > 2 GHz, lambda < 2 mum) have been propagated on high mobility AlxGa1-xAs/GaAs heterostructures, allowing measurement of the finite-wave-vector conductivity sigma(xx) (q,omega). We find enhanced sigma(xx) (q,omega) at multiple even-denominator filling factors nu = 1/2, 3/2, 1/4, and 3/4 with sigma(xx) (q,omega) increasing linearly in q. The enhanced sigma(xx) (q,omega) at nu = 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 nu.