Direct evidence of the roton-minima in the spectrum of the Fractional Quantum Hall Effect gap excitations.

Igor Kukushkin

Russian Academy of Sciences, Russia

Dispersions of neutral gap excitations under condition of the Fractional Quantum Hall Effect at filling factors 2/5, 3/7 and 4/9 are measured by combined microwave SAW resonance. Roton minima in the gap-excitation spectra are directly established and it is shown that the number of such roton-minima is dictated by the numerator of the studied fraction. The characteristic masses of roton-excitations are evaluated and they are found to increase with electron density. The values of the FQHE gap-excitations at zero momentum are evaluated for the 3/7 and 4/9 fractional states.