Inelastic Light Scattering by Gap Excitations of Fractional Quantum Hall States at 1/3 <= \nu <= 2/3

Abstract

We report observations of collective gap excitations of the fractional quantum Hall (FQH) states at filling factors \nu=p/(2p+1) (p=integer), for 1/3 <= \nu <= 2/3, by inelastic light scattering. The collective gap energies at \nu= 1/3, 2/5 and 3/7 show a drastic decrease as the value \nu =1/2 is approached. These energies and the one at \nu=3/5 display the linear scaling with (e^2/\epsilon l_o)/|2p+1| that is characteristic of composite fermions in Chern-Simons gauge fields. In a narrow range of \nu centered at 1/2, where the FQH gaps collapse, we observe a new excitation mode which exists only at temperatures below 150 mK.