Inelastic light scattering by gap excitations of fractional quantum hall states at 1/3 ≤ v ≤ 2/3

We report observations of collective gap excitations of the fractional quantum Hall (FQH) states at filling factors v = p/(2p + 1)(p = integer), for 1/3≤v≤2/3, by inelastic light scattering. The collective gap energies at v = 1/3, 2/5, and 3/7 show a drastic decrease as the value v = 1/2 is approached. These energies and the one at v = 3/5 display the linear scaling with (e²/εl₀)/[2p + 1] that is characteristic of composite fermions in Chern-Simons gauge fields. In a narrow range of v centered at 1/2, where the FQH gaps collapse, we observe a new excitation mode which exists only at temperatures below 150 mK.