Inelastic light scattering by gap excitations of fractional quantum Hall states at 1/3 <= v <= 2/3
17 January 2000
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 less than or equal to nu less than or equal to 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(0))/textbackslash{} 2p + 1 textbackslash{} 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. PACS numbers: 73.40.Hm, 73.20.Mf, 78.30.-j.