Inelastic Light Scattering by Gap Excitations of Fractional Quantum Hall States at $1/3\le \mathit{\nu }\le 2/3$

We report observations of collective gap excitations of the fractional quantum Hall (FQH) states at filling factors $\nu =p/(2p+1)$ ( $p=\mathrm{integer}$), for $1/3\le \nu \le 2/3$, by inelastic light scattering. The collective gap energies at $\nu =1\mathrm{}/3$, 2/5, and 3/7 show a drastic decrease as the value $\nu =1\mathrm{}/2$ is approached. These energies and the one at $\nu =3\mathrm{}/5$ display the linear scaling with $\left(e^2/\epsilon l_0\right)/|2p+1|$ 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.