Magnetorotons in the dispersions of collective gap excitation modes of fractional quantum Hall liquids are measured in resonant inelastic light scattering experiments. Two deep magnetoroton minima are observed at $\nu =2/5\mathrm{}$, while a single deep minimum is resolved at $\nu =1/3\mathrm{}$. The observations are the first evidence of multiple roton minima in gap excitations of the quantum liquids. The results support Chern-Simons and composite fermion calculations that predict multiple roton minima for states with $\nu >1/3\mathrm{}$.

• Received 5 October 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.2637