Observation of Multiple Magnetorotons in the Fractional Quantum Hall Effect

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 ν0ex0ex=0ex0ex2/5, while a single deep minimum is resolved at ν0ex0ex=0ex0ex1/3. 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 ν>1/3.