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Gapped Excitations of unconventional FQHE states in the Second Landau Level

We report the observation of low-lying collective charge and spin excitations in the second Landau level at ν = 2 + 1/3 and also for the very fragile states at ν = 2 + 2/5, 2 + 3/8 in inelastic light scattering experiments. These modes exhibit a clear dependence on filling factor and temperature substantiating the unique access to the characteristic neutral excitation spectra of the incompressible FQHE states. A detailed mode analysis reveals low energy modes at around 70 μeV and a sharp mode slightly below the Zeeman energy interpreted as gap and spin wave excitation, respectively. The lowest energy collective charge excitation spectrum at ν = 2 + 1/3 exhibits significant similarities and a universal scaling of the energies with its cousin state in the lowest Landau level at ν = 1/3 suggesting similar underlying physics. The observed excitation spectra facilitate to distinguish between theoretical descriptions of the nature of those FQHE states. A striking polarization dependence in light scattering is discussed in the framework of anisotropic electron phases that allow for the stabilization of nematic FQHE states.