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## Anomalous spin exciton with a magnetoroton minimum in a quantum Hall ferromagnet at a filling factor ν = 2

In ZnO-based two-dimensional electron systems with strong Coulomb interaction, the anomalous spin-exciton

branch is revealed. As probed by inelastic light scattering, in a ferromagnetic quantum-Hall state with the filling

factor ν = 2, a spin exciton has a negative momentum dispersion with steepness dependent on the electron

density. The negative dispersion of the spin exciton is associated with its interaction with higher-energy spin-flip

modes that exist near ν = 2. Surprisingly, the anti-Stokes light scattering on spin excitons at ferromagnetic state

ν = 2 is amplified by orders of magnitude, indicating the macroscopic accumulation of these excitations. The

experimental findings are confirmed by the exact diagonalization method—these calculations show a magnetoroton

minimum in the dispersion of spin excitons and also the attractive interaction between magnetoroton spin

excitations.