Light Scattering Spectroscopy of Low Energy Excitations in Fractional Quantum Hall Fluids

Low-energy neutral excitations encode the collective dynamics of correlated quantum phases. In the fractional quantum Hall (FQH) effect, long-wavelength neutral modes such as magnetorotons serve as distinctive fingerprints of the ground state. Their spectral features and dispersions reveal the quantum geometry and topological order of the incompressible fluids, and can be directly accessed through resonant inelastic light scattering (RILS) spectroscopy. Recently, condensed-matter analogues of gravitons called chiral gravitons are proposed under a quantum geometrical description of FQH states. They arise from fluctuations of the internal quantum metric and manifest as chiral spin-2 long-wavelength magnetorotons. Using circularly polarized RILS spectroscopy, we provide strong evidence for chiral gravitons in FQH fluids. At several FQH states in the lowest Landau level, we observed gapped modes that emerge under a specific polarization scheme corresponding to angular momentum S = -2 or +2. The modes have characteristic energies and sharp peaks with marked temperature and filling-factor dependence, corroborating the assignment of long-wavelength magnetorotons. I will further discuss opportunities to investigate exotic physics in the FQH effect from the aspect of quantum geometry.