Lingjie Du

Gravitons are hypothetical neutral particles possessing total spin of 2. By analogy, the quanta of the geometric fluctuations of the fractional quantum Hall (FQH) states can be viewed as graviton modes. At filling factor v = 1/3 of the FQH liquid, a graviton mode characterized by spin angular momentum -2, i.e., being chiral, was proposed to emerge as a neutral chiral excitation in the long-wavelength limit of magnetoroton. However, experimental identifications of graviton modes in the FQH liquids are still absent. Here, we observe the chiral long-wavelength magnetoroton at /v/ = 1/3 by circularly polarized resonant inelastic light scattering. We find that the long-wavelength magnetoroton mode emerges under a specific polarization scheme corresponding to a well-defined spin of -2, while a finite-momentum mode at the energy minimum of magnetoroton does not carry certain spins. The chiral mode has a sharp resonance peak with width < 30 μeV, confirming its long-wavelength nature. Moreover, it rapidly quenches with raising temperatures and changing filling factors. At v = 2/3, we observe the long-wavelength magnetoroton mode carries a spin of +2. The results reveal the chiral nature of the long-wavelength magnetoroton at v = 1/3 and v = 2/3, providing experimental evidence of the chiral graviton modes in the FQH liquids. Our method probing the spin component of graviton modes would provide a powerful way to identify the nature of the v = 5/2 state.