Recently, the team of Professor Lingjie Du from the School of Physics published a research paper in the journal Nature, with the title “Evidence for chiral graviton modes in fractional quantum Hall liquids”. They utilized resonant inelastic scattering of circularly polarized light to study low-energy collective excitations of the fractional quantum Hall liquids in a gallium arsenide quantum well, and for the first-time observed graviton modes that are condensed-matter analogues of gravitons (hypothetical spin-2 bosons as pointed out by Fierz and Pauli in 1939). The graviton modes in fractional quantum Hall liquids manifest as chiral spin-2 long-wavelength magnetorotons. This significant discovery also has crucial importance for understanding new correlated quantum physics and for the realization of topological quantum computers. Mr. Jiehui Liang and Dr. Ziyu Liu are the first authors contributing equally, and the corresponding author is Prof. Lingjie Du. (https://www.nature.com/articles/s41586-024-07201-w)

The abstract of the paper is as follows:

Exotic physics could emerge from interplay between geometry and correlation. In fractional quantum Hall (FQH) states, novel collective excitations called chiral graviton modes (CGMs) are proposed as quanta of fluctuations of an internal quantum metric under a quantum geometry description. Such modes are condensed-matter analogues of gravitons that are hypothetical spin-2 bosons. They are characterized by polarized states with chirality of +2 or −2, and energy gaps coinciding with the fundamental neutral collective excitations (namely, magnetorotons) in the long-wavelength limit. However, CGMs remain experimentally inaccessible. Here we observe chiral spin-2 long-wavelength magnetorotons using inelastic scattering of circularly polarized lights, providing strong evidence for CGMs in FQH liquids. At filling factor v = 1/3, a gapped mode identified as the long-wavelength magnetoroton emerges under a specific polarization scheme corresponding to angular momentum S = −2, which persists at extremely long wavelength. Remarkably, the mode chirality remains −2 at v = 2/5 but becomes the opposite at v = 2/3 and 3/5. The modes have characteristic energies and sharp peaks with marked temperature and filling-factor dependence, corroborating the assignment of long-wavelength magnetorotons. The observations capture the essentials of CGMs and support the FQH geometrical description, paving the way to unveil rich physics of quantum metric effects in topological correlated systems.

Link to the paper: https://www.nature.com/articles/s41586-024-07201-w