Observation of fractionally quantized anomalous Hall effect

The integer quantum anomalous Hall (QAH) effect is a lattice analogue of the quantum Hall effect at zero magnetic field^1-3. This phenomenon occurs in systems with topologically non-trivial bands and spontaneous time-reversal symmetry breaking. Discovery of its fractional counterpart in the presence of strong electron correlations, that is, the fractional QAH effect^4-7, would open a new chapter in condensed matter physics. Here we report the direct observation of both integer and fractional QAH effects in electrical measurements on twisted bilayer MoTe₂. At zero magnetic field, near filling factor ν = −1 (one hole per moiré unit cell), we see an integer QAH plateau in the Hall resistance R_xy quantized to h/e² ± 0.1%, whereas the longitudinal resistance R_xx vanishes. Remarkably, at ν = −2/3 and −3/5, we see plateau features in R_xy at 3/2 h /e²±1 % and 5/3 h /e²±3 % , respectively, whereas R_xx remains small. All features shift linearly versus applied magnetic field with slopes matching the corresponding Chern numbers −1, −2/3 and −3/5, precisely as expected for integer and fractional QAH states. Additionally, at zero magnetic field, R_xy is approximately 2h/e² near half-filling (ν = −1/2) and varies linearly as ν is tuned. This behaviour resembles that of the composite Fermi liquid in the half-filled lowest Landau level of a two-dimensional electron gas at high magnetic field^8-14. Direct observation of the fractional QAH and associated effects enables research in charge fractionalization and anyonic statistics at zero magnetic field.