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 nu = -1 (one hole per moire unit cell), we see an integer QAH plateau in the Hall resistance R-xy quantized to h/e(2) +/- 0.1%, whereas the longitudinal resistance R-xx vanishes. Remarkably, at nu = -2/3 and -3/5, we see plateau features in R-xy at 3/2h/e(2) +/- 1% and 5/3h/e(2) +/- 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/e(2) near half-filling (nu = -1/2) and varies linearly as nu 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.