Reentrant Landau levels in a Dirac topological insulator

Pentatellurides have emerged as an ideal platform for exploring topological phase transitions and their electronic properties. Positioned at the boundary between a strong and weak topological insulator phase, their low carrier density and linear dispersion enable a three-dimensional Dirac Hamiltonian description. A complete understanding of these materials remains elusive due to unexplained sample variability. In particular, these materials have been shown to host anomalous magnetoresistance including $\log \left(B\right)$ -periodic signals, non- $\log \left(B\right)$ and non-1/B oscillations, in addition to the conventional 1/B quantum oscillations. Differing interpretations invoke many-body effects and there is currently no unified understanding of these oscillations. Here we report non-1/B oscillations in ZrTe₅ down to 700 mK and up to 60 T, with a temperature and magnetic field dependence that violates the Lifshitz-Kosevich framework. We show that nonlinear Landau-level back-bending, arising from the interplay of cyclotron energy and strong spin-orbit coupling in a non-interacting Dirac system, captures all observed regimes and provides a foundation for understanding the electron dynamics of Dirac topological insulators beyond the quantum limit.