The recent discovery of the 3D quantum Hall effect in HfTe5 has also revealed puzzling signatures of possible 3D fractionalization. Beyond the first plateau associated with the lowest Landau band, Hall conductivity exhibits a second plateau with a value of about 3/5 of the first, accompanied by a suppressed longitudinal resistivity. Here, we attribute this second plateau to an insulating ground state arising from spin-density-wave order. We show that a magnetic-field-driven Lifshitz transition causes the spin-down holelike zeroth Landau band to cross the Fermi energy and that the resulting nesting between the lowest spin-up and spin-down Landau bands induces a spin-density wave. We calculate the Hall and longitudinal resistivity and reproduce the experimental behaviors. Our renormalization-group analysis further supports this insulating ground state. Our work reveals that the tunability of Landau bands along the magnetic-field direction endows the 3D quantum Hall effect with a broader phenomenology than its 2D counterpart and merits further exploration.
@article{arxiv.2605.02188,
title = {3D Quantum Hall Effect with Two Distinct Plateaus},
author = {Jun-Hong Li and Yi-Yuan Chen and Peng-Lu Zhao and Hai-Zhou Lu and X. C. Xie},
journal= {arXiv preprint arXiv:2605.02188},
year = {2026}
}