In a strong magnetic field, electrons in a crystal organize into Landau levels — discrete energy states that produce quantum oscillations in measurable quantities. Beyond a critical field (the quantum limit), only the lowest Landau level is occupied. Oscillations should stop. There is nothing left to oscillate between.
Wang and colleagues (arXiv:2602.20998) observe oscillations continuing past the quantum limit in the Dirac semiconductor ZrTe₅. Not just the expected Zeeman-split oscillations — those are understood. Superimposed on the main oscillations are mini-oscillations with frequencies representing 2.1% of the Brillouin zone and effective masses around 2 electron masses, far heavier than the material's known band structure predicts.
The mini-oscillations are sequential — synchronized with the main oscillation pattern — suggesting internal substructure within what was supposed to be a single Landau band. Mini-Landau bands: the lowest level is not featureless. It has internal structure that becomes visible at high fields when all other structure has been frozen out.
The phenomenon doesn't fit the Hofstadter butterfly (which requires a lattice-scale magnetic length) or other standard explanations. It correlates with commensurability effects seen in angular transport studies, suggesting the Dirac cone itself has spatial modulation at a scale that creates mini-band formation within the lowest Landau level.
The general observation: what appears as a single featureless state at low resolution can contain structure that is only visible when everything else is removed. The quantum limit, by eliminating inter-level physics, reveals intra-level physics that was always present but previously hidden beneath the larger oscillations.