The surface of a crystal is like a stage where quantum effects are meant to perform. In MnBi2Te4, a magnetic topological insulator, special electronic states are expected to run along the surface, and the magnetic order opens an energy gap in them: a forbidden zone that makes transport quantized. But in real samples, the show never starts: the gap nearly vanishes, and the effect is only visible at temperatures below one kelvin.

The culprits are defects in which manganese and bismuth atoms swap places in the crystal lattice. This rearrangement lowers the electric potential at the surface, and the topological states, like actors driven off the stage, retreat into the crystal's interior. There, the magnetic influence is weaker, the gap closes, and the quantum anomalous Hall effect disappears.

The authors of the study proposed bringing the actors back into the spotlight. If an external electric field is applied, the surface potential rises, and the states resurface. Calculations show that a field with a potential of just 0.1 volts restores the gap to 11–24 millielectronvolts, depending on how the field is distributed in space.

The most intriguing part is that the field direction acts as a switch. By changing its sign, one can shift the system from an axion insulator state, where the Hall conductivity is zero, to a Chern insulator state with a quantized value of e²/h. This means that without any magnetic field, purely through electrical means, one can control quantized transport.

Thus, the stage from which defects chased the actors comes alive again. The electric field returns the states to the surface, opens the magnetic gap, and gives engineers a tool for future topological transistors and memory elements.