In a laboratory at Tohoku University, physicists have for the first time made an unusual quantum phenomenon obey the commands of an electric current. The phenomenon in question is the quantum-geometric nonlinear Hall effect — a state in which the voltage in a material grows not in proportion to the current, but according to a more complex law. Until now it was believed that this "geometry" of electrons cannot be restructured from the outside, just as one cannot redraw a city map without demolishing the buildings.
The researchers worked with thin films of the antiferromagnet Mn₃Sn. In this material, the atomic spins form a special structure closely tied to the quantum geometry of the electrons. At room temperature, the scientists registered the third order of the nonlinear Hall effect: the transverse voltage turned out to be proportional to the cube of the current. Theoretical calculations confirmed that the effect is indeed generated by quantum geometry rather than by ordinary classical mechanisms.
The key step was electrical switching. When short current pulses were passed through an adjacent layer of heavy metal, a spin-orbit torque arose that rotated the magnetic structure of Mn₃Sn. Along with it, the sign of the nonlinear response also changed. A positive pulse gave one sign, a negative pulse the opposite. Thus the quantum-geometric state became controllable for the first time, like an ordinary transistor.
Imagine a river whose course can be bent instantly with a light press of a button: the water begins to flow not only forward but also sideways according to a new law. This is exactly how electrons behave in Mn₃Sn under the action of a current. This analogy helps to understand why the effect is called nonlinear: the familiar proportionality of "current to voltage" is broken, opening the way to new types of signal processing.
The discovery connects quantum geometry, nonlinear electronics and spintronics. Instead of treating quantum geometry as an immutable characteristic of a material, the scientists showed how it can be restructured by current. This opens the way to compact devices — frequency multiplexers and rectifiers that will work faster and more economically thanks to the principles of magnetoresistive memory.
The experiment was carried out on epitaxial films at room temperature and published in the journal Newton. The authors emphasize that it is now possible not only to observe but also to purposefully change quantum-geometric responses in real time.
The work lays the foundation for future electronics in which the quantum properties of materials will become not merely an object of study but a working tool controlled by an ordinary current.
