​Graphene is a two-dimensional electronic system made up of a monoatomic layer of carbon. If a strong magnetic field is applied to it, the electrons spread out across well identified energy levels that are fully spin polarized, in a regime known as the quantum Hall effect.

By adjusting the electron density and the magnetic field, it is possible to obtain a perfect two-dimensional ferromagnet in which all electron spins point in the same direction. In this case, the electron propagation is limited to the edges of the sample, and the inside becomes globally insulating.

In this kind of ferromagnet, it is possible to produce, by applying a well-chosen potential, spin waves or magnons (which are oscillations of electronic spins), without displacing the electric charges. According to the theoreticians, these magnetic excitations should exhibit an electric dipole.

To verify this, the Iramis researchers probed magnons with an electronic Mach-Zehnder interferometer, carried out in a graphene p-n junction.

The desired dipolar effect unbalances the two arms of the interferometer, introducing a clearly visible phase shift. Furthermore, the deterioration in the contrast of the interference pattern indicates the random emission of magnons.

The upshot of this experimental proof is that it has become possible to control magnons with an electric field, and not just with a magnetic field, which is less easy to manipulate. This actually opens up a whole new experimental field, based on a new class of quantum circuits.

This work was a collaboration between the IPhT and Japanese researchers (NTT Basic Research Laboratories and the National Institute for Materials Science).