Unveiling excitonic properties of magnons in a quantum Hall ferromagnet

TL;DR

This paper provides the first experimental evidence of electric dipole moments in magnons within a quantum Hall ferromagnet, demonstrating their potential for quantum circuit applications through interferometry in graphene.

Contribution

It reports the direct detection of magnon electric dipole moments in a graphene quantum Hall ferromagnet using a Mach-Zehnder interferometer, revealing their influence on quantum interference.

Findings

Magnons carry an electric dipole moment affecting interferometric phase and visibility.

The emission energy threshold of magnons approaches zero near filling factor v=0.

Evidence suggests a gapless mode in the canted-antiferromagnetic phase at v=0.

Abstract

Magnons enable transferring a magnetic moment or spin over macroscopic distance. In quantum Hall ferromagnet, it has been predicted in the early 90s that spin and charges are entangled, meaning that any change of the spin texture modifies the charge distribution. As a direct consequence of this entanglement, magnons carry an electric dipole moment. Here we report the first evidence of the existence of this electric dipole moment in a graphene quantum Hall ferromagnet using a Mach-Zehnder interferometer as a quantum sensor. By propagating towards the interferometer through an insulating bulk, the magnon electric dipole moment modifies the Aharonov-Bohm flux through the interferometer, changing both its phase and its visibility. In particular, we relate the phase shift to the sign of this electric dipole moment, and the exponential loss of visibility to the flux of emitted magnons. Finally, we probe the emission energy threshold of the magnons close to filling factor v=1. Approaching v=0, we observe that the emission energy threshold diminishes towards zero, which might be linked to the existence of gapless mode in the canted-antiferromagnetic (CAF) phase at v=0. The detection and manipulation of magnons based on their electric dipole open the field for a new type of coherent magnon quantum circuits that will be electrostatically controlled.