Magnetic monopole field exposed by electrons

Abstract

Magnetic monopoles continue to be elusive. However, an experiment now shows that the interaction of an electron beam with the tip of a nanoscopically thin magnetic needle—a close approximation to a magnetic monopole field—generates an electron vortex state, as expected for a true magnetic monopole field. The experimental search for magnetic monopole particles1,2,3 has, so far, been in vain. Nevertheless, these elusive particles of magnetic charge have fuelled a rich field of theoretical study4,5,6,7,8,9,10. Here, we created an approximation of a magnetic monopole in free space at the end of a long, nanoscopically thin magnetic needle11. We experimentally demonstrate that the interaction of this approximate magnetic monopole field with a beam of electrons produces an electron vortex state, as theoretically predicted for a true magnetic monopole3,11,12,13,14,15,16,17,18. This fundamental quantum mechanical scattering experiment is independent of the speed of the electrons and has consequences for all situations where electrons meet such monopole magnetic fields, as, for example, in solids. The set-up not only shows an attractive way to produce electron vortex states but also provides a unique insight into monopole fields and shows that electron vortices might well occur in unexplored solid-state physics situations.