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Curated by RSF Research Staff

A new way to create a magnetic monopole

In Maxwell’s theory, there was only one piece missing for a perfect symmetry between the electric and magnetic forces, the magnetic monopoles. As theoretically demonstrated by Dirac, the existence of this single magnetic monopole would explain quantization of electric charge everywhere in the Universe. Since then, the search of such magnetic monopoles—as real elementary particles or effective quasiparticles—has been a great preoccupation for physicists and it’s of great importance. Their existence or discovery could lead to the unification of the fundamental interactions, another big problem of our century.

Despite the lack of experimental evidence for elementary monopoles in nature, magnetic monopoles can emerge indirectly or mathematically. For example, nuclear rotation of a diatomic molecule can be seen as a charged particle interacting with the field of a magnetic monopole. More recently, it was shown that a magnetic monopole fields could emerge from the angulon, a new quasiparticle concept representing a quantum impurity exchanging orbital angular momentum with a many-particle bath. This angulon serves as a reliable model for the rotation of molecules in superfluids.

The quasiparticle concept allows physicists to describe complex, many-body interactions in terms of the behavior of a single particle-like entity. Usually these particles turn up in condensed-matter systems such as semiconductors, but a new type of quasiparticle known as an angulon has been proposed to describe the rotation of an atomic or molecular impurity within a solvent. First proposed theoretically two years ago, angulons have now been shown to explain the curious behavior of a range of different molecules rotating within liquid helium.

The concept of quasiparticle is a great tool for physicists. Like the polaron used to describe the behavior of an electron traveling through a crystal lattice, describing the process with only one particle help to simplify the problem and thus to solve it without too many complex calculation and simulations. The recent introduction of angulon supported by strong evidence of its existence is of great help. Thanks to this new tool, a team from Austria demonstrated that the “angulon quasiparticle” can be interpreted as a quantum particle on the two-sphere interacting with a field of a magnetic monopole. Intuitively, in the corotating frame, the cloud of bosons rotates around the molecule fast, and this rotation induces a gauge field of a magnetic monopole, similar to the case of electrons orbiting the nuclei. These results pave the way for studying topological phenomena in experiments on molecules trapped in superfluid helium nanodroplets, as well as on other realizations of orbital impurity problems.

The appearance of a magnetic monopole in superfluid helium droplets is very different from the other, previously studied, systems. "The difference is that we are dealing with a chemical solvent. Our magnetic monopoles form in a fluid rather than in a solid crystal, and you can use this system to study magnetic monopoles more easily.

Professor Mikhail Lemeshko, Institute of Science and Technology Austria.

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