Quantum electrodynamics (QED) is the fundamental theory that describes the interactions between electrons and photons. Its success has led some to wonder if quantum field theories, like QED, can describe quasiparticles in a solid. These collective excitations include phonons, which describe lattice vibrations, and magnons, which are waves in a magnetic material but might also be of a more exotic nature. In a recent collaboration, Sylvain Capponi and his colleagues have shown that QED in two spatial dimensions can be observed in frustrated antiferromagnets.
An antiferromagnet is a material where neighboring electron spins in the crystal lattice would like to point in opposite directions. However, on certain geometries, such as a triangular lattice, it is impossible to have all neighboring spins align precisely the opposite way. This is called geometric frustration and can lead to strong disorder in the system.
This disorder is not featureless, however. In fact, we show that the quasiparticles of such a spin soup, known as a quantum spin liquid, are related one-to-one to excitations of QED. Importantly, even the elusive magnetic monopoles, among a wide variety of other particle-hole excitations, are observed.
The precise understanding of the spin-liquid state with magnetic monopoles as elementary excitations is a key step to discovering these exotic quasiparticles in antiferromagnetic materials. It is unlikely that the founders of QED would have predicted such a surprising emergence in condensed matter.
For more details, see the publication A. Wietek, S. Capponi, A. Läuchli, Phys. Rev. X 14, 021010 (2024) or the related viewpoint or the related commentary by Joel Moore.
