The lack of symmetry between electric and magnetic fields is one of the oldest puzzles in physics. Why is it possible to isolate positive and negative electric charges, but not north and south magnetic poles? Dirac linked the existence of magnetic monopoles with the quantization of electric charge - another puzzle that is still not fully understood - but they have never been detected in an experiment.

Magnetic monopoles are also predicted by some theories that seek to unify the electroweak and strong interactions. However, the monopole masses that are predicted by these so-called grand unified theories are much too large - about 1016 giga-electronvolts – to be detected in experiments.

Instead of searching for magnetic monopoles in real space, Yoshinori Tokura of the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba and co-workers turned to momentum space - the mathematical space in which condensed matter physicists construct Fermi surfaces, Brillouin zones and so on. The team was motivated by recent theoretical work which suggested that the behaviour of magnetic monopoles in momentum space is closely related to the anomalous Hall effect.

Tokura and co-workers placed a high-quality crystal made of strontium, ruthenium and oxygen in a magnetic field that pointed in the z direction, and then measured the transverse resistivity – the resistivity in the y direction - as a current flowed in the x direction. They found that the resistivity did not change linearly with temperature, as expected, but varied non-monotonously and even changed sign (figure 1).

The researchers also measured the transverse optical conductivity of a thin film of the crystal using a technique known as high-resolution Kerr microscopy and found a sharp peak at low energies. According to Tokura and co-workers, this peak can only be explained by the presence of monopoles in the band structure of the crystal.

The Japan-China-Switzerland team believe that both of these anomalous effects are “fingerprints” for the existence of magnetic monopoles. The team now plans to study materials that show even larger anomalous effects. “The laws of electromagnetism are the starting point for every area of physics,” says team member Kei Takahashi of the University of Geneva. “From this view point, we have proved that we can investigate most physics subjects - including particle physics and cosmology – in experiments on solid crystals.”