Universiteit Leiden

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Peculiar particles show paradoxical behavior

Theoretical physicists research a special class of particles; Weyl fermions. They have found them to exhibit paradoxical behavior, in contradiction to a thirty-year old fundamental theory in electromagnetism. A possible application is a new kind of electronics—spintronics. Publication in Physical Review Letters.

Physicists divide the world of elementary particles into two groups. On one side we have force-carrying bosons, and on the other there are so-called fermions. The latter group comes in three different flavors. Dirac fermions are the most famous and make up all matter, including your computer screen and even your own body. Majorana fermions have recently been discovered, and might form the basis of future quantum computers. Lastly, Weyl fermions complete the group, and could find applications in a new kind of electronics: spintronics. Their weird behavior in for example electromagnets has sparked the interest of Prof. Carlo Beenakker’s theoretical physics group.


Conventional electromagnets work on the elegant interplay between electrical currents and magnetic fields. Inside a bicycle dynamo, a rotating magnet generates electricity. And vice versa, moving electrical charges in a wire wrapped around a metal bar will induce a magnetic field. It seems impossible that, in addition, an electric current could be produced within the bar in the same direction. That would invoke a magnetic field around it, in turn generating a current in the opposite direction, and the whole concept would collapse.

Strange behavior

Oddly enough, Beenakker and his group have found cases where this does actually happen. Following an idea from collaborator Prof. İnanç Adagideli (Sabanci University), PhD student Thomas O’Brien built a computer simulation showing that materials harboring Weyl fermions exhibit this weird behavior. This has been seen before, but only at artificially short timescales, when the system didn’t get time to correct for the anomaly. The Leiden/Sabanci collaboration showed that in special circumstances—at temperatures close to absolute zero when materials become superconducting—the strange scenario takes place indefinitely.


Until now, physicists have considered this to be impossible due to underlying symmetries in the models used. That gives the discovery fundamental significance, which is also the Leiden researchers’ main drive. ‘We study Weyl fermions mainly out of a fundamental interest,’ says O’Brien. ‘Still, this research gives more freedom in the use of magnetism and materials. Perhaps the additional flexibility in a Weyl semimetal will be of use in future electronics design.’


T. E. O'Brien, C. W. J. Beenakker, I. Adagideli, ‘Superconductivity provides access to the chiral magnetic effect of an unpaired Weyl cone’, Physical Review Letters

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