Magnetic excitations can be held together by repulsive interactions

A group of physicists specialized in solid-state physics from the University of Cologne and international collaborators have examined crystals made from the material BaCO2V2O8 in the Cologne laboratory. They discovered that the magnetic elementary excitations in the crystal are held together not only by attraction, but also by repulsive interactions. However, this results in a lower stability, making the observation of such repulsively bound states all the more surprising. The results of the study ‘Experimental Observation of Repulsively Bound Magnons’ were published in Nature. 
The working group of Professor Dr Thomas Lorenz from the University of Cologne’s Institute of Physics II succeeded in producing artificial crystals made from BaCo2V2O8, which contain screw chains of magnetic cobalt atoms.
Together with researchers from Augsburg, Bonn, Dortmund, Dresden, Geneva and Prince George, the BaCo2V2O8 crystals were irradiated with electromagnetic terahertz waves in order to study the collective magnetic excitations in the crystal structure in high magnetic fields. In addition to the usual elementary magnetic low-energy excitations, the so-called magnons, the researchers also discovered two- and three-magnon bound states. 
The peculiarity of these multi-magnon bound states is that they are held together not by attraction, but by repulsive interactions. “The discovery of these states is the result of the very successful collaboration of experimental and theoretical working groups within the framework of our Collaborative Research Center 1238 ‘Control and Dynamics of Quantum Materials’, based in Cologne,” said Professor Lorenz.
Since 2016, the Collaborative Research Center (CRC) 1238 ‘Control and Dynamics of Quantum Materials’ has united a team of experts from experimental and theoretical physics as well as crystallography in Cologne, complemented by groups from the University of Bonn and Forschungszentrum Jülich. The aim is to discover, understand and control new collective phenomena and new functionalities in quantum materials.