Atoms consist of a positively charged nucleus and a number of negatively charged electrons. They form a stable object since the nucleus and the electrons attract each other.
The same working principle can also be seen in the interaction of many particles: in bodies that are solid at room temperature, for example, the individual molecules are held in specific positions by binding forces. This ensures that the solid remains “in shape” and does not slosh around like a liquid, for instance. One example of this would be table salt (sodium chloride), which forms a crystal at room temperature. Its positively charged sodium ions and negatively charged chloride ions attract one another.
So it is perhaps surprising to learn that even repelling forces can form stable objects. “In this case, the binding force is generated because the complex object is highly excited and is unable to dissipate its energy,” explains Professor Corinna Kollath from the Theoretical Quantum Physics team at the University of Bonn. Since the object is unable to reduce its energy, its components are bound together. This mechanism only comes into play if the object exchanges hardly any energy with its environment.
“For solids, which have numerous constituents such as electrons and nuclei, engineering a sufficient degree of isolation to achieve repulsively bound states was thought to be impossible,” says Professor Kollath, who is also a member of the Matter Transdisciplinary Research Area and the ML4Q Cluster of Excellence at the University of Bonn. The physicist is part of an international research team that has now discovered a specific bond—known as BaCo2V2O8—in which this exotic, highly excited quantum state of matter can be observed.
The researchers used terahertz light waves to excite the spins of the electrons in this bond (“spin” here means the intrinsic angular momentum carried by particles) by subjecting BaCo2V2O8 to an extremely strong magnetic field of up to 60 Tesla. As well as the magnetic quasiparticle excitation at low energy levels (the “magnon”), they also identified two- and three-magnon bound states as high-energy excitations.
The magnon is a quasiparticle that, in layperson’s terms, can be understood as a spin pointing in the opposite direction to all the other spins. It is the terahertz light waves that cause the spin to flip its orientation. The bound two- and three-magnon bound states thus consist of two or three of these flipped spins respectively, which are held together by the repulsive interaction. The University of Bonn researchers analyzed the data from the experiments and identified and characterized the various bound states.
“The study is the first evidence that repulsively bound states can be observed in a solid-body system,” Corinna Kollath points out. However, she adds, that some fundamental research is still needed to understand how these exotic states form in more complex quantum systems. The authors expect that exploring the potential applications of these bounds states in particular in the field of quantum information technology will take several years.