Prince George, B.C. - How can a structure hold together when its individual components repel each other? 

In collaboration with a team of international researchers, UNBC Physics Assistant Professor Dr. Jean-Sébastien Bernier recently answered this question demonstrating that such exotic highly excited quantum states of matter exist in a solid compound. 

Bernier, along with colleagues from Augsburg, Bonn, Cologne, Dortmund, Dresden and Geneva published their findings in the prestigious journal Nature. The paper, titled Experimental observation of repulsively bound magnons, was published Wednesday.

“Attractively acting forces are well known to form stable bound objects. For instance, atoms consisting of a positively charged nucleus and negatively charged electrons are bounded together via the Coulomb interaction,” Bernier explains. “In contrast, forming bound objects via repulsive forces appears totally counterintuitive. However, this can happen if the composite object is highly excited and cannot reduce its energy.”

In their study, Bernier and collaborators, identified a compound, known by the chemical formula BaCo₂V₂O₈, in which, to the general surprise, exotic repulsively bound objects are observed. This compound is a realization of a chain of interacting spins, a paradigmatic model of quantum many-body physics. 

One can think of the electron’s spin as a quantum analogue to the needle of a compass which aligns with an external magnetic field. If the direction of one of the spins is flipped compared to others in the chain, this realizes a magnon, a magnetic quasiparticle excitation.

The researchers used terahertz light waves to excite spins in BaCo₂V₂O₈ creating magnons that repel each other and investigated their dynamics in applied magnetic fields up to 60 Tesla. Their analysis revealed the presence of quantum states in which two and even three flipped spins are held together by their repulsive interactions.

“This study provides the first evidence that repulsively bound states can be observed in a solid-state system,” Bernier says. “Understanding these exotic states in even more complex quantum systems and exploring their potential for quantum information applications will be of great interest in the years to come.”