Laboratoire de Physique Théorique
Toulouse - UMR 5152

Similar to how sound waves propagate through everyday materials, some materials can carry magnetic waves, which are collective excitations of electronic magnetic moments (or spins). These “spin waves” can be detected in neutron-scattering experiments. Theoretically, it is believed that the spin waves can sometimes split up (or fractionalize) into a different kind of excitation, called a spinon, which carries only half of the magnetic moment associated with a normal spin wave. Recent experiments have revealed anomalous spin-wave behavior in antiferromagnetic materials, where the electron spins are oriented antiparallel to their neighbors in the crystal lattice, like the colors of a checkerboard. This behavior suggests that spin waves propagating in certain directions are unstable and fractionalize into spinons. a collaboration involving Sylvain Capponi from the lab has demonstrated that this proposal is only partially correct.

Using a theoretical quantum spin model known to be an accurate representation of the experimental system and a novel computer-simulation method for extracting the dynamical signatures, we show that the anomalous spin waves are not quite broken up into spinons but exhibit a “mixed personality,” fluctuating between spinons and spin waves. However, by changing the parameters of the model, we can cause the spin wave to fully fractionalize. We also present a simplified theoretical picture in which the mechanism of fractionalization can be captured and understood.

Our achievement demonstrates that spinons are more common than previously thought. We argue that further signs of spin-wave fragility are present in existing neutron-scattering data, and we suggest experiments to test our proposed fractionalization mechanism.

Référence : H. Shao, Y. Q. Qin, S. Capponi, S. Chesi, Z. Y. Meng, A. W. Sandvik, Phys. Rev. X 7, 041072 (2017)