THE SPLIT

Electron fractionalization in a one-dimensional structure. a , Schlappa and colleagues’ system 6 consists of a chain of copper oxide units (CuO 3 ). In the ground state, all units contain 3 d electrons in the same in-plane orbital, and the direction of the local 3 d spins (arrows) alternates between neighbouring units. Shining X-ray photons on the chain excites the electrons in one of the units into an out-of-plane orbital of higher energy. b , The authors show that the X-ray irradiation leads to an immediate splitting of the excitation into two separate quasiparticles of different velocities: a spinon, which is associated with a local perturbation in the spin arrangement, and an orbiton, which carries the orbital excitation.