Interacting electrons that are confined to move in a one-dimensional structure do not simply jam together like cars in rush hour. Inelastic X-ray scattering shows that the electrons act as if they split into separate fractional entities.

In physics, a phenomenon is often best explained by reducing it to its simplest version. For example, to understand the quantum-mechanical motion of non-interacting electrons in a crystalline solid, the case of a one-dimensional crystal lattice suffices to introduce the concept of electronic band structure, which describes the range of energies that the electrons may have in the solid. But when it comes to interacting electrons, this approach fails. Coulomb repulsion between any two electrons is much stronger in one-dimensional solids than in their higher-dimensional counterparts, and many-body effects emerge that lead to an apparent fractionalization of the electron^{1, 2, 3, 4, 5}. With this fractionalization, the electron's spin and charge seem to form separate quasiparticles — spinons and holons, respectively — that move independently of each other and have different velocities. In a paper published on Nature's website today, Schlappa et al.⁶ describe an experiment that takes the idea of electronic fractionalization a step further by revealing that electrons can split into a third form of quasiparticle, the orbiton.^*

Schlappa and colleagues observe orbitons in a copper oxide (Sr₂CuO₃) containing a one-dimensional chain of copper oxide building blocks (CuO₃), in which the valence electrons reside in the 3d electronic shell of the central copper atoms (Fig. 1). In a free atom there are five 3d orbitals, which are energetically degenerate — that is, they all have the same energy. However, in a CuO₃ unit, the electrical field of the oxygen ions around each copper atom lifts this degeneracy, generating an intra-shell energy spectrum. In the chain's ground state, the 3d electrons in all of the CuO₃ units are in the same, lowest-energy 3d state, with their spin orientation alternating between neighbouring units as a result of antiferromagnetic interactions.

a, Schlappa and colleagues' system⁶ consists of a chain of copper oxide units (CuO₃). In the ground state, all units contain 3d electrons in the same in-plane orbital, and the direction of the local 3d 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.