Electrons in atoms can be described by three quantum numbers: spin, charge, and orbital. In an experiment performed at the Paul Scherrer Institute in Switzerland, these properties have now been separated. In one-dimensional systems, it is predicted that the electrons can separate into independent quasi-particles, which cannot leave the material in which they have been produced. While quasi-particles carrying either spin (spinons) or charge (holons or chargons) have already been identified, an international team of researchers led by experimental physicists from the Paul Scherrer Institute, Switzerland, and theoretical physicists from the IFW Dresden, Germany, have now succeeded in separating quasi-particles carrying the orbital degree of freedom (orbitons). These results are reported in the May 3 issue of Nature (DOI: 10.1038/nature10974; p. 82).

The electron’s breakup into two new particles—spinons and orbitons—has been gleaned from measurements on the copper-oxide compound Sr2CuO3, a one-dimensional Mott insulator. This material has the distinguishing feature that the particles in it are constrained to move in one direction only, either forward or backward. Using x-rays, scientists have lifted some of the electrons belonging to the copper atoms in Sr2CuO3 to orbitals of higher energy, corresponding to the motion of the electron around the nucleus with higher velocity. By comparing the properties (energy and momentum) of the x-rays before and after the collision with the material, the properties of the newly produced particles can be traced.

“These experiments not only require very intense x-rays, with an extremely well-defined energy, to have an effect on the electrons of the copper atoms, ” said Thorsten Schmitt, head of the experimental team, “but also extremely high-precision x-ray detectors.”

“It had been known for some time that, in particular materials, an electron can in principle be split,” said Jeroen van den Brink, who leads the theory team at the IFW Dresden, “but until now the empirical evidence for this separation into independent spinons and orbitons was lacking. Now that we know where exactly to look for them, we are bound to find these new particles in many more materials.”

Observation of the electron splitting may also have important implications for high-temperature superconductivity, according to the researchers. Due to the similarities in the behavior of electrons in Sr₂CuO₃ and in copper-based superconductors, understanding the way electrons decay into other types of particles in these systems might offer new pathways toward improving the theoretical understanding of high-temperature superconductivity.