Building on physicist F. Duncan M. Haldane 's 1981 theory, experts from the Universities of Cambridge and Birmingham proved experimentally in 2009 that a mass of electrons artificially confined in a small space together will split into spinons and holons due to the intensity of their mutual repulsion (from having the same charge). A team of researchers working at the Advanced Light Source (ALS) of the U.S. Department of Energy's Lawrence Berkeley National Laboratory observed the peak spectral structures of spin–charge separation three years prior.