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In condensed matter physics, the spin–charge separation is an unusual behavior of electrons in some materials under some special conditions that allows the electrons to behave as a bound state of two independent particles, the spinon and the chargon, also known as the holon. The spinon only carries the spin of the electron but not the charge while the chargon/holon has spin equal to zero but its electric charge equals to the charge of the electron. Spin–charge separation is one of the most unusual manifestations of the concept of quasiparticles. This property is counterintuitive, because the "bare" quasiparticles are electrons, holes, phonons and photons, all of which have the property that bosons are evenly charged and fermions are oddly charged; and naively, it would seem that any quasiparticle formed from these particles would also satisfy this property.[dubious ]
Since the original electrons in the system are fermions, one of the spinon and chargon has to be a fermion, and the other one has to be a boson. However, it is arbitrary to choose which either one to be fermionic. The formalism with bosonic chargon and fermionic spinion is usually refereed to as the "slave–fermion" formalism, while the formalism with fermionic chargon and bosonic spinon is called the "Schwinger boson" formalism. Both approaches have been used for strongly correlated systems, but neither has been proved to be completely successful. One difficulty of the spin–charge separation is that while spinon and chargon are not gauge–invariant quantities, i.e. unphysical objects, there are no direct physical probes to observe them. Therefore more often than not one has to use thermal dynamical or macroscopic techniques to see their effects. This implies that which formalism we choose is irrelevant to real physics, so in principle both approaches should give us the same answer. The reason we obtain radically different answers from these two formalisms is probably because of the wrong mean field solution we choose, which means that we are dealing with the spin–charge separation in a wrong way.
Another proposal has been put forward in the framework of ultracold atoms. In a two-component Bose gas in 1D, strong interactions can produce a maximal form of spin-charge separation.
Using a method first proposed by physicist Duncan Haldane in 1981, 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 also observed peak spectral structures of spin–charge separation around the same time.