Az elektromos töltés és a spin az elektron intrinsic komponensei, kivéve -az elmélet szerint- az egydimenziós szilárdtestekben (vékony drót, nanotubes), ahol a rendszer elektronjainak kollektív gerjesztése két új részecske megjelenéséhez vezet, amit spinonnak és holonnak neveztek el. A spinon hordozza az elektron spin tulajdonságát míg a holon az elektron elektromos töltés tulajdonságát tartalmazza valamint ezt elkülönült és független létezőként teszi.
In one-dimensional electron systems, theory predicts that collective excitations of electrons produce, instead of the quasiparticles in ordinary Fermi liquids, two new particles known as 'spinons' and 'holons'. Unlike ordinary quasiparticles, these particles surprisingly do not carry the spin and charge information of electrons together. Instead, they carry spin and charge information separately and independently. This novel and exotic phenomenon was predicted theoretically decades ago and is commonly known as spin-charge separation. An experimental observation of spin-charge separation, thus spinons and holons, would mean a completely new paradigm for the physics of electron systems where the electrons have been 'fractionalized'.
In a paper published in the June 2006 issue of the journal Nature-Physics, researchers have reported the observation of distinct spinon and holon spectral signals in one-dimensional samples of copper oxide SrCuO2 , using the technique known as ARPES, for angle-resolved photoemission spectroscopy.
There have been claims of observing the two peak spectral structures of spin-charge separation in the past, but they turned out to be wrong or have plenty of ambiguity. This was primarily because those results were obtained from complicated materials and were not theoretically backed up," said Kim, who has spent several years investigating the spin-charge separation phenomenon. "Our observations using ARPES are direct and the results are unambiguous because they were obtained from a simple material that left little room for misinterpretation. Also, our results are theoretically backed up.
The idea behind spin-charge separation is that electrons behave differently when their range of motion is restricted to a single dimension, as opposed to three or even two dimensions. When moving through one dimension, for example, the electrons are lined up head-to-tail, making the repulsive force between their negative electrical charges overridingly dominant. The restricted movement of electrons through one-dimensional material was expected to give rise to collective effects that would be strong enough to break the information flow of spin and charge from a single electron.
ARPES is an excellent tool for observing spin-charge separation and other collective effects involving electrons. In this technique, x-rays are flashed on a sample causing electrons to be emitted through the photoelectric effect. Measuring the kinetic energy of emitted electrons and the angles at which they are ejected identifies their velocity and scattering rates. This in turn yields a detailed picture of the electron energy spectrum. Ordinarily, the removal of an electron from a crystal creates a hole, a vacant positively-charged energy space. This hole carries information on both the spin and the charge, as observed in a single peak of an ARPES spectrum. If spin-charge separation occurs, the hole decays into a spinon and a holon and two peaks in the ARPES spectrum are observed.
ALS Beamline 7.0.1 utilizes a state-of-the-art undulator magnetic insertion device to generate beams of x-rays with properties similar to that of a laser. These coherent and tunable x-ray beams are a hundred million times brighter than those from the best x-ray tubes and provide an exceptionally high degree of angular resolution for ARPES experiments.
Said Rotenberg, who manages the beamline and oversees research at the ESF experimental station, "At the ESF we have the advantage of being able to survey relatively large amounts of reciprocal space to locate where the interesting correlated effects are occurring. Our data not only shows a clear separation of ARPES spectral peaks, it can also be compared to theory to obtain spectral functions, which, in principle, can provide detailed information about the dynamics of spinons and holons."
High-temperature superconducting copper oxides, or cuprates, with their ability to lose all electrical resistance at transition temperatures far above those of metal superconductors, have become valuable tools for research even though scientists still do not know why they work. Central to many of the leading theories that attempt to explain high-temperature superconductivity in cuprates is the existence of spin-charge separation in one-dimensional systems.
Another area in which spinons and holons could play an important role is in the development of nanowires, one-dimensional hollow tubes through which the movement of electrons is so constrained that quantum effects dominate.
Nanowires are expected to be key components in future nanotechnologies, including optoelectronics, biochemical sensing, and thermoelectrics.
Said Rotenberg, "The transport of electrons through nanowires will be subject to spin-charge separation and it will be very helpful to have experimental as well as theoretical understanding of this phenomenon as nanowire technology advances."
Holon-spinon, Holon-spinon, Spinon
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