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First Direct Observations of Spinons and
Holons
Berkeley, CA, July 13: The theory has been around for more than 40 years,
but only now has it been confirmed through direct and unambiguous experimental
results. Working at the Advanced Light Source (ALS)
of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, a team
of researchers has observed the theoretical prediction of electron “spin-charge
separation” in a one-dimensional solid. These results hold implications for
future developments in several key areas of advanced technology, including
high-temperature superconductors, nanowires and spintronics.
Just as the body and wheels of a car are thought to be intrinsic parts of a
whole, incapable of separate and independent actions, i.e., the body goes right
while the wheels go left, so, too, are electrical charge and spin intrinsic
components of an electron. Except, according to theory, in one-dimensional
solids, where the collective excitation of a system of electrons can lead to the
emergence of two new particles called “spinons” and “holons.” A spinon carries
information about an electron’s spin and a holon carries information about its
charge, and they do so as separate and independent entities. Numerous
experiments have tried to confirm the creation of spinons and holons, referred
to as spin-charge separation, but it took the technological advantages offered
at ALS Beamline 7.0.1, also known as the Electronic Structure Factory (ESF), to
achieve success.
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. The research was
led by Changyoung Kim, at Yonsei University, in Seoul, Korea, ALS scientist Eli
Rotenberg, and Zhi-Xun Shen of Stanford University, a leading authority on the
use of ARPES technology. Co-authoring the Nature-Physics paper with them were
Bum Joon Kim and Hoon Koh, plus S.J. Oh, H. Eisaki, N. Motoyama, S. Uchida, T.
Tohyama, and S. Maekawa.
“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.”
Said Shen, “Our results confirming the idea of spin-charge separation are
important because they reveal deep insights into the quantum system - and the
beauty and subtleties associated with it. From this study we know more about how
the collective behavior of a system of particles can be so fundamentally
different from that of the constituent individuals.”
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.
Said Kim, “Our experimental confirmation of this spin-charge separation should
provide more confidence in these theories.”
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.”
The creation of spinons and holons in one-dimensional systems is also expected
to have an impact on the future of spintronics, a technology in which the
storage and movement of data will be based on the spin of electrons, rather than
just on charge, as with our current electronic technology. Spin-based electronic
devices promise to be smaller, faster and far more versatile than today’s
devices.
Berkeley Lab is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research and is
managed by the University of California. Visit our Website at
www.lbl.gov.
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