An experiment has confirmed that spinons, particle-like magnetic excitations,
can be confined in a magnetic insulator similar to the way elementary quarks
are confined within individual protons and neutrons. The finding, in a well-described
magnetic system, may offer new ways to explore Quantum Chromodynamics, the theory
that describes the fundamental interactions of quarks.
The observations of spinon confinement were made at the Science and Technology
Facilities Councils Rutherford Appleton Laboratory in the United Kingdom by
an international team of physicists. The team realized serendipitously that
a theory developed 12 years earlier by theoretical physicist Alexei Tsevelik,
now at the U.S. Department of
Energy's Brookhaven National Laboratory, and collaborators accurately predicted
the current findings. Together, the scientists describe the theory and their
new observations in the November 29th issue of Nature Physics.
"The concept of confinement is one of the central ideas in modern physics,
being at the core of the theory of nuclear forces," Tsvelik said. "In
certain systems, when constituent particles are bound together by an interaction
whose strength increases with increasing particle separation, individual particles
cannot exist in a free state and therefore can be observed only indirectly."
The most famous example of confinement is of quarks which are held together
in protons and neutrons, for example, by the strong force, a force that grows
stronger with increasing distance.
"It has been interesting for us that a similar situation of confinement
can be modeled in condensed matter systems," Tsvelik said. "Instead
of quarks being confined in protons and neutrons, we have other quantum entities
that act just like particles -- elementary excitations of magnetic systems called
In the case of the current experiment, the spinons exist on parallel chains
of copper-oxide separated by inert calcium. Spinons on individual chains are
not confined, but as soon as two chains are brought together to form ladder-like
arrangements, the inter-ladder interactions confine the spinons.
"That is, the spinons can appear now only in pairs and cannot fly away
from each other too far," Tsvelik said. "The result of this confinement
is a particle we call a magnon. It is like two quarks pairing up to form a meson."
The original theory paper published by Tsvelik and collaborators 12 years ago
described the magnetic excitation spectrum of such a system in detail. The team
performing the experiments at Rutherford observed a signature that fit that
"Now that we have an example of confinement in a condensed matter system,
our next step is to check further predictions of the theory to make sure there
are no unpleasant surprises," Tsvelik said. The scientists will also measure
the responses in other compounds to see if they observe similar effects.