In 1853, two German physicists, Gustav Wiedemann and Rudolf Franz,
studied the thermal conductivity (a measure of a system's ability
to transfer heat) of a number of elemental metals and found that
the ratio of the thermal to electrical conductivities was
approximately the same for different metals at the same
temperature.
The origin of this empirical observation did not become clear
however until the discovery of the electron and the advent of
quantum physics in the early twentieth century. Electrons have a
spin and a charge. When they move through a metal they cause an
electrical current because of the moving charge. In addition, the
moving electrons also carry heat through the metal but now it is
via both the charge and the spin. So a moving electron must carry
both heat and charge: that is why the ratio does not vary from
metal to metal.
For the past 150-plus years, the Wiedemann-Franz law has proved to
be remarkably robust, the ratio varying at most by around 50 per
cent amongst the thousands of metallic systems studied.
In 1996, two American physicists Charles Kane and Matthew Fisher
made a theoretical prediction that if you confine electrons to
individual atomic chains, the Wiedemann-Franz law could be strongly
violated. In this one-dimensional world, the electrons split into
two distinct components or excitations, one carrying spin but not
charge (the spinon), the other carrying charge but not spin (the
holon). When the holon encounters an impurity in the chain of atoms
it has no choice but for its motion to be reflected. The spinon, on
the other hand, has the ability to tunnel through the impurity and
then continue along the chain. This means that heat is conducted
easily along the chain but charge is not. This gives rise to a
violation of the Wiedemann-Franz law that grows with decreasing
temperature.
The experimental group, led by Professor Nigel Hussey of the
Correlated Electron Systems Group at the University of Bristol,
tested this prediction on a purple bronze material comprising
atomic chains along which the electrons prefer to travel.
Remarkably, the researchers found that the material conducted heat
100,000 times better than would have been expected if it had obeyed
the Wiedemann-Franz law like other metals. Not only does this
remarkable capability of this compound to conduct heat have
potential from a technological perspective, such unprecedented
violation of the Wiedemann-Franz law provides striking evidence for
this unusual separation of the spin and charge of an electron in
the one-dimensional world.
Professor Hussey said: "One can create purely one-dimensional
atomic chains on substrates, or free-standing two-dimensional
sheets, like graphene, but in a three-dimensional complex solid,
there will always be some residual coupling between individual
chains of atoms within the complex that allow the electrons to move
in three-dimensional space.
"In this purple bronze, however, nature has conspired to limit this
coupling to such an extent that the electrons are effectively
confined to individual chains and thus creating a one-dimensional
world inside the three-dimensional complex. The goal now is to find
a way, for example, using pressure or chemical substitution, to
increase the ability of the electrons to hop between adjacent
chains and to study the evolution of the spin and charge states as
the three-dimensional world is restored within the material."
###
'Gross violation of the Wiedemann-Franz law in a
quasi-one-dimensional conductor' by Nicholas Wakeham, Alimamy F.
Bangura, Xiaofeng Xu, Jean-Francois Mercure, Martha Greenblatt and
Nigel E. Hussey in Nature
Communications
University of Bristol
Thanks to University of Bristol for this article.
