Download Superconductors and neutrons at Grenoble - Institut Laue

metals become perfect conductors (superconductors) but
1
only at temperatures too low to be really useful.
The press has given considerable publicity to the
unexpected discovery of new superconductors at more
accessible temperatures2, and the determined efforts by
scientists to go ever higher. To do this it is necessary to
understand the structure of these materials (arrangement
of the atoms), and this is where the neutrons come in.
©1996 - Institut Laue-Langevin
Superconductors and neutrons
at Grenoble
What is a superconductor ?
When electric current circulates in a normal
conductor, there is a loss of energy and emission of heat
because of its electrical resistance. Copper, the best
conductor known at ambient temperature, still has a
resistance which is not negligible, and this is why EDF
tries to limit losses by building enormous high voltage
lines.
With superconductors low voltage would be
sufficient, as they have no electrical resistance and there
would therefore be no loss of energy. Unfortunately very
low temperatures are expensive to maintain!
produced, creating opposing forces, which repel the
magnetic surface. A vehicle gliding in this way without
friction over a magnetic surface, rather than on wheels,
would need much less energy to move. It is clear that
'high temperature' superconductors have all sorts of
potential applications. They are already present in
scanners using nuclear magnetic resonance (NMR),
which are increasingly replacing X-rays in hospitals for
the examination of the interior of the human body. But it
will no doubt take years to develop magnetic suspension
trains and loss-free electrical cables.
The quest for new superconductors
These new superconductors were not discovered by
chance, but using ideas and theories resulting from
fundamental research.
New materials
Figure 1. Superconductor in levitation above a magnetic
surface.
Electricity authorities loses 10% of the electricity
produced because the metal of the electricity cables heats
up as the current passes. This phenomenon, known as the
Joule effect, is sometimes useful (electric radiator) but is
often a considerable disadvantage. Can it be avoided ?
Yes and no. Since 1911 it has been known that some
In 1986, G. Bednorz and K.A. Müller in Zurich
discovered that a new class of materials first synthetized
by B. Raveau in France become superconductors at a
much higher temperature, above that of liquid air. This is
still cold (- 190°C), but liquid air can be produced and
stored relatively inexpensively.
Fig. 1 shows a cold superconductor floating above a
magnetic surface. As soon as the magnetic force field
penetrates the superconductor, electrical currents are
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2
Lower than -250 ºC
Temperatures were achieved of - 238°C (BaLaCuO), then 183°C (YBa2Cu3O7). The current record is - 148°C
Figure 2. The temperature of the superconducting transition
(Tc) in YBaCuO depends on its oxygen composition (x).
The superconductor YBaCuO is a mixture of four
atoms (e.g. yttrium, barium, copper, oxygen), with
properties which are remarkably sensitive to slight
variations in crystal structure. Figure 2, for example,
shows how the superconducting temperature depends on
the proportion of oxygen. It is obviously important to
understand why.
differently with atoms and does not care whether they are
light or heavy. Fig. 3 shows that neutrons are necessary
to see lightweight atoms, such as oxygen in
superconductors.
And where do neutrons come in ?
Neutrons, yes but how ?
X-rays, the electron microscope and neutrons are the
three techniques most used to examine the atomic
structure of materials. Which is the most appropriate for
the study of superconductors ?
The experiments carried out on superconductors are
relatively simple (Fig. 4). The ILL high flux reactor is
used to produce beams of neutrons and a precise energy
or wavelength is selected by a monochromator crystal
and aimed at the sample.
Figure 3. Apparent size (visibility) of the different atoms of a
superconductor, as seen by X-rays, an electron microscope
and neutrons.
Figure 4. Diagram of a neutron scattering experiment using a
powder
Oxygen, whose rôle in superconductivity is so
important, is a lightweight atom barely "visible" to Xrays, especially because it is associated here with much
heavier atoms (copper, yttrium, barium). In the human
body the heavy atoms are concentrated in the bones, and
a medical X-ray clearly shows that the lightweight atoms
(skin, muscles) appear "transparent" to X-rays.
Neutrons are another type of radiation which interacts
The scattered neutrons are collected by a detector, and
the variations in intensity are analysed as a function of
angle by computers, to deduce the atomic arrangement,
and to situate the oxygen in the superconductor.
oxygen, and on its position in the structure (Fig. 5). This
work by a joint ILL-CNRS team resulted in the
experiment most frequently cited in the year following
the discovery of the superconductor YBaCuO.
Since then, scientists from the Bell Labs in the USA
working in Grenoble with the CNRS and ILL have
shown - an essential point - that the superconductivity
temperature depends directly on the Cu-O distances
which measure the state of oxidation of the copper.
Figure 5. Atomic structure of the superconductor YBaCuO. It
shows the copper oxide chains and planes which are now wellknown.
Provisional conclusion ...
What have we discovered with neutrons ?
At ILL we have shown that the oxidation of copper in
the superconductor plays an essential rôle: the
superconductivity depends on the precise quantity of
These neutron experiments have thus enabled us to
understand what is important in superconducting oxides.
They have shown the direction of research needed to
obtain better materials, but there is still a great deal to do.