example, certain materials are better conductors than
others.
The technique used is known as inelastic scattering
and we shall see its contribution to the study of
superfluid helium.
Liquid helium, superfluid helium
©1996 - Institut Laue-Langevin
Helium, a rare, very light gas, is the chemical
substance which liquefies at the lowest temperature (269°C), only 4.2 degrees above absolute zero.
This very cold liquid has unusual properties in
Liquid helium and neutrons
relation to other liquids, and physicists have been
studying them enthusiastically for eighty years. The
most remarkable of these properties is the complete
At very low temperatures liquid helium is indeed a
absence of viscosity which appears if the temperature
strange liquid. If it is put in a bottle it climbs the sides
is reduced further to - 271°C. The liquid can then flow
and comes out again immediately ! It is said to be
in a tube without being slowed down by the walls; in
superfluid, but where does this odd property come
other words, it has perfect fluidity, hence its name of
from ? This is a question to ask neutrons.
superfluid helium.
Neutrons and atomic movements
viscosity
A spectacular experiment showing the absence of
Whatever its state (gas, liquid or solid) matter
is
the
beaker
experiment,
Figure 1. a) If an empty beaker is put in a bath of superfluid
helium, it may be observed that the beaker fills progressively
until the levels are equalised.
b and c). Once full, if the beaker is lifted above the surface of
the liquid, it empties completely. It is essentially because it is
superfluid that the helium can so easily "climb" the walls of
the beaker to reach the lowest level, to which it tends
naturally.
shown
diagrammatically in Fig. 1.
Inelastic scattering of neutrons
consists of atoms which, even at very low temperature,
One can understand intuitively that the superfluid
Neutrons are particles of dimension and mass close
are always moving. This phenomenon is known as
state is associated with microscopic movements of
to those of the nucleus of a helium atom. Carrying out a
thermal motion, as it is due to the ambient heat. The
atoms, very different from those in the viscous liquid.
neutron inelastic scattering experiment is rather like
fact that atoms can move also explains that solids
But which ?
playing bowls with a moving jack (the bowl is the
Neutrons, because they penetrate into the heart of
neutron, and the jack is the helium atom) and deducing
Neutrons are very useful for the study of atomic
matter, and are very sensitive to atomic movements, are
the movement of the jack from the deflection of the
movements in matter (dynamics of solids and liquids).
the only way of showing the real nature of the
bowl.
Neutron radiation is the best way of studying this
superfluid state.
transmit sound waves and conduct heat.
motion on the atomic scale and understanding why, for
A neutron inelastic scattering experiment thus
consists in this case of directing a beam of neutrons all
of the same velocity at a liquid helium target, and
In a liquid, the atoms are also interlinked, but less
analysing the velocity and the direction of the neutrons
strongly than in a solid, and the atoms are disorganised
after collision with the atoms of this target. From the
because of a greater thermal agitation. An acoustic
change in velocity of the neutrons at various angles of
wave of the same type as that in a solid will also be
deflection, it is possible to deduce the frequency of
propagated through a liquid, but will quickly damped
oscillation of the helium atoms, and whether the
out.
movements of the atoms are correlated.
Superfluid helium is a case of a liquid with no
This type of measurement is generally done on a
rigidity (the complete opposite of a solid), and yet
three-axis spectrometer (Fig. 2) or a time-of-flight
neutrons show that the acoustic waves are propagated
instrument and requires a high neutron flux, since the
without damping (i.e. even better than in a solid)!
effect to be observed is small in comparison with other
This unusual behaviour stops as soon as the
neutron interactions (e.g. diffraction).
temperature rises and the liquid becomes normal and
therefore viscous.
Superfluid but almost solid !
This remarkable and apparently contradictory result
Neutrons show that paradoxically the liquid,
can only really be understood by means of theories
superfluid from the viewpoint of the atomic
based on quantum mechanics1. It can be said that,
movements, behaves in certain respects like a solid
although the atoms of superfluid helium are disordered
which has been deprived of all its rigidity. Let us
(typical of a liquid) they all move at the same time in
consider why this is strange.
the same direction and at the same velocity, and this
In a crystalline solid the atoms are positioned at the
does not require any energy. This is similar to the
nodes of a three-dimensional lattice (a grid in three
movement of a crowd of people. If all the people in the
dimensions) and can be thought of as being linked
together by springs, stronger or weaker depending on
the atoms concerned and their respective distances. The
atoms, kept in place by these forces, make small
oscillations around their equilibrium position, provoked
either by thermal agitation or by external perturbations.
Figure 2. ILL three-axis spectrometer for the measurement
of movements of atoms in matter. The sample (liquid helium)
is inside a refrigerator or "cryostat" (white arrow), which can
go down to 0.3 Kelvin (- 272.85°C).
If an atom is pulled out of the surface and then
released, it is pulled back by the "springs" which link it
crowd, without necessarily being in line, move
synchronously, the crowd moves easily and without
problem. If everyone moves in different directions,
there will be no progress, but a lot of friction,
collisions, excitation. In other words a great deal of
energy is wasted.
to the neighbouring atoms, and this produces an overall
movement (acoustic wave), which propagates in the
whole of the solid with little damping.
1
This means that liquid helium belongs to a larger family of
fluids known as "quantum fluids".