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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".