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The multifarious superfluid
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The theoretician who first succeeded in explaining the properties of the new superfluid in a decisive way was Anthony Leggett, who in the 1970s was working at the University of Sussex in England. His theory helped experimentalists to interpret their results and provided a framework for a systematic explanation. Leggett's theory, which was first formulated for superfluidity in 3He, has also proved useful in other fields of physics, e.g. particle physics and cosmology.
As superfluid, 3He consists of pairs of atoms, its properties are much more complicated than those of the 4He superfluid. In particular the pairs of atoms of the superfluid have magnetic properties, which means that the liquid is anisotropic, it has different properties in different directions. This fact was used in experiments in which studies were made of the liquid immediately after its discovery. By means of magnetic measurements it was revealed that the superfluid has very complex properties, exhibiting a mixture of three different phases. These three phases have different properties and the proportions in the mixture are dependent on temperature, pressure and external magnetic fields (fig. 5).
| Fig. 5. Superfluid 3He can exist in three phases called A, A1, and B. The type of phase is determined by pressure, temperature and magnetic field according to the figure's phase diagram | 
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Superfluid 3He is a tool that researchers can use in the laboratory to study other phenomena as well. In particular the formation of turbulence in the superfluid has recently been used to study how order can turn into chaos (fig. 6). This research may lead to a better understanding of the ways in which turbulence arises — one of the last unsolved problems of classical physics.
Fig. 6. It has recently been shown that if a vortex is created in a rotating vessel containing superfluid 3He (a), the result can critically depend on the temperature. Above a critical temperature the vortex lines up along the axis of rotation (b). Below the critical temperature a confusion of vortices occurs (c)
(http://www.nobelprize.org/nobel_prizes/physics/laureates/2003/public.html)
Exercises
1) The outline of the history of discoveries concerning superconductivity and superfluids contains a number of names. Can you remember what the achievements of these people are? Consult the text if necessary
Heike Kamerlingh Onnes, John Bardeen, Leon Cooper, Robert Schreiffer, Pyotr Kapitsa, Lev Landau, Georg Bednorz, Alex Müller,Eric Cornell, Wolfgang Ketterle, Carl Wieman, David Lee, Douglas Osheroff, Robert Richardson, Alexei Abrikosov, Vitaly Ginzburg, Anthony Leggett
2) Translate the terms into Russian and explain what they mean in English
type-I superconductors, type-II superconductors, BCS theory, Cooper pairs, the Meissner effect, order parameter, wave function, vortex core, strong superconducting magnet, Bose-Einstein condensation, microscopic theory of superconductivity, anisotropic liquid, external magnetic field, confusion of vortices
3) Answer the questions.
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a) What does the phenomenon of superconductivity imply?
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b) What is the difference between type-I and type-II superconductors?
c) Can the BCS theory describe the specific properties of type-II superconductors?
d) What was Alexei Abrikosov’s starting point in investigating superconductivity?
e) What breakthrough did Abrikosov make in his research in the late 1950s?
f) Why was it necessary to introduce an order parameter describing the density of the superconductive material?
g) What revolutionary applications has the knowledge of superconductivity led to?
h) What forms can helium take and what is the difference between them?
i) Why was it crucial to turn to quantum physical effects in explaining the properties of liquid helium?
j) How did Pyotr Kapitsa, Lev Landau and Richard Feynman contribute to the study of liquid helium?
k) What three different phases does the superfluid (³He) exhibit?
l) How does turbulence form in the superfluid?
Banquet Speech
Listen as you read the banquet speech of Anthony J. Leggett at http://www.nobelprize.org.
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