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Superfluids
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Superfluids

Super-properties

In a glass, superfluid helium climbs up the walls by capillarity and forms a droplet at the outside bottom; Alfred LeitnerSuperfluidity was discovered in liquid helium below about 2 kelvins (-271°C) at the end of 1937, in Cambridge (J.F. Allen and A.D. Meisener) and independently in Moscow (P. Kapitza). The following year, in Paris, F. London understood that this strange behaviour was a consequence of a phenomenon predicted by Einstein in 1925: in some fluids, atoms can adopt a coherent collective behaviour; together they merge in a wave of matter that moves with no friction. This wave is called a “Bose-Einstein condensate”.

All particles cannot form such a state: Einstein’s prediction only works for particles belonging to the “boson” family. Indeed, because of different symmetries, quantum mechanics classifies particles in two families with very distinct properties: bosons and fermions. Fermions cannot form a condensate. A helium-4 atom is a boson; it was in that isotope of helium that superfluidity was discovered in 1937. Electrons are fermions; however, a pair of electrons is a boson: today, we know that in a superconductor, electrons merge in a collective wave similar to superfluidity but, in order to do that, they have to make pairs. Because the nucleus of a helium-3 atom has one less neutron than helium-4, the helium-3 atom is a fermion. In 1973, it was discovered that liquid helium-3 also becomes superfluid, but at a temperature ten thousand times lower than its heavier isotope (helium-4) and only if the helium-3 atoms form pairs, just like superconducting electrons.

The absence of viscosity in a superfluid was explained thanks to the works of Lev Landau in 1941 and Bogoliubov in 1947. Landau noticed that in order for a superfluid to lose energy, quantum states of energy superior to that of the fundamental state had to be excited, and that this could only happen below a minimum speed. This phenomenon has been confirmed in details in liquid helium since 1995, but also in many superfluid gases such as cold atomic vapours (rubidium, sodium, hydrogen, cesium, etc.).

 

CNRSSociété Française de PhysiqueTriangle de la physique
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CNRSSociété Française de PhysiqueTriangle de la physique