10 April, 2016

Superfluidity and Bose-Einstein Condensation

A few times I heard people from the outside of the atomic physics community wondering why was the discovery of Bose-Einstein Condensation (BEC) in alkali gases so special, since the existence of BEC in superfluid helium was considered to be an accepted fact.

Of course, very soon after a BEC of ultracold atoms was created, the implication of the employed technology (as well as of several related developments) became crystal clear. By now, the field of ultracold gases grew into one of the mainstream areas of physics; it already allowed to use our knowledge about atoms and molecules to understand solid state physics, photonics, and even chemistry better.

However, a direct experimental observation of the BEC, as a novel state of matter, was of crucial importance.

Almost immediately after the discovery of superfluidity by Kapitza and Allen and Misener,* Fritz London suggested that this phenomenon was closely related to Bose-Einstein condensation. László Tisza, who was together with London in Paris (no pun intended) at the time, got excited and quickly elaborated on these ideas. Tisza developed the basis for what is known as the "two-fluid model." He conjectured that one can understand the superfluid phase of helium as a mixture of two components. The first, superfluid component, represents a Bose condensate of the atoms occupying the same single-particle quantum state. This results in a macroscopic coherence allowing a flux without friction or viscosity. The second, normal component, whose fraction depends on temperature, behaves as a regular viscous fluid.

Three years later, Lev Landau derived his version of the two-fluid model, based on the quantization of classical hydrodynamics equations. His theory was phenomenological and didn't require the particles to obey Bose statistics. Moreover, he started the paper by bluntly opposing the ideas of Tisza:

...Tisza’s well-known attempt to consider helium II as a degenerate Bose gas cannot be accepted as satisfactory – even putting aside the fact that liquid helium is not an ideal gas, nothing could prevent the atoms in the normal state from colliding with the excited atoms; i.e., when moving through the liquid they would experience friction and there would be no superfluidity at all.

As a matter of fact, it took several decades to unify the ideas of Landau with the ones of London and Tisza. In the end of the 1950's and beginning of the 1960's, several hard-core many-body calculations allowed to theoretically prove that superfluidity is indeed accompanied by Bose-Einstein condensation of helium atoms.**

However, from the experimental side, establishing the existence of the BEC state in superfluid helium turned out to be extremely challenging. Namely, it was possible to obtain only indirect evidence that about 10% of the atoms form a condensate, based on high-energy neutron scattering and spectra of atoms evaporated from the helium surface.

Thus, it was the observation of a BEC in ultracold gases and later experiments on their superfluidity which allowed to establish a connection between the two concepts beyond all possible doubt.

Take care,


* While these two papers appeared back-to-back in Nature, only Kapitza was awarded a Nobel Prize for this discovery (and only 40 years later!). Quite unfortunately, even nowadays the contribution of Allen and Misener remains widely disregarded. There are several great articles discussing this peculiar story; there are even gossips that Kapitza refused to accept the prize together with Allen which made the Nobel committee postpone the decision for decades.

** Since the phenomenological theory of Landau happened to successfully reproduce the experimental data, the contributions of London and Tisza were not acknowledged as widely as they should have been. Among other things, it seems that Fritz London was the first person ever to recognize the effects of quantum mechanics at the macroscopic scale, and think about the emergence of quantum phenomena in many-particle systems. Phil Anderson wrote a nice essay about London's forgotten contributions, which resonates with his own "More is different"  very well.

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