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Concept
Superfluid film
Summary
Superfluidity is a phenomenon where a fluid, or a fraction of a fluid, loses all its viscosity and can flow without resistance, which can form thin films.
Superfluid helium, for example, forms a 30-nm-thick film on the surface of any container. The film's properties cause the helium to climb the walls of the container and, if this is not closed, flow out.
Superfluidity and superconductivity are macroscopic manifestations of quantum mechanics. There is considerable interest, both theoretical and practical, in these quantum phase transitions. There has been a tremendous amount of work done in the field of phase transitions and critical phenomena in two dimensions. Much of the interest in this field is because as the number of dimensions increases, the number of exactly solvable models diminishes drastically. In three or more dimensions one must resort to a mean field theory approach. The theory of superfluid transitions in two dimensions is known as the Kosterlitz-Thouless (KT) theory. The 2D XY model - where the order parameter is characterized by an amplitude and a phase - is the universality class for this transition.
In looking at phase transitions in thin films, specifically helium, the two main experimental signatures are the superfluid fraction and heat capacity. If either of these measurements were to be done on a superfluid film in a typical open container, the film signal would be overwhelmed by the background signal from the container. Therefore, when studying superfluid films, it is of paramount importance to study a system of large surface area as to enhance the film signal.
There are several ways of doing this. In the first, a long thin strip of material such as PET film is rolled up into a "jelly roll" configuration. The result is a film that is a long continuous plane, referred to as a planar film. A second way is to have a highly porous material such as porous gold, Vycor, or Aerogel. This results in a multiply connected film where the substrate is much like Swiss cheese with the holes interconnected.
Official source
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