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superfluidity

superfluidity

[soo-per-floo-id]
superfluidity, tendency of liquid helium below a temperature of 2.19°K; to flow freely, even upward, with little apparent friction. Helium becomes a liquid when it is cooled to 4.2°K;. Special methods are needed to cool a substance below this temperature, which is very near absolute zero (see Kelvin temperature scale; low-temperature physics). When the temperature reaches 2.19°K;, the properties of liquid helium change abruptly, so much so that ordinary helium is known as helium I and helium below this temperature is known as helium II. The transition temperature between helium I and helium II is known as the lambda point because a graph of certain properties of helium takes a sharp turn at this temperature and resembles the Greek letter lambda (Λ). Liquid helium II flows easily through capillary tubes that resist the flow of ordinary fluids (see capillarity) and a Dewar flask filled with helium II from a larger container will empty itself back into the original container because the liquid helium flows spontaneously in an invisible film over the surface of the flask. The behavior of helium II can be partially understood in terms of certain quantum effects (see quantum theory). Helium stays a liquid down to absolute zero because its zero-point energy is such that it cannot become a solid without giving up an amount of energy that is less than that allowed by the quantum theory. Similarly, quantum restrictions keep helium II from behaving like a normal fluid because the energy interactions associated with friction and viscosity in normal fluid flow involve amounts not possible for helium II.

Unusual property of liquid helium cooled below −455.75 °F (−270.97 °C). At such low temperatures, helium exhibits an enormous rise in heat conductivity and rapid flow through capillaries or over the rim of its container. To explain such behaviour, the substance is described in terms of a “two-fluid” mixture model consisting of normal helium and superfluid helium. In normal helium the atoms are in excited states (see excitation), whereas in superfluid helium they are in their ground state. As the temperature is lowered below −455.75 °F, more of the helium becomes superfluid. It is assumed that the superfluid component can move through the container without friction, thereby explaining the unusual behaviour.

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In the physics of superfluidity, a boojum is a geometric pattern on the surface of one of the phases of superfluid helium-3, whose motion can result in the decay of a supercurrent. A boojum can result from a monopole singularity in the bulk of the liquid being drawn to, and then "pinned" on a surface. Although superfluid helium-3 only exists within a few thousandths of a degree of absolute zero, boojums have also been observed forming in various liquid crystals at room temperature.

The boojum was named by David Mermin of Cornell University in 1976. He was inspired by Lewis Carroll's poem The Hunting of the Snark. As in the poem, the appearance of a boojum can cause something (in this case, the supercurrent) to "softly and suddenly vanish away". Other, less whimsical names had already been suggested for the phenomenon, but Mermin was persistent. After an exchange of letters that Mermin describes as both "lengthy and hilarious" , the editors of Physical Review Letters agreed to his terminology. Research using the term "boojum" in a superfluid context was first published in 1977, and the term has since gained widespread acceptance in broader areas of physics. Its Russian phonetic equivalent is "budzhum", which is also well accepted by physicists.

The plural of the term is "boojums", a word initially disliked by Mermin (who at first used "booja") but one which is defined unambiguously by Carroll in his poem.

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