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|A superfluid helium fountain|
When helium-4 is chilled to below about 2.2 K, it starts to behave in some very weird ways. The fluid passes through narrow tubes with almost no friction, and even climbs up walls and overflows its container. Though there were early suggestions of odd behavior, it took 30 years after helium had been liquefied before its superfluidity was discovered.
In 1908, Heike Kamerlingh Onnes first liquefied helium at the University of Leiden in the Netherlands. Soon there were several hints at the strange behavior of liquid helium. By 1924 Onnes had made precise measurements of liquid helium’s density, and found that as the temperature lowers, the density goes through a sharp maximum at about 2.2 K. In 1927 Willem Keesom and Mieczyslaw Wolfke concluded that that liquid helium undergoes a phase transition at about 2.2 K. This temperature is called the lambda point because the graph of specific heat versus temperature resembles the Greek letter lambda. The two phases are called helium I and helium II.
Though these were interesting results, they were not so surprising that anyone paid much attention at the time. The truly remarkable result, that helium II is a superfluid, was first discovered in 1937 and published in January 1938, by Pyotr Kapitsa in Moscow, and independently by John F. Allen and Donald Misener at the University of Toronto.
Kapitsa, the son of a military engineer, was born in 1894 in Kronstadt, near Leningrad. He studied engineering at Petrograd Polytechnical Institute, graduated in 1918, and stayed on as a lecturer there for several years, during which time he carried out research on magnetic fields.
After losing his first wife and two young children to an influenza epidemic in 1921, Kapitsa moved to Cambridge to work with Ernest Rutherford at the Cavendish Laboratory. Kapitsa first worked on magnetic field research, developing ways to produce extremely strong magnetic fields. After several years he turned his attention to low temperature research, and in 1934 he developed a new method for liquefying large amounts of helium, which paved the way for continued experiments with the strange fluid.
In 1934, Kapitsa traveled to Russia on a visit, expecting to return to Cambridge. For reasons that are not clear, he was detained and had his passport seized on Stalin’s orders. When it became clear that Kapitsa could not return to Cambridge, Rutherford helped arrange for most of his apparatus from his lab at Cambridge to be sent to him, and Kapitsa set up a new research facility, the Institute of Physical Problems, in Moscow.
In 1937, while investigating the thermal conductivity of liquid helium, Kapitsa measured the flow as the fluid flows through a gap between two discs into a surrounding bath.
The results were striking: above the lambda point, there was little flow, but below the lambda temperature, the liquid flowed with such great ease that Kapitsa drew an analogy with superconductors, and wrote in his paper in Nature on January 8, 1938 “the helium below the lambda point enters a special state that might be called a ‘superfluid'."
At the same time, Allen and Misener at the University of Toronto performed similar studies on liquid helium, using a slightly different setup. They measured the flow through a narrow glass tube, and also observed the extremely low viscosity. They noted that the flow was almost independent of pressure and that therefore “any known formula cannot, from our data, give a value of viscosity which would have any meaning.” Their paper appeared in Nature back-to-back with Kapitsa’s article.
It is now understood that helium II can be described as a two-fluid mixture–part a normal fluid, and part a superfluid, in which atoms have condensed into a single quantum state. This two fluid model explains Kapitsa’s and Allen and Misener’s results.
Kapitsa continued his research in low temperature physics for several years. During World War II he built an apparatus for producing large amounts of liquid oxygen for the soviet steel industry. In the 1940s he turned his attention to plasma physics and fusion. In 1946 he refused to work on the Soviet atomic bomb, and thus fell out of favor with Stalin. He lost his position at the Institute of Physical Problems, and was not reinstated until after Stalin’s death.
Thirty years after his discovery of superfluidity, and long after he had moved on to other research topics, Kapitsa was awarded the Nobel Prize in Physics for his low temperature research. He shared the 1978 prize with Arno Penzias and Robert Wilson, who won for their discovery of the cosmic microwave background radiation.
Allen and Misener, though they made essentially the same discovery as Kapitsa, did not receive a Nobel Prize, and Kapitsa is generally the one credited with the discovery of superfluidity.
The work on liquid helium and the understanding of the weird properties of the superfluid state have been fundamental to the field of low temperature physics, which is still an exciting area of research today, as ever more exotic low temperature states continue to be produced.
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