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Superfluid Helium

Dateline: 05/29/00

By Alan Bruzel

We are most familiar with helium as the gas used to inflate balloons, blimps, and dirigibles. Although helium exists as two stable isotopes (helium-3 and helium-4), helium-3 occurs but rarely in nature; it is commercially available as a by-product from nuclear weapons plants. Helium-4 is by far the most abundant isotope of helium – more than 99% of naturally occurring helium is helium-4.

Liquid helium-4 has been known for some time. In the early 1900's, Heike Kamerlingh-Onnes first liquefied helium. He also invented supercooling devices, discovered superconductivity, and received the 1913 Nobel Prize in Physics. But it wasn't until the late 1930's that Pyotr Kapitsa (who was to receive the 1978 Nobel Prize in Physics for this work) observed an interesting phenomenon upon cooling helium-4 below its 4.2 K (-269 ^oC, -452 ^oF) boiling point.

Kapitsa found that at 2.17 K (absolute zero, 0 K, is the lowest temperature possible) helium-4 further condensed and exhibited behavior unlike that of any other known liquid. At 2.17 K and below, helium-4 becomes a superfluid: an entity without viscosity and thus capable of flowing without friction. This explains superfluid helium's ability to creep up the walls of any vessel containing it. Superfluid helium moves from cool regions to warmer regions. Because it moves without friction – and is such a lightweight atom – superfluid helium travels along the thin film of superfluid helium already coating the walls of a container, effortlessly moving from the cooler region at the bottom to the warmer region at the top.

Also, unlike conventional liquids which pass freely in either direction through porous materials, superfluid helium atoms pass through porous plugs made of ceramic or metal (whose pores are so small that the plug is impervious to other liquids) and, when they have reached the other side and have become warmer than 2.17 K, are then unable to pass back again. In other words, the colder, frictionless, superfluid helium (known as helium II) upon warming becomes a warmer, unremarkable, ordinary liquid helium (known as helium I) and assumes a measurable viscosity.

One further remark is in order. Helium-3 can also be a superfluid, but only at temperatures far lower than that for superfluid helium-4. In 1972, Lee, Osheroff, and Richardson succeeded in preparing and identifying superfluid helium-3 (for which they received the Nobel Prize in Physics for 1996) by reducing the temperature of helium-3 to 0.002 K.

Now, a superfluid is believed to be an aggregate of components all in the same quantum state. Bosons, such as helium-4, contain an even number of particles and are able to exist in the same quantum state. (The nucleus of helium-4 consists of four particles: two protons and two neutrons.) Fermions, such as helium-3, contain an odd number of particles and, because they must obey the Pauli Exclusion Principle, cannot all exist in the same quantum state. (The nucleus of helium-3 has three particles: two protons and one neutron.) Helium-3 needs a vastly lower temperature to achieve superfluidity (as compared to helium-4) because only at this temperature will helium-3 fermions begin to pair up and act as bosons. These bosons can then enter the same quantum state and form a superfluid.

What the Web Has to Say about:
Superfluid Helium

Behavior of Superfluid Helium
Two-fluid model and differences between helium-3 and helium-4. From J.C. Davis, University of California at Berkeley.

Introduction to Liquid Helium
With a description of the Superfluid Helium On-Orbit Transfer project (part of the June, 1993 Space Shuttle mission). From Cryogenics and Fluids Branch, NASA.

Liquid Genius
New Scientist article by Michael Brooks describes quantum whistle in superfluid helium.

Nobel Prize in Physics 1913
To Heike Kamerlingh-Onnes, first to prepare liquid helium and discoverer of superconductivity.

Nobel Prize in Physics 1978
To Pyotr L. Kapitsa for research into the properties of superfluid helium II. Shared with Arno A. Penzias and Robert W. Wilson for their work in cosmic microwave background radiation.

Nobel Prize in Physics 1996
To David M. Lee, Douglas D. Osheroff, and Robert C. Richardson for their discovery of superfluidity in helium-3.

Physics News Graphics: Superfluidity in Helium-3
Representations of condensed helium-3 and helium-4 atoms. From the American Institute of Physics.

Superfluids
Introduction to some properties of superfluid helium. From James Schombert, University of Oregon.

Superfluid Helium As a Vacuum
Molecules other than helium, when dissolved in superfluid helium, act as if they are in a vacuum. From David Ceperley, University of Illinois at Urbana.

Superfluid Helium: What Is It?
Phase diagram of helium-4 shows its existence as a liquid down to absolute zero. From M.S. Cramer, Virginia Polytechnic Institute and State University.

Why Does Supercooled Helium Run Uphill?
It doesn't, as explained in this MAD Scientist Network reply, but it is exceedingly responsive to pressure and temperature changes.

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