Science · Physics

Superfluidity

Discovered 1937

Physics — low-temperature and condensed matter physics

Superfluidity is the state in which a liquid flows with exactly zero viscosity, so that once set moving it never stops. It was found in helium-4 cooled below 2.17 K, the lambda point, in experiments completed in 1937 and published simultaneously in January 1938 by Pyotr Kapitsa in Moscow and by John F. Allen and Don Misener in Cambridge. The liquid does impossible-looking things: it climbs the walls of its container in a film and siphons itself out, it squeezes through pores no ordinary liquid can enter, it conducts heat so well that it stops boiling and goes glassy, and it can be made to squirt upwards in a fountain when a heater is switched on. Helium-3, whose atoms are fermions, becomes superfluid too, but only at around two thousandths of a degree above absolute zero.

How to spot them

▸A liquid with exactly zero viscosity that crawls up and over the wall of its beaker
▸Helium-4 below 2.17 K, the temperature known as the lambda point
▸At the transition the liquid abruptly stops bubbling and goes eerily still
▸A heater plunged into it makes a fountain spurt upwards
▸It keeps the magnets of the Large Hadron Collider at 1.9 kelvin

Key facts

The state in which liquid helium creeps up the walls of a beaker and siphons itself out

A Rollin film about 30 nanometres thick flows over any barrier at the same rate.

Reached below the lambda point of 2.17 K, where the liquid suddenly stops bubbling

Its thermal conductivity becomes so enormous that no hot spots form to nucleate bubbles.

Demonstrated by the fountain effect, where a heater makes the liquid spurt upward (1938)

The thermomechanical effect: heat drives flow through a fine capillary.

Found in 1937 by Kapitsa in Moscow and by Allen and Misener in Cambridge at the same time (1937)

Their papers appeared back to back in Nature on 8 January 1938.

In the rarer helium isotope it requires about 2.5 millikelvin and paired fermionic atoms (1972)

Osheroff, Lee and Richardson found it at Cornell; Cooper-like pairs make bosons of fermions.

What cools the 1,232 dipole magnets of CERN's biggest ring to 1.9 K

Its huge heat conductivity carries away energy no ordinary coolant could.

Key ideas

Below the lambda point a macroscopic fraction of the helium atoms occupy the same quantum state, closely related to Bose–Einstein condensation, so the liquid has a single wavefunction and no way to dissipate energy in small steps.
The two-fluid model treats the liquid as a mixture of a normal, viscous component and a frictionless superfluid component, whose proportions depend on temperature.
Rotation cannot be shared out smoothly, so the liquid carries angular momentum in quantised vortices — tiny whirlpools with a fixed circulation.

Quick-fire facts

•Discovered in liquid helium-4 below the lambda point, 2.17 K, and named 'superfluidity' by Kapitsa by analogy with superconductivity
•Pyotr Kapitsa, and independently John F. Allen and Don Misener, published in Nature on 8 January 1938
•The transition is called the lambda point because the heat-capacity curve is shaped like the Greek letter λ
•Helium is the only element that stays liquid down to absolute zero at ordinary pressure; solidifying it needs about 25 atmospheres, because zero-point energy keeps the atoms jiggling
•Lev Landau won the 1962 Nobel Prize in Physics for the theory of condensed matter, especially liquid helium
•Kapitsa shared the 1978 Nobel Prize in Physics for his low-temperature work
•Superfluid helium-3 was found in 1972 by Douglas Osheroff, David Lee and Robert Richardson, who shared the 1996 Nobel Prize in Physics
•Heat travels through superfluid helium as a wave, a phenomenon called second sound
•The Large Hadron Collider runs on about 120 tonnes of helium, much of it superfluid at 1.9 K
•Neutron stars are thought to contain superfluid neutrons, which may explain the sudden spin-ups called glitches

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Sources

The Nobel Foundation — The Nobel Prize in Physics 1996 For the discovery of superfluidity in helium-3.
The Nobel Foundation — The Nobel Prize in Physics 1962 Landau, for his theories of condensed matter, especially liquid helium.
CERN — Cryogenics: low temperatures, high performance