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9 October 1996

The Royal Swedish Academy of Sciences has decided to award the 1996 Nobel Prize in Physics to

Professor David M. Lee, Cornell University, Ithaca, New York, USA,

Professor Douglas D. Osheroff, Stanford University, Stanford, California, USA and

Professor Robert C. Richardson, Cornell University, Ithaca, New York, USA

for their discovery of superfluidity in helium-3.

A breakthrough in low-temperature physics

When the temperature sinks on a cold winter’s day water vapour becomes water and water becomes ice. These so-called phase transitions and the changed states of matter can be roughly described and understood with classical physics. What happens when the temperature falls is that the random heat movement in gases, liquids and solid bodies ceases. But the situation becomes entirely different when the temperature sinks further and approaches absolute zero, -273.15°C. In samples of liquid helium what is termed superfluidity occurs, a phenomenon that cannot be understood in terms of classical physics. When a liquid becomes superfluid its atoms suddenly lose all their randomness and move in a coordinated manner in each movement. This causes the liquid to lack all inner friction: It can overflow a cup, flow out through very small holes, and exhibits a whole series of other non-classical effects. Fundamental understanding of the properties of such a liquid requires an advanced form of quantum physics, and these very cold liquids are therefore termed quantum liquids. By studying the properties of quantum liquids in detail and comparing these with the predictions of quantum physics low-temperature, researchers are contributing valuable knowledge of the bases for describing matter at the microscopic level.

David M. Lee, Douglas D. Osheroff and Robert C. Richardson discovered at the beginning of the 1970s, in the low-temperature laboratory at Cornell University, that the helium isotope helium-3 can be made superfluid at a temperature only about two thousandths of a degree above absolute zero. This superfluid quantum liquid differs greatly from the one already discovered in the 1930s and studied at about two degrees (i.e. a thousand times) higher temperature in the normal helium isotope helium-4. The new quantum liquid helium-3 has very special characteristics. One thing these show is that the quantum laws of microphysics sometimes directly govern the behaviour of macroscopic bodies also.

The isotopes of helium
In nature the inert gas helium exists in two forms, isotopes, with fundamentally different properties. Helium-4 is the commonest while helium-3 occurs only as a very small fraction. Helium-4 has a nucleus with two protons and two neutrons (the 4 stands for the total number of nucleons, i.e. protons and neutrons). The nucleus is surrounded by an electron shell with two electrons. The fact that the number of particles constituting the atom is even makes helium-4 what is termed a boson. The nucleus of helium-3 also has two protons, but only one neutron. Since its electron shell also has two electrons, helium-3 consists of an odd number of particles, which makes it what is termed a fermion. Since the two isotopes of helium are built up of different numbers of particles, dramatic differences in their behaviour arise when they are cooled to temperatures near absolute zero.

The properties of the isotopes
Bosons such as helium-4 follow Bose-Einstein statistics which, among other things, means that under certain circumstances they condense in the state that possesses the least energy. A phase transition process in which this occurs is termed Bose-Einstein condensation. The first person to manage to cool helium-4 gas to such low temperatures that it liquidised was Heike Kamerlingh-Onnes (Nobel Prize in Physics 1913). This happened at the beginning of the 1900s. He noted even then that when the temperature came closer to absolute zero than about 2 degrees something special happened in the liquid. But it was not until the end of the 1930s that Pjotr Kapitsa (Nobel Prize in Physics 1978) discovered experimentally the phenomenon of superfluidity in helium-4, a phenomenon first explained schematically by Fritz London and then in detail by Lev Landau (Nobel Prize in Physics 1962). The explanations are based on the fact that the superfluid liquid, which appears at a phase transition when the temperature is only 2.17° above absolute zero, is a kind of Bose-Einstein condensate of helium atoms.

Fermions such as helium-3 follow Fermi-Dirac statistics and should not actually be condensable in the lowest energy state. For this reason superfluidity should not be possible in helium-3 which, like helium-4, can be liquidised at a temperature of some degrees above absolute zero. But fermions can in fact be condensed, but in a more complicated manner. This was proposed in the BCS theory for superconductivity in metals, formulated by John Bardeen, Leon Cooper and Robert Schrieffer (Nobel Prize in Physics 1972). The theory is based on the fact that electrons are fermions (they consist of one particle only, an odd number) and therefore follow Fermi-Dirac statistics just as helium-3 atoms do. But electrons in greatly cooled metals can combine in twos to form what are termed Cooper pairs and then behave as bosons. These pairs can undergo Bose-Einstein condensation to form a Bose-Einstein condensate. Starting with the experience of superfluidity in helium-4 and superconductivity in metals, it was expected that the fermions in liquid helium-3 should be capable of forming boson pairs and that superfluidity should be obtainable in very cold samples of the isotope helium-3. Although many research groups had worked with the problem for years, particularly during the 1960s, none had succeeded and many considered that it would never be possible to achieve superfluidity in helium-3.

The discovery
The researchers at Cornell University were low-temperature specialists and had built their apparatus themselves. With it they could produce such low temperatures that the sample was within a few thousands of a degree of absolute zero. David Lee and Robert Richardson were the senior researchers while Douglas Osheroff was a graduate student in the team. Actually they were looking for a different phenomenon: A phase transition to a kind of magnetic order in frozen helium-3 ice. To find this phase transition, they were studying the pressure measured within the sample as a function of the time during which the volume was slowly increased and reduced. It was Osheroff’s vigilant eye that noted small extra jumps in the curve measured (Fig. 1). It is easy to consider such small deviations as more or less inexplicable characteristics of the apparatus, but this student and his older co-workers became convinced that it was a true effect. In a first report published in 1972 the result was interpreted as a phase transition in the solid helium-3 ice which can also form at these low temperatures. But since the interpretation did not correspond precisely with the results of measurement, a rapid series of supplementary measurements was undertaken and in the same year the researchers were able to show in a second publication that there were in fact two phase transitions in liquid helium-3. The discovery heralded the start of intensive research on the new quantum liquid. A particularly important contribution was made by the theoretician Anthony Leggett, who assisted in the interpretation of the discovery. This thus assumed great significance for our knowledge of how the laws of quantum physics, formulated for microscopic systems, sometimes directly govern macroscopic systems also.

Fig. 1. The figure shows the pressure inside a sample containing a mixture of liquid helium-3 and solid helium-3 ice. The sample is first subjected to increasing external pressure for about 40 minutes, whereafter the external pressure is reduced. Note the changes in the slope of the curve at A and B and the temperatures at which these occur. The graph resembles that published by D.D. Osheroff, R.C. Richardson and D.M. Lee in Physical Review Letters 28, 885 (1972) in which the new helium-3 phase transitions were first reported. It is taken from an article by N.D. Mermin and D.M. Lee in Scientific American 1976 (see Further Reading).

Superfluidity in helium-3
That the new liquid really was superfluid was confirmed soon after the discovery, among others by a research team under Olli Lounasmaa at the Helsinki University of Technology. They measured the damping of an oscillating string placed in the sample and found that the damping diminished by a factor of one thousand when the surrounding liquid underwent the phase transition to the new state. This shows that the liquid is without inner friction (viscosity).

Later research has shown that helium-3 has at least three different superfluid phases, of which one occurs only if the sample is placed in a magnetic field. As a quantum liquid helium-3 thus exhibits a considerably more complicated structure than helium-4. It is, for example, anisotropic, which means that it has different properties in different spatial directions, which does not occur in classical liquids but more resembles the properties of liquid crystals (cf. Nobel Prize in Physics 1991 to Pierre-Gilles de Gennes).

If a superfluid liquid is caused to rotate at a speed exceeding a critical value, microscopic vortices arise. This phenomenon, which is also known from superfluid helium-4, has in helium-3 led to extensive research since its vortices can assume more complicated forms. Finnish researchers have developed a technique using optical fibres to observe directly how vortices affect the surface of rotating helium-3 at temperatures only one thousandth of a degree from absolute zero.

A fascinating application of superfluidity in helium-3
The phase transitions to superfluidity in helium-3 have recently been used by two experimental research teams to test a theory regarding how what are termed cosmic strings can be formed in the universe. These immense hypothetical objects, which are thought possibly to have been important for the forming of galaxies, can have arisen as a consequence of the rapid phase transitions believed to have taken place a fraction of a second after the Big Bang. The research teams used neutrino-induced nuclear reactions to heat their superfluid helium-3 samples locally and rapidly. When these were cooled again, balls of vortices were formed. It is these vortices that are presumed to correspond to the cosmic strings. The result, which must not be taken as proof of the existence of cosmic strings in the universe, is that the theory tested appears to be applicable to vortex formation in superfluid helium-3.

David M. Lee
born 1931 in Rye, NY, USA. American citizen. Doctoral degree in physics 1959, Yale University. Lee has received among other awards the Institute of Physics Sir Francis Simon Memorial Prize 1976 and the Oliver E. Buckley Solid State Physics Prize (American Physical Society) 1980 for the discovery of superfluidity in helium-3.

Professor David M. Lee
Department of Physics
Cornell University
Ithaca, NY 14853
USA

Douglas D. Osheroff
born 1945 in Aberdeen, WA, USA. American citizen. Doctoral degree in physics 1973 at Cornell University. Osheroff has received among other awards the Institute of Physics Sir Francis Simon Memorial Prize 1976 and the Oliver E. Buckley Solid State Physics Prize (American Physical Society) 1980 for the discovery of superfluidity in helium-3.

Professor Douglas D. Osheroff
Department of Physics
Stanford University
Stanford, CA 94305
USA

Robert C. Richardson
born 1937 in Washington, DC, USA. American citizen. Doctoral degree in physics 1966 at Duke University. Richardson has received among other awards the Institute of Physics Sir Francis Simon Memorial Prize 1976 and the Oliver E. Buckley Solid State Physics Prize (American Physical Society) 1980 for the discovery of superfluidity in helium-3.

Professor Robert C. Richardson
Department of Physics
Cornell University
Ithaca, NY 14853
USA