The Nobel Prize in Physics 1996
David M. Lee, Douglas D. Osheroff, Robert C. Richardson
Presentation Speech by Professor Carl
Nordling of the Royal
Swedish Academy of Sciences.
Translation of the Swedish text
Your Majesties, Your Royal Highness, Ladies
The air that we breathe not only contains oxygen and nitrogen, but also small quantities of other gases. One of these - often mentioned in connection with the global greenhouse effect - is carbon dioxide. Another is the noble gas helium, which makes up five pails per million of our atmosphere.
This chemical element exists in two forms, or isotopes: a heavier one called helium-4 and a lighter one called helium-3. The heavier variety accounts for nearly all helium. Helium-3 makes up only one millionth of the total quantity of helium, which is already very insignificant to begin with. Yet this year's Nobel Prize in Physics is all about helium-3.
David Lee, Douglas Osheroff and Robert Richardson used a few cubic centimeters of liquid helium-3 to perform the experiments that would lead to the discovery being rewarded with this year's Nobel Prize. They changed its pressure, temperature and volume, carefully monitoring the mutual dependence of these valuables.
In the resulting diagram, they observed two small jogs in the curve, a few thousandths of a degree above absolute zero. Many researchers would probably have shrugged their shoulders at these deviations, dismissing them as minor imperfections in their measuring apparatus.
Not these three researchers. Might new magnetic states be manifesting themselves in this way? What the three were actually looking for was magnetism in solid helium-3. At first, they also believed that this was what they had seen. But there was not a perfect correspondence with the measurement data. By means of measurements employing the same technique that has since come to be used in hospital magnetic resonance imaging systems, Lee, Osheroff and Richardson were able to show that the phenomenon was occurring in liquid helium-3, not in solid helium-3.
In other words they had discovered two new forms of liquid helium-3, both superfluid. The team eventually discovered three superfluid phases. And as is so often the case in basic research, they had discovered something other than what they were looking for!
Superfluidity is a remarkable and unusual property that had previously been observed only in helium-4. It manifests itself in different ways: A super-fluid liquid lacks viscosity and cannot be stored in an unglazed ceramic vessel, because it seeps out through the microscopic pores in the ceramic. If an empty beaker is lowered part way into the liquid, the liquid flows upward, over the edge and down into the beaker.
To describe the phenomenon of superfluidity at a fundamental, atomic level, we usually say that the atoms have undergone a Bose-Einstein condensation. This means that all the atoms join in a common quantum state. Such a condensation is only possible in a category of particles called bosons. In the category called fermions, this type of condensation is not possible.
As it happens, helium-3 atoms are fermions and should thus not be capable of undergoing a Bose-Einstein condensation and form a superfluid liquid. Yet they can. The explanation is that the atoms form pairs, where the atoms orbit each other. Such a pair behaves like a boson and - presto - Bose-Einstein condensation can now occur and the liquid becomes superfluid!
Lee's, Osheroff's and Richardson's discovery triggered intensive research activity in all the low-temperature laboratories of the world. The phase transitions to superfluidity in liquid helium-3 showed that the quantum laws of microphysics sometimes also govern the behavior of macroscopic quantities of matter. They are being used to define the temperature scale at very low temperatures. They are contributing to our growing understanding of "warm" superconductors, and they were recently used as the model for how "cosmic strings" may have been formed in the universe.
Professor Lee, Professor Osheroff,
You have been awarded the 1996 Nobel Prize in Physics for your discovery of superfluidity in helium-3. Your discovery has greatly enlarged our knowledge of the possible states of condensed matter. On behalf of the Royal Swedish Academy of Sciences, I wish to congratulate you on this achievement, and I now ask you to step forward and receive your Prize from the hands of His Majesty the King.
From Les Prix Nobel. The Nobel Prizes 1996, Editor Tore Frängsmyr, [Nobel Foundation], Stockholm, 1997
Copyright © The Nobel Foundation 1996