News FocusCondensed-Matter Physics

The Quirks and Culture of Helium

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Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 39
DOI: 10.1126/science.309.5731.39

Ordinarily an inert gas so light it floats off into space, helium might seem to hold little interest for condensed-matter physicists. But since it was liquefied by Dutch physicist Heike Kamerlingh Onnes in 1908, the odd stuff has revealed much about the physics of liquids and solids. “Throughout history, it has provided a variety of new paradigms,” says Jason Ho, a theorist at Ohio State University in Columbus.

Since 1938, physicists have known that below 2.17 kelvin the most common isotope of helium, helium-4, becomes a “superfluid” that flows without resistance, as about 9% of the atoms crowd into a single quantum wave. In 1972, physicists discovered that helium-3 also becomes a superfluid at just a few thousandths of a kelvin. Because of the way they spin, helium-3 atoms cannot pile into a single quantum wave. Instead, they form pairs that glide without resistance, as electrons do in superconductors.

Experiments with helium-3 validated much of the “Fermi liquid theory” that also describes electrons in metals. The superfluid transition in helium-4 provided the primary test bed for the theory of “second-order phase transitions,” which describes, for example, the onset of magnetism in materials.

While helium has helped theorists develop key concepts, experimenters working with ultracold helium have developed a reputation for old-fashioned ingenuity. Their experimental devices are usually mechanical contraptions that shake, spin, and squeeze helium to produce subtle but telling signals. By tradition, “you don't buy your instrumentation; you invent it,” says John Goodkind, an experimenter at the University of California, San Diego. “You make it, you leak-check it, and you fix it.”

Helium physicists are also known for seat-of-the-pants problem solving, slathering their refrigerators with soap and glycerin to plug elusive leaks so small only superfluid helium squeezes through, or using a condom to regulate the flow of gas.

Never very big, the field of helium physics has contracted since its heyday in the 1970s. But researchers trained in helium physics have become leaders in high-temperature superconductivity, nanomechanical devices, two-dimensional electron systems, and other areas. “The people in the field are willing to take risks,” says Richard Packard, an experimenter at the University of California, Berkeley. “They aren't afraid of making new devices, and when they go out into other fields, that same state of mind goes with them.”

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