SUPERFLUIDITY

an unusual set of properties that occur only in liquid helium when it is cooled to near absolute zero (0 K [-273.15 C]). Superfluidity occurs in both of the stable isotopes of helium: helium-3 and helium-4. Superfluidity in helium-4 was discovered in 1938 by the Soviet physicist Pyotr Leonidovich Kapitsa. Helium-4 exhibits superfluidity when it is cooled below 2.18 K (-270.97 C), which is called the lambda (l) point. At these temperatures, helium-4 exhibits the characteristics of two distinct fluids, one of which appears to flow without friction. An extensive series of experiments showed that in this state of helium, called helium II (He II), there is an apparent enormous rise in heat conductivity, at an increase rate of about three million. Another unusual property of He II is its mobile, rapid flow through capillaries or over the rim of its containment vessel as a thin film that exhibits no measurable viscosity and appears unaffected by the forces of gravity or evaporation and condensation. In order to account for such behaviour, the two fluid model, as proposed by Laszlo Tisza, described He II as a mixture of normal helium (helium I) and superfluid helium. The normal component is attributed to helium atoms in excited energy states, whereas the superfluid component is attributed to atoms all in the ground state (having lowest or zero-point energy). As the temperature continues to be lower than the lambda point, more of the He II becomes superfluid. It is assumed that this superfluid component is able to move through its container without friction, thereby explaining most of the distinctive properties of helium II. Superfluidity was first discovered in helium-3 by American physicists David M. Lee, Douglas D. Osheroff, and Robert C. Richardson. It occurs at temperatures a few thousandths of a degree above absolute zero and is distinguished by either an A phase or a higher-pressure, lower-temperature B phase. Helium-3 is anisotropic, which means it displays different properties when measured in different directions, and as such, its study has become valuable to scientists in the fields of big-bang theory and superconductivity.