Semi-superfluidity

Low-temperature superconductors resemble low-temperature superfluid Helium-4; in the first, pairs of electrons condense into a macroscopically coherent quantum state, which manifests itself as a resistanceless fluid, while in the other helium-4 atoms condense into an analogous state which manifests itself as a frictionless fluid. Helium-3 can also form a coherent quantum state and exist as a superfluid when helium-3 atoms pair up, but things are more complicated because the He-3 pairs are magnetic, while He-4 atoms and electron pairs are not.

In recent years one way of studying how He-3 atoms interact is to loose a sample of the fluid into a sample of aerogel, the nearly-as-light-as-air solid consisting of a diaphanous forest of silicon pillars, some only 5 nm across. The pillars serve as a sort of impurity and the superfluid properties alter appropriately. The main change is that the temperature at which this "dirty" He-3 becomes superfluid is depressed relative to the case for unadulterated He-3.

However, a new experiment at the National Center for Scientific Research in Grenoble, France (Yuriy Bunkov, bunkov@labs.polycnrs-gre.fr, 011-33-476-88-1252) reveals that He-3 shows some of the properties of the quantum state at temperatures in between the critical points for the pure and dirty cases. Studying the helium using nuclear magnetic resonance (NMR) techniques, the Grenoble venture to suggest that one reason for the anomalous behavior might be the existence of a new type of superfluidity, one involving the coupling of not two but four helium-3 atoms. (Bunkov et al., Physical Review Letters, 16 Oct; Select Articles.)