Superfluids
Superconductors
Helium is the only gas that makes a good superfluid because it has such weak intermolecular forces. Superfluidity was first demonstrated in Helium-4 in 1962 by Landau. Helium condenses to a liquid at 4.2 K, and turns into a superfluid at 2.17 K. The point at which it becomes superfluid is called the critical point, or lambda point, becaue its specific heat graph looks kind of like a lambda. The point is characterized by a jump in specific heat, and an discontinuity in its density graph.
Quantized vortices and second and third sound were discovered first in Helium 4. Helium 4 was good to work with because it was fairly easy to cool to its lambda point, and it was easy to come by. Helium 3 was thought by some physicists to work, but it was so rare that a pure sample was way to expensive in the quantities needed to study vortices and other effects. Besides, Helium 3's lambda point was much lower.
In the forties and fifties, helium-3 became fairly common because it was a major by-product of nuclear weapons production. Interest in helium-3 was sparked, but no one was able to cool it enough to see any superfluid properties. Finally, in 1996, David M. Lee and Robert C. Richardson of Cornell University, and Douglas D. Osheroff of Stanford University, cooled Helium 3 enough to turn it into a superfluid. The temperature they cooled it to was 2 mK, one thousand times colder than Helium 4's lambda point. They figured that Helium-3 had to pair up so that the pairs became bosons, and in order to form these pairs the atoms had to be moving very slow. Once the helium-3 pairs were bosons, they system could exhibit Bose-Einstein properties and assume the same quantum state, thereby becoming a superfluid.