2003 Nobel prize in physics for theoretical work on superfluid ³He

The element helium comes in two (stable) forms, ⁴He and ³He; at low temperatures and pressures both form liquids rather than solids. The liquid phase of the common isotope, ⁴He, was realized nearly a century ago, and since 1938 has been known to show, at temperatures below about 2 K, the property of superfluiditythe ability to flow through the narrowest capillaries without apparent friction. The light isotope, ³He, is believed to be of quite a different nature; however, because of its similarity to the electrons in metals, which at low temperatures sometimes form "Cooper pairs" and thereby become superconducting, theorists in the 1960s and early 1970s had speculated that something similar might happen in liquid ³He, which would then also show superfluidity though for reasons rather different than ⁴He. In 1972 nuclear magnetic resonance (NMR) experiments at Cornell University revealed the existence, below 3 millidegrees, if two new phases, one of which displayed extraordinary NMR properties. Anthony Leggett is one of the theorists who succeeded in fitting the experimental properties into the "Cooper- pairing" scenario; in particular, he explained the NMR behavior and predicted further novel NMR phenomena which were subsequently found.