Superfluid liquid hydrogen?

Recent work by Peter Toennies’ group in Göttingen has revealed convincing evidence for superfluidity in liquid hydrogen – thus adding a third liquid to the two, liquid He and liquid He, in which superfluididy is already known. Superfluids are numbered among the most peculiar and counter-intuitive of all materials. They have no viscosity at all. Thus, an object travelling in a pure superfluid moves frictionlessly. Its behaviour is inertial, much as it would be in a vacuum (apart from an increased effective mass). Similarly, superfluids can flow effortlessly through narrow channels or pores that are virtually impermeable to conventional liquids. Superfluids are relatively rare, and inaccessible. The first liquid in which superfluidity was recognised was He, in 1938. It was found to undergo a phase transition to the superfluid state below the so-called lambda temperature Tλ = 2.12K. Fritz London suggested at the time that the phenomenon was associated with Bose-Einstein condensation (BEC), which is expected to occur in any assembly of freely mobile bosons (particles with zero or integer spin) if cooled below a critical temperature TB. In BEC, a macroscopic fraction of all the particles in the assembly congregate together in the zero-momentum ground state. In 1972 superfluidity was also discovered in the rare isotope, liquid He, below a critical temperature about a thousand times smaller, at Tc = 2.4mK. The mechanism here is somewhat different because, with half-integral spin (being composed of an odd number of fundamental particles), the He atom is a fermion, not a boson. But, at Tc, the atoms can link up to form Cooper pairs. Because each pair is a boson (being composed of an even number of fundamental particles), the system can then undergo BEC like its liquid He cousin. Where else may superfluididy exist? The most long-standing example is the electron gas in a superconductor – the original instance of a Cooper pair system (electrons being fermions). More recently, superfluidity has been observed in laser-cooled alkali atomic gases. It is also believed that the neutron and proton fluids in neutron stars are superfluid but the idea is, to say the least, rather difficult to test experimentally. Although it is generally agreed that there is a close association between BEC and superfluidity, their exact relationship has yet to be established – the ground state condensate