Superfluid Helium Three: The First Unconventional BCS States Available to Purchase

The ordered phases of liquid 3He represent the first known unconventional BCS states, in which the Cooper pairs contain internal structure, most notably intrinsic spin and orbital angular momentum. They are also the only observable BCS states in which the fermions order in the absence of an underlying lattice structure, although it is believed that similar order also occurs in neutron stars. Because of this, the 3He superfluids are free to exhibit the symmetry that truly minimizes their free energy, resulting in liquid crystal‐like textures over macroscopic distances. The ‘spontaneously broken spin‐orbit symmetry’ of the Cooper pairs leads to a remarkably complex set of spin dynamical properties, which allow one to probe the nature of these condensates with remarkable ease. These ordered states and some features of the superfluid 3He behaviour are discussed.