M i l e s to n e s

DOI: M iles tone 1 4 10.1038/nphys869 Sticking together

The fact that a dirty metal can support is a boson and 3He is a fermion, an electric current flowing without it was not clear how 3He could resistance might sound exotic to condense until the BCS theory some, but it is a textbook property of came along. However, the mag- ‘conventional’ superconductors. When netic interactions between the 3He spins are involved, however, the super- particles are strong, and so they conductors do become ‘exotic’. cannot pair up as The conventional mechanism electrons do. Instead, the pairing of superconductivity was explained glue must come from another by John Bardeen, Leon Cooper and source. Robert Schrieffer (in what is known The first authors to propose as the BCS theory) back in 1957. ferromagnetic fluctuations of the For decades, it was a mystery how spins as such a glue were A. Layzer electrons, which are classified as and D. Fay. When one particle ‘fermions’, could be forced into a single whizzes through the liquid, its spin ground state that was more typical (pointing upwards, for instance) for ‘bosons’ (as when they undergo attracts other spin-up particles Bose–Einstein condensation). and repels spin-down particles. Fermions cannot all pile into the same The effective spin-up polarization ground state because only two — one can then attract another spin-up superconductor that carries charge with its spin pointing upwards and particle, leading to a spin-up–spin- and mass, the spin supercurrent the other pointing downwards — can up pair. When coupled in this way, carries only spin and there is no mass occupy each quantum state; bosons do the 3He atoms are able to form a flow. 3He is truly exotic, because of not heed such conventions. As it turns superfluid. its spin. out, the only way for fermions to form Amazingly, all the theoretical May Chiao, Senior Editor, a condensate is for them to pair up, groundwork, including that by Nature Physics with the help of the crystalline lattice: Philip Anderson and Pierre Morel, when one electron passes through the and by Roger Balian and Richard ORIGINAL RESEARCH PAPERS Bardeen, J., Cooper, L. N. & Schrieffer, J. R. Microscopic theory lattice, the positive ions are slightly Werthamer, was laid down before of superconductivity. Phys. Rev. 106, 162–164 attracted to the passing negative elec- the experimental confirma- (1957) | Anderson, P. W. & Morel, P. Generalized tron; if a second electron comes along, tion of a superfluid state in 3He Bardeen–Cooper–Schrieffer states and the proposed low-temperature phase of liquid He3. it will sense the deformed lattice and — that came in 1972 and earned its Phys. Rev. 123, 1911–1934 (1961) | Balian, R. & be attracted to the net positive charge, authors, Douglas Osheroff, Robert Werthamer, N. R. Superconductivity with pairs in a relative p wave. Phys. Rev. 131, 1553–1564 and hence to the original electron. Richardson and David Lee, the Nobel (1963) | Layzer, A. & Fay, D. Superconducting This kind of lattice-assisted coupling prize in Physics in 1996. pairing tendency in nearly ferromagnetic is weak, but it is strong enough for the Although superfluidity was antici- systems. Int. J. Magn. 1, 135–141 (1971) | Osheroff, D. D., Richardson, R. C. & Lee, D. M. Evidence for a paired electrons to drop down to a pated, measurements revealed several new phase of solid He3. Phys. Rev. Lett. 28, collective ground state. unique superfluid phases in 3He. 885–888 (1972) | Osheroff, D. D., Gully, W. J., Naturally, people considered Moreover, because of the non-zero Richardson, R. C. & Lee, D. M. New magnetic phenomena in liquid He3 below 3 mK. Phys. Rev. whether such pairing could glue spin of the pairs (in conventional Lett. 29, 920–923 (1972) | Borovik–Romanov, A. S., other fermions together, in particular superconductors the net spin is zero), Bun’kov, Yu. M., Dmitriev, V. V. & Mukharskii, 3 4 3 Yu. M. Observation of phase slippage during the He. As He exhibits superfluidity He also yielded some unexpected flow of a superfluid spin current in 3He-B. JETP — that is, the liquid can flow without properties. In 1987, a team working Lett. 45, 124–128 (1987) viscosity below a certain temperature in Moscow discovered a pure Further reading Leggett, A. J. A theoretical 3 3 description of the new phases of liquid He — it was believed (hoped) that He spin supercurrent. Unlike the Rev. Mod. Phys. 47, 331–414 (1975) would do likewise. Yet, given that 4He supercurrent in a conventional

nature MILESTONES | Spin march 2008 © 2008 Nature Publishing Group