A normal metal placed next to an s-wave superconductor develops superconducting correlations because of the "proximity effect" -- Cooper pairs penetrate some distance into the normal metal, for basically the same reason as the eigenstates in a finite square well leak some distance out of the well. (I think this is true; the point is that the Cooper pairs have a longish lifetime in the normal metal.) This shouldn't work for a ferromagnetic material, however, as the Cooper pairs -- being a spin-singlet -- ought to be broken and aligned with the ferromagnet if they stray into it. Efetov and coworkers made the rather surprising prediction (Bergeret et al, PRL 86, 4096 (2001)) that under certain conditions a singlet superconductor can induce a triplet proximity effect in the ferromagnet, with pair correlations (i.e., <>) that are odd in time (Bergeret et al, Rev. Mod. Phys. 77, 1321 (2005)). Norman Birge's group has a new preprint out claiming to have seen this proximity effect:
Observation of spin-triplet superconductivity in Co-based Josephson Junctions
arXiv:0912.0205
Authors: Trupti S. Khaire, Mazin A. Khasawneh, W. P. Pratt, Jr., Norman O. Birge
Abstract: We have measured a long-range supercurrent in Josephson junctions containing Co (a strong ferromagnetic material) when we insert thin layers of either PdNi or CuNi weakly-ferromagnetic alloys between the Co and the two superconducting Nb electrodes. The critical current in such junctions hardly decays for Co thicknesses in the range of 12-28 nm, whereas it decays very steeply in similar junctions without the alloy layers. The long-range supercurrent is controllable by the thickness of the alloy layer, reaching a maximum for a thickness of a few nm. These experimental observations provide strong evidence for induced spin-triplet pair correlations, which have been predicted to occur in superconducting/ferromagnetic hybrid systems in the presence of certain types of magnetic inhomogeneity.
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A group of Prof. dr. M.G. Blamire from Cambridge University also observed the supercurrent through a Josephson junction fabricated using Co/Ho as ferromagnetic multilayer and Nb as superconductor. It has been claimed that supercurrent is triplet in nature that is penetrating over a distance of 15nm, half of the Khaire’s observations.
ReplyDeleteRecently some interesting observations have been done with CrO2 as a half metallic ferromagnet and MoGe superconductor. The supercurrnet is observed over 700nm lateral junction length (http://arxiv.org/abs/1003.4446).
These measurements along with a recent one with Cu2MnAl (http://arxiv.org/abs/1003.2082), we can confidently say that this year 2010 is the year of triplet superconductivity.