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Robinson Group

 

Below the critical temperature of a superconductor, electrons form Cooper pairs with an antiparallel spin alignment, and are in a singlet spin state. In contrast to superconductivity, ferromagnetism leads to a parallel alignment of electron spins, so when electrons pass through a ferromagnetic material the electrical current is spin-polarised meaning that a singlet spin state is energetically unfavourable.

 

At a superconductor / ferromagnet (S/F) interface, a superconducting state can be induced in the ferromagnet, but in general only over a distance of a few nanometres. However, if the magnetisation vectors at the S/F interface are non-collinear with repect to the magnetisation vector in the ferromagnet, the superconducting state in the ferromagnet should be extended through a radically different electron interaction process. The non-collinear magnetism allows the electrons to remain as Cooper pairs, but with each electron of a pair having the same spin so they are in a triplet rather than singlet spin state; see figures A to C - solid and dashed curves represent the spatial dependence of the singlet and triplet pair correlations in F. Spin-aligned triplet pairs can pass deeply in a ferromagnet and possibly over hundreds of nanometres if the mean free path for spin-flip and scatter are sufficiently long, such as in a half-metallic  material in which only one spin channel is present. This project builds on our recent success in demonstrating evidence for triplet supercurrents in Nmagnetic Josephson junctions. The aim is to investigate triplet proximity effects in a variety of SC/FM material systems with a view to understanding them better so that the effects can be optimised and possibly exploited to make novel device architectures.

 

This research programme includes the following partners: Professor Mark Blamire (Cambridge); Professor James Annett (Bristol); Dr Sebastian Bergeret (San Sebastian, Spain); and Professor Oded Millo (Racah Institute of Physics, Israel). The research is funded by the Royal Society through a URF.