Superconducting spin currents

Lead Research Organisation: University of Cambridge
Department Name: Materials Science & Metallurgy

Abstract

In almost all superconductors the pairs of electrons which carry the charge are in the so-called "singlet" state in which the quantum spin of the two electrons is anti-parallel. This is true whether the materials are high temperature oxide superconductors in which case the pairing symmetry is described as d-wave, or conventional metallic superconductors which have s-wave pairing. There are only a few known compounds which show so-called p-wave superconductivity in which the electron spins within a pair are parallel and hence in a "triplet" state.

During the past five years there has been increasing evidence that proximity coupling between singlet superconductors and ferromagnets can sometimes generate triplet pairs within the ferromagnet - the evidence being that supercurrents can be passed through ferromagnetic materials over length scales which are simply too large for singlet pairs to survive. Earlier this year, in parallel with two other international groups, we made a breakthrough in demonstrating how this triplet state can be created in a controlled way. Together, the results have opened the way for a rich new field of triplet superconductivity in which the potential ability of a supercurrent to carry spin can be allied with standard spin electronics ("spintronics").

In this project the experimental team at Cambridge will develop an existing collaboration with the Condensed Matter Theory Group at Bristol to cement their lead in this field and to explore how triplet currents can be controlled by magnetic elements within a device so that the spin supercurrent can be directly measured. As well as demonstrating superconducting spintronic devices, this project also aims to investigate the potential of creating artificial p-wave superconductors by exploiting materials which are predicted to have a favourable p-wave coupling but which are not themselves superconductors. The results from this programme will inevitably stimulate the broader scientific community interested in unconventional superconductivity and spintronics. We believe that, by establishing the fundamental processes at work in generating spin supercurrents and controlling them, this work will pave the way for important new research directions exploiting this novel phenomenon.

Planned Impact

By analogy with the research into conventional SFS devices, we would expect the basic science to be performed in this project to gradually inform applications areas. The work on generating and controlling spin currents closely parallels activities in the wider spintronics community. Whether S/F multilayer devices ultimately have application in spintronics (eg for current induced switching) is not our primary aim, however our work may inform future research aimed at developing such applications.

Both the participating Groups publish their results widely. Over the past five years we have published over twenty papers on S/F and Andreev systems in major refereed journals. Over the same period, the investigators have given many plenary and invited presentations at major conferences in addition to research seminars at UK and overseas universities and institutions as well as a variety of UK and European network meetings. Wherever possible we will raise the impact of these publications by issuing press-releases.

As well as continuing to present our results at high-profile conferences, as part of this programme we will organise our own meetings in unconventional superconductivity. For example, MGB has recently been awarded a Royal Society Discussion Meeting to take place in September 2011 The main meeting will be on the general properties of oxide interfaces, but we have also been invited to organise a satellite meeting at the Kavli Royal Society International Centre which we will use to showcase leading international research on superconductor / ferromagnet interfaces to a UK audience as well as to make the community aware of the potential of our own programme. External funding (for example from ESF) will be sought for further such meetings during the project.

The groups involved in this programme have a high profile in publicising scientific developments. CAM already participate in public engagement activities such as the annual Cambridge Science Week and "Physics at Work" exhibitions. JFA is chair of the IoP branch for the South West UK and organizes a variety of activities for the public and school students JFA has organized a number of graduate schools and other training events including Electronic Structure Training courses on the leading industry standard electronic structure computer codes, which they can later use both in their PhD research and in their future careers. .

The work of this programme similarly directly impacts the knowledge transfer of device processing and computational electronic structure techniques to industry. The postdoctoral research personnel will learn state of the art experimental and computational techniques (eg in multilayer system growth, ion beam etching, measurement, and in electronic structure methods), all of which can be applied in appropriate industrial settings.

Publications

10 25 50
 
Description The current best understanding of the triplet superconducting state and its formation is that there are four possible electron pairing channels (one singlet and three triplet) between a conventional superconductor (S) and and a half-metallic ferromagnet (F). The key requirement is for a "spin-mixing" layer to be present at the S/F interface that mediates the transformation of a singlet pair into a triplet pair. Of the triplet states, only the spin-aligned pairs can be long-ranged in a ferromagnet. At the start of the grant there was no theoretical prediction for the net polarisation of a supercurrent in a F with both majority and minority states available; a primary objective of the research was therefore to confirm directly that the supercurrent carries spin. This was was achieved for the first time in this project by creating spin valve devices in which the critical temperature could be controlled by the selective transfer of triplet pairs into adjacent F layers. The project has also demonstrated a direct, active, method of controlling the transfer of singlet into triplet pairing. Together, these capabilities offer the prospect of creating superconductor-based spin electronics.
Exploitation Route Many research groups around the world are now exploring superconducting electronics and combining superconductors and ferromagnets to create memory and active devices to b e used at low temperatures. In particular, there is a US cryogenic computing initiative which is seeking to create low power digital electronics for logic applications.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics

URL http://www.dmg.msm.cam.ac.uk/research.php?page=Superconducting%20Spintronics
 
Description The basic ideas developed in this research are now being explored for their potential in cryogenic memory applications. The dissemination of the results has been aided by a Royal Society meeting organised in 2014
First Year Of Impact 2014
Sector Digital/Communication/Information Technologies (including Software)
Impact Types Economic

 
Description European Research Council
Amount € 1,800,000 (EUR)
Funding ID Superspin 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 03/2012 
End 02/2017
 
Description Leverhulme Visiting Professorship
Amount £91,620 (GBP)
Funding ID VP1-2016-043 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 12/2017 
End 12/2018
 
Description Programme Grant
Amount £2,715,071 (GBP)
Funding ID EP/N017242/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2016 
End 12/2020
 
Description Standard Research - NR1
Amount £684,502 (GBP)
Funding ID EP/P026311/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2017 
End 03/2022