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Leverhulme International Network

Spin-Polarised Superconductor Proximity Effects

This new network brings together five leading groups from four different countries, which combine expertise in the theory of the superconductor proximity effect (CSIC, Spain), thin-film growth (CAM, UK), scanning tunneling spectroscopy (HUJI, Israel; UKON, Germany), and molecular electronics (HUJI and UKON).

We are merging three generally distinct areas of research: spintronics, superconductivity, and molecular electronics, via the proximity effect in nanoscale systems involving three different spin-filtering materials (ferromagnets, nanoparticles and chiral molecules). We are motivated to do this because we have identified a unifying theme that would enable these areas to be combined in a complimentary way in order to open up a whole new area of research based on spin-polarised superconductivity.

 

Motivation

It is well known that the scaling road map on which semiconductor device development is based will reach its physical limits within the next fifteen years. Current computing architectures mostly rely on semiconductor integrated technologies, and, in particular, on logic circuits built using metal-oxide-semiconductor field-effect-transistors, which are used as control switches. The high level of energy dissipation associated with this technology is fundamentally related to the switching energy and finite-size effects.

To be credible, new concepts of computing must address the issues of energy dissipation, performance, and integration. Identifying a viable technology with real potential to solve all-three problems is far from obvious. Two of the most promising schemes that can in principle outperform current electronic devices are superconducting- and spin-electronic (spintronics)-based technologies. While both these offer the potential for much higher operating frequencies because of lower switching energies, there are serious fundamental problems in transforming these into practical computing devices.

Combining superconductivity and spin-electronics (spintronics) has the potential to overcome significant limitations of logic circuits based separately on superconductivity and spintronics. We are specifically investigating the superconductor proximity effect in the following spin-polarised systems:

1. Magnetic spin-filters such as Fe, Co, and Ni

2. Self-assembled linker-molecule / magnetic nanoparticle structures

3. Spin-filtering chiral molecules such as DNA

 

 

Scientific Background

The proximity effect (PE) between a superconductor (S) and a normal metal (N) is very well understood: due to Andreev reflection at an S/N interface, superconducting pair correlations are induced in N that can support supercurrents, and a gap in the quasiparticle density of states (DoS) opens in the range ±δ around the Fermi energy. Because δ is smaller than the superconductor gap (Δ), it is named the ‘mini-gap’ ⎯ its nature can be probed by tunnelling spectroscopy, such as by a scanning tunnelling microscope (STM) or by conductance measurements in junctions containing a tunnelling barrier. Through such techniques a detailed understanding of singlet PEs in metals and ferromagnets (F) has been achieved. Only very recently has tunnelling experiments been used to probe the (odd frequency) triplet state at S/F interfaces, and in spin-polarised PEs in general. This is because experimental results consistent with a triplet PE have been hard to observe. This changed in 2010 when several groups reported devices that enabled triplet PEs to be observed in a reliable way. As yet, however, the triplet state is far from understood.

A spin-triplet state is induced in S-F hybrids if the magnetisation at the interface and in the F layer are non-collinearly aligned. In simple terms, such a  ‘spin-mixing’ interface converts singlet pairs into triplet pairs and vice-versa. Currently, the best way to detect a triplet state involves measuring long-ranged supercurrents in F materials; however, such experiments provide limited information about the triplet state and so probing it by measuring the local DoS in F or N material represents the most important challenge to the field.

Even less well understood are PEs in S/linker-molecule/nanoparticle (NP) systems. In a recent study of such a system, the Israeli team observed a novel PE in which the critical temperature and current of a thin Nb film increased upon chemically linking gold NPs. Concomitantly, the tunnelling DoS in the gold NPs was modified, showing either proximity-induced gaps or zero-bias conductance peaks implying a triplet PE. Within this project we plan to study spin-polarised PEs in similar systems that contain magnetic (Co) NPs instead of Au. In parallel, we will also study the transport through the magnetic NPs and molecule/magnetic-NP assemblies using superconducting break-junctions.

In both systems discussed above the triplet state was due to the presence of either an F layer or a NP. We also plan to examine an alternative route to generate triplet pairs without a magnetic spin-mixer, by using chiral molecule spin-filters. Chiral molecules will be used either as linkers in S/linker/NP hybrids, or replacing the F layer in the planer junctions. Note that chiral molecules were recently found to be effective spin-filters implying that, like chiral ferromagnets such as holmium, they could be effective spin-mixers and thus good generators of triplet pairs.

This programme aims to deepen the understanding of proximity-induced triplet superconductivity, and may pave-the-way to novel applications in spintronics.

 

Publications

1. J Linder & JWA Robinson, Nature Physics 11, 307–315 (2015)

2. N Banerjee, CB Smiet, RGJ Smits, A Ozaeta, FS Bergeret, MG Blamire, JWA Robinson*, Nature Communications 5, 3048 (2014).

2. Y Kalcheim, O Millo, T Kirzhner, G Koren, M Egilmez, A di Bernardo, MG Blamire, JWA Robinson*, Phys. Rev B Rapid 89, 180506 (2014).

3. XL Wang, A Di Bernardo, N Banerjee, A Wells, FS Bergeret, MG Blamire, JWA Robinson*, Phys. Rev. B Rapid 89, 140508 (2014).

4. JWA Robinson*, N Banerjee, MG Blamire, Phys. Rev. B 89, 104505 (2014).

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