Spin-Orbit Coupling-Driven Superconducting Spintronics

Ferromagnetism arises from parallel alignment of electron spins in a material and is much stronger than conventional singlet superconductivity resulting from the pairing of electrons with opposite spins. Therefore, the coupling or the proximity effect between superconductivity and ferromagnetism is short-ranged and is restricted to only a few nanometres from the interface of the two materials.
However, in the last ten years surprising results from the interaction of superconductivity and ferromagnetism has challenged this conventional picture. At suitably engineered superconductor/ferromagnet interfaces, it is possible to generate superconductivity mediated by equal spin-paired electrons (triplet pairs) enabling a superconductor to carry a dissipationless current with a non-zero spin. The finite spin-polarisation and the zero dissipation integrates the rich potentials of spin based electronics (spintronics) and superconducting electronics into a rich new field of superconducting spintronics. Although attractive both fundamentally and in its reach for potential applications, generating triplet pairs at superconductor/ferromagnet interfaces require prohibitively complex magnetic structures.
Strikingly, a dramatic simplification can be achieved by incorporating spin-orbit coupling in superconductor/ferromagnet heterostructures. Our recent experiments have indicated that using heavy-metals at superconductor/ferromagnet interfaces, it is possible to generate and control triplet pairs using a simple homogeneous ferromagnet. This not only radically simplifies structures used in superconducting spintronics but opens-up the possibility to study a new class of effects predicted to occur due to the coexistence of superconductivity, ferromagnetism and spin-orbit coupling.
In this proposal, we will establish our understanding by focusing on three key areas: understanding the role of the materials and interfaces, maximise the triplet generation efficiency and finally, demonstrate a functional device working on triplet generation and control using spin-orbit coupling. With these objectives, we hope to achieve i) spin-orbit coupling as an efficient source and controlling factor for triplets thereby making superconducting spintronics ideas practically feasible ii) a functional device that is far simpler than any previously designed and serves as a blue-print for future device designs with radically new functionalities.

While the world's most energy efficient supercomputer in Japan ranks 259th in terms of computational power, the world's fastest supercomputer in China consumes a staggering 15 MW power - equivalent to providing power to 19,000 average UK houses for a day. Clearly, energy efficiency and computational speed does not go hand in hand. With the demand for more powerful supercomputers increasing to solve complex problems like mapping the Zika virus to predicting climate change, it is difficult to think of a world where further progress in computing power would be impossible. However, that seems a reality since the exascale computer (10^18 flops/second) scheduled to launch in 2021 will require approximately 2000 MW - 50% of power output of UK's largest power station.

Overcoming this challenge is not a simple energy efficiency problem and fundamental changes in ways to store, process and transmit information is needed. To tackle this problem, we need a cohesive approach with government, universities and private sectors working closely together. Several countries including UK has launched initiatives but it is widely believed that without a fundamentally different information processing technology, any progress would be impossible. In line with this, USA has initiated the C3 program to build the supercomputer entirely out of superconductors. Superconductors can fundamentally alter the way information is transmitted and processed but magnetic storage technology is not compatible with superconductivity.
The current proposal provides a tantalizing possibility which, if realised, will go much beyond the aims of the C3 program both in terms of functionality and complexity of the computing architectures. This proposal unites three fundamental phenomena in condensed matter physics - superconductivity, ferromagnetism and spin-orbit coupling enabling the construction of devices with three key features: ultra-low dissipation, radically new functionalities with device structures that are practically realizable and scalable.

The results from this proposal could directly complement EPSRC's ambitious initiative like the Scalable, Energy-Efficient, Resilient and Transparent Software Adaptation (SERT) project which looks into the design of software for next-generation exascale computing assuming severe limitations coming from energy inefficient hardware. Whereas this is a reactive measure, a more proactive solution is to directly tackle and fundamentally alter the underlying physics of information processing. The simple functional device - the triplet spin-orbit coupled Josephson junction proposed here can serve as a fundamental switching element in the superconducting circuit. Realising such a junction will provide us with the blueprint to design more complex logic and memory elements forming the superconducting computing architecture.
Long-term economic impact will include business sectors adopting new technologies and this can directly improve public services. For example, exascale computing and beyond would open-up new possibilities in healthcare, defence and climate prediction including advanced modelling to predict earthquakes. The greatest impact will be environmental through the introduction of novel technology resulting from this research.