Superconducting devices with spin-orbit coupling

Electron spin flip scattering within materials leads to energy dissipation and ultimately this irreversible process limits the operating efficiency of devices based on spin-electronics (spintronics). Dissipationless spin currents can be generated by the quantum hall effect in material systems in which the electrons are confined to two dimensions, but only at low temperatures and by applying large magnetic fields. Do materials exist which allow long-lived spin carriers to be generated and propagated across a wide range of temperatures and in the absence of a magnetic field? This research proposal is exciting because it is based on understanding a recently discovered class of material named a ‘topological insulator’ which can generate long-lived spin carriers and without a magnetic field [MZ Hasan and CL Kane, Rev. Mod. Phys. 82, 3045 (2010)]. This is possible because although these materials are insulating in the bulk their surface / edge states are gapless and support highly mobile charge and spin carriers that are protected from back scattering (by time reversal symmetry). These exotic materials were predicted to exist in 2005 and following their discovery in 2007 [for a review see Rev. Mod. Phys. 82, 3045 (2010)], a number of topological insulators have been identified (e.g. Bi2Se3 and Bi2Te3).

Why combine topological insulators with superconductors?

At the interface with a superconductor, the surface / edge states of a topological insulator can acquire superconducting-like properties due to the proximity effect. In this situation, the surface / edge states are predicted to host Majorana fermions [L Fu and CL Kane, Phys. Rev. Lett. 100, 096407 (2008)] ⎯a much sought-after particle that is its own antiparticle. Proving the existence of these particles would be a colossal scientific achievement, but before this is possible a better understanding of the materials science issues at the interface of a superconductor with a topological insulator is required. Because the surface states of a topological insulator are supposed to be highly conducting, the penetration of superconductivity along the surface should be long-ranged and similar to the penetration of superconductivity into a normal metal (>1 μm at ~4 K). This means that the interaction between superconductivity and surface topological states can be investigated by making Josephson junctions in which two superconductors are coupled via a topological insulator. This research theme therefore aims to investigate the proximity coupling of superconductivity into topogical insulators.

This research programme includes the following partners: Professor Andrea Ferrari (Cambridge, UK), Dr Alexander Brinkman (The Netherlands), and Dr Jagadeesh Moodera (MIT). The research is funded by the Royal Society.