Unconventional superconductors: new paradigms for new materials

Superconductivity is one of the strangest states of matter. The fact that electrical current can flow indefinitely in a closed circuit, potentially for thousands of years, seems miraculous. The complete expulsion of magnetic flux from a superconductor, the Meissner effect, is even more surprising. The fact that an entire passenger train can be levitated using this effect is a beautiful consequence of the quantum physics of materials and also a key future technology. In 2015 the SCMaglev train at Yamanashi, Japan achieved a record speed of 603km/h.

The 2015 discovery of superconductivity at 203K in hydrogen sulphide under extremely high pressures shows that room temperature superconductivity may not be an impossible dream, although such very high pressure systems are not likely to be directly useful in applications. Nevertheless, high-temperature superconducting wires and magnets operating at 77K are now widely available commercially and have found applications ranging from electricity power grids, to motors for submarines. More familiar applications of low-temperature superconductivity include the particle beam bending magnets in the LHC at CERN or the MRI scanners in all major hospitals.

The theoretical physics of superconductivity is also a surprising consequence of many-particle quantum physics. Essentially it is a state of matter where the myriad electrons in a material act coherently, like photons in a laser field. This allows the electrons to act together, amplifying quantum behaviour until it becomes visible on the scale of everyday objects. The theory of superconductors has led to other fundamental advances in widely different fields of physics. For example the Higgs mechanism was inspired by research into the theory of superconductivity.

The research to be carried out in this project is related to the phenomenon of 'unconventional superconductivity'. This refers in general to any superconductor which cannot be explained within the broad framework set out in the 1957 theory of Bardeen, Cooper and Schrieffer (BCS). This theory was immensely successful in explaining superconductivity in most materials. But it cannot explain high temperature superconductivity up to 160K in the cuprates, or above 55K in the iron based superconductors (pnictides). All superconductors are metals in which the electrons lower their energy by forming pairs, called Cooper pairs. Unconventional superconductors have Cooper pairs with complicated structures. Many of them have anisotropic pairing, one of whose consequences is often that the energy gap vanishes when the electron momentum points in certain directions. In some of them, called triplet superconductors, the Cooper pairs may have finite spin.

Our project aims to advance our understanding of new classes of unconventional superconductors. The superconductors we will study were discovered to have an intrinsic magnetic structure inside the superconducting state, termed 'spontaneous time reversal symmetry breaking'. We have worked closely with the experimentalists at RAL and Warwick who made these discoveries, as well as interested scientists in Japan, USA and elsewhere. In order to truly understand these systems we need to develop a much fuller theoretical description of them. This requires applying a powerful a range of theoretical tools to the problem, ranging from DFT electronic structure calculation, to model building and solving these models using many-body theory. We especially want to examine possible links between the novel Cooper pair orders and other additional useful and/or interesting properties, such as magnetism or so-called "topological order". In addition, we will explore possible novel applications of these unusual systems, for example the possibility of superconductors which naturally resist the 'quench' phenomenon. It was such a superconducting quench in one of the CERN bending magnets in 2008 which shut down the whole of the LHC for several months.

Since pre-history, technological evolution has been closely tied to the harnessing of new materials: flint stone, bronze, iron, coal and steel, and silicon. Increasingly, curiosity-driven research into the fundamental properties of new materials precedes, and opens the way for, the identification and exploitation of their functionalities. A case in point are liquid crystals, whose discovery in 1888 led to several decades of curiosity-driven research followed by the proposal to use them in displays in the 1960's leading to today's ubiquitousness of LCD technology.

A similar evolution from discovery, through basic understanding to the invention of new technologies and their generalisation has taken place with superconductors: their discovery in 1911 and the emergence of the first successful theory in 1957 led to the first major application of superconductivity being proposed a decade later, namely medical Magnetic Resonance Imagin (MRI). Today, MRI is one of the main diagnostic tools available in every major hospital.

In recent years the range and pace of development of superconductor-based technologies have increased dramatically. This has been driven by the emergence of new societal demands. To name a few examples: underground superconducting power lines to deal with increasing population and urbanisation; superconducting logic and superconducting memories to break the exa-FLOP barrier as high-performance computer reaches CMOS technology's natural limits; and superconducting fault current limiters and magnetic energy storage devices to lend resilience to grids increasingly reliant on intermittent sources of renewable energy.

The above technologies rely largely on fundamental advances made in the 1980's and 1990's, particularly the discovery, characterisation, and theoretical advances towards understanding of Copper-based high-temperature superconductors. PI JFA made an important contribution to these efforts by helping to establish the d-wave symmetry of their order parameter [see Case for Support: Previous Research Track Record].

The work we propose aims to advance our fundamental understanding of new superconductors with quite distinct properties and therefore potentially new uses not mentioned above. Among the potentially-useful phenomena that we expect to uncover and/or explain we find: novel couplings between magnetism, lattice polarisation and superconductivity which may extend the range of available multi-ferroic materials to include superconductors; superconductivity where unconventional pairing occurs without the disadvantages of an ungapped spectrum; high-Tc triplet superconductors; and topological node-reconstruction transitions at which the material may become more resistant to quenches.

The last of these possible applications is the area where our project has the highest potential impact. A quench is a phenomenon whereby an initially small region of a superconductor becomes normal, leading to dissipation, heating and a runaway instability where potentially the whole system may catastrophically go into the normal state - as happened at CERN on 19 February, 2008. This affects negatively nearly all applications of superconductors. Part of our work will be specifically targeted towards establishing whether the unusual properties of superconductors near node reconstruction transitions can be exploited to slow down, or even prevent quenches. The results will be communicated directly to the industrial sector at appropriate applied-superconductivity conferences.

Finally, we will also engage in outreach efforts exploiting the popular appeal of superconductivity as a field and exploring possibilities to connect with other communities such as the arts world (with which we have some experience already).