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. That an entire passenger train can be levitated using superconductors 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. Many of the superconductors we will study were discovered to have an intrinsic magnetic structure inside the superconducting state, termed ‘spontaneous time reversal symmetry breaking’. 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.
Official information about the project on our funder’s website: