Novel unconventional superconductivity in non-centrosymmetric superconductors
Non-centrosymmetric superconductors are currently the subject of intense theoretical and experimental interest. The crystal structure of these materials lacks a centre of inversion, and this has profound implications for the symmetry of the superconducting state (i.e. the internal structure of the Cooper pairs of electrons responsible for superconductivity).
Many novel properties have been predicted, including field-tuned Fermi surface topology, topologically protected spin currents, a novel mixed-state (vortex lattice) and complex phase diagrams involving superconductivity and magnetism. However, despite these intense theoretical and experimental efforts, the issue of symmetry breaking in some of these systems remains uncertain.
Recently, we have shown using muon spin relaxation (µSR) that LaNiC2 breaks time-reversal symmetry on entering its superconducting state. This makes LaNiC2 one of a very small group of superconductors (and the first among the non-centrosymmetric superconductors) where this rare form of unconventional pairing has been detected directly. A group-theoretic analysis reveals the possible symmetries of the superconducting state and suggests that our observations of LaNiC2 imply non-unitary pairing. Moreover we have obtained evidence in favour of the coexistence in this system of singlet and triplet pairing .
We propose to further investigate non-centrosymmetric superconductors, including LaNiC2 and the materials obtained by doping with other elements that substitute for La, Ni and C. The work will be carried out using a combination of large facilities-based techniques with further theoretical analyses. The new large-facility-based experiments will include new µSR studies with Dr A.D. Hillier at ISIS. These will concentrate on probing the flux line lattice and determining the fundamental superconducting parameters. From these measurements we will also determine the nature and size of the superconducting gap.
In addition, zero-field µSR will be used to search for other examples of broken time-reversal symmetry. Neutron and X-ray scattering measurements will also be used to probe for structural changes and measure the relative strength of phonon and magnetic interactions. The theory will be developed with Dr J. Quintanilla in the Theory Group.
We will carry out further group theoretic analyses to classify the possible pairing symmetries in each material. This will enable us to extract the maximum amount of information from each experiment. We will also explore different scenarios for the pairing mechanism in the light of the results of the experiments: for example, in connection with LaNiC2, we will look at the consequences of non-unitary pairing and investigate the possible role of relativistic effects (spin-orbit coupling).
 “Evidence for Time-Reversal Symmetry Breaking in the Noncentrosymmetric Superconductor LaNiC2.” A. D. Hillier, J. Quintanilla and R. Cywinski, Physical Review Letters 102, 117007 (2009).
For further information contact Professor Jon Goff.