For example, a square looks the same when it has been turned through 90o. The concept of symmetry is very important in physics, as is the idea of symmetry breaking, in which a system no longer obeys a particular symmetry. Superconductivity provides a paradigm for symmetry breaking, and in some superconductors a variety of symmetries can be broken. For example, in cuprate high-temperature superconductors, which have a layered structure with planes made of square CuO2 'plaquettes', the 90o rotation symmetry of the plaquettes is broken on entering the superconducting state. An even more exotic possibility is failure of time-reversal symmetry in the superconducting state. This can be detected through an increase in the muon spin relaxation rate produced by the spontaneous onset of magnetic fields below Tc (such as in Sr2RuO4). Non-centrosymmetric superconductors – materials whose crystal structure has no central symmetry point – are particularly interesting as the way the electrons in the material pair up in the superconducting state can take unusual forms.
One example of a non-centrosymmetric superconductor is LaNiC2 (critical temperature Tc=2.7K). We have shown that the muon spin relaxation rate in zero applied field increases as the material is cooled through the transition. This is the first direct proof of broken time-reversal symmetry in any non-centrosymmetric superconductor.
AD Hillier, J Quintanilla (ISIS), R Cywinski (University of Huddersfield)
Contact: Dr Adrian Hillier, firstname.lastname@example.org; Dr Jorge Quintanilla, email@example.com
Further reading: AD Hillier et al., Phys Rev Lett 102 (2009) 117007