Research seminar at the MPI for the Physics of Complex Systems in Dresden, 27 November 2014 (abstract below):
The concept of broken symmetry is one of the cornerstones of modern physics, for which superconductors stand out as a major paradigm. In conventional superconductors electrons form isotropic singlet pairs that then condense into a coherent state, similar to that of photons in a laser. We understand this in terms of the breaking of global gauge symmetry, which is the invariance of a system under changes to the overall phase of its wave function. In unconventional superconductors, however, more complex forms of pairing are possible, leading to additional broken symmetries and even to topological forms of order that fall outside the broken-symmetry paradigm.
In this talk I will discuss such phenomena, making emphasis on triplet pairing and the spontaneous breaking of time-reversal symmetry in some superconductors. I will pay particular attention to large-facility experiments using muons to detect tiny magnetic fields inside superconducting samples and group-theoretical arguments that enable us to constrain the type of pairing present in the light of such experiments. I will also address the possibility of mixed singlet-triplet pairing without broken time-reversal symmetry in superconductors whose crystal lattices lack a centre of inversion, and predict bulk experimental signatures of topological transitions expected to occur in such systems.