Physics research seminar at Cardiff University.
Superconductivity is a macroscopic quantum phenomenon where the electrons in a metal behave coherently, like photons in a LASER. The essential feature of the superconducting state, compared to a normal metal, is that it is more ordered – in other words, some symmetries are broken. There is a consensus that all superconductors break global gauge symmetry, meaning that the phase of the wave function is locked to a fixed, randomly-chosen value. On the other hand many so-called unconventional superconductors break additional symmetries. Unfortunately it has been difficult, even after decades of research in some cases, to establish the nature of their ordered states. In this talk I will discuss the issue in the light of recent experimental discoveries. These have uncovered evidence of broken time-reversal symmetry (the same form of symmetry breaking occurring in magnets) in materials that were expected to be conventional superconductors. I will explain how we have been piecing together a theoretical understanding of these new materials. The picture that emerges is one of complexity, these systems have complicated band structures affording new degrees of freedom which have to be fully taken into account. When this is done, novel types of superconducting state become possible. I will describe the exotic nature of these states and conclude by speculating on the possibility to engineer analogues in quantum dot arrays made of conventional superconductors.