A view from inside iron-based superconductors

Muon spin spectroscopy is one of the most powerful tools to investigate the microscopic properties of superconductors. In this paper, an overview on some of the main achievements obtained by this technique in iron-based superconductors (IBS) are presented. It is shown how the muons allow to probe the whole phase diagram of IBS, from the magnetic to the superconducting phase, along with their sensitivity to unravel the modifications of the magnetic and the superconducting order parameters, as the phase diagram is spanned either by charge doping, by an external pressure or by introducing magnetic and non-magnetic impurities. Moreover, it is highlighted that the muons are unique probes for the study of the nanoscopic coexistence between magnetism and superconductivity taking place at the crossover between the two ground-states.