Muon Spin Rotation (uSR) investigation of electron-doped iron-oxypnictide superconductors

In the last ten years, the Fe-based superconductors have been extensively investigated both from the experimental and theoretical point of view, and many studies have been devoted to investigate the intimate interplay between the magnetic and superconducting phases. However, many aspects of their phenomenology are still unclear. In this thesis work we focused onto a specific material family, namely the LaFeAsO1−xFx (1111), to perform a systematic experimental investigation of the superconducting and magnetic microscopic properties as a function of the Fluorine doping. We considered the doping range 0.04 < x < 0.15, namely from the emergence of superconductivity to the so-called overdoped regime. We used Muon Spin Rotation (µSR) spectroscopy, which is a very powerful local probe to study the microscopic superconducting and magnetic properties of materials. In particular, we systematically studied the doping dependence of i) the superconducting gap and the London penetration depth, related to the superconducting density ns, via the analysis of the muon depolarization rate, and of ii) the local magnetic susceptibility, through the local field probed at the muon site. The results suggest a possible change from s-wave to d-wave character when the superconducting critical temperature Tc is reduced. The normalized Tc as a function of the superfluid density for different Ln-1111 families collapses onto a single phenomenological universal curve that indicates a crossover from the so called-Uemura (Tc∝ns) to the BCS (Tc∿const) regime. This is a phenomenological original outcome of this work. Another important result of this thesis is the observation of a rehentrant paramagnetic behavior in all the samples, more evident for compositions with high Tc. We argued that it may arise from residual magnetic correlations which survive up the to optimally doped regime and beyond. The interplay between this anomalous rehentrant behavior and superconductivity deserves further investigation.