The superconducting gap structure in iron-based high-temperature superconductors (Fe-HTSs) is non-universal. In contrast to other unconventional superconductors, in the Fe-HTSs both d-wave and extended s-wave pairing symmetries are close in energy. Probing the proximity between these very different superconducting states and identifying experimental parameters that can tune them is of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth in the optimally doped nodeless s-wave Fe-HTS Ba_0.65Rb_0.35Fe₂As₂. Upon pressure, a strong decrease of the penetration depth in the zero-temperature limit is observed, while the superconducting transition temperature remains nearly constant. More importantly, the low-temperature behaviour of the inverse-squared magnetic penetration depth, which is a direct measure of the superfluid density, changes qualitatively from an exponential saturation at zero pressure to a linear-in-temperature behaviour at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the superconducting gap.

The interplay between magnetism and superconductivity in LaFeAsO${}_{0.945}$F${}_{0.055}$ was studied as a function of hydrostatic pressure up to $p\simeq 2.4$ GPa by means of muon-spin rotation ($\mu$SR) and magnetization measurements. The application of pressure leads to a substantial decrease of the magnetic ordering temperature, reduction of the magnetic phase volume and, at the same time, to a strong increase of the superconducting transition temperature and the diamagnetic susceptibility. From the volume-sensitive $\mu$SR measurements it can be concluded that the superconducting and the magnetic areas, coexisting in the same sample, are inclined toward spatial separation and compete for phase volume as a function of pressure.

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe1-xRuxAsO0.85F0.15 for 0.1$\le${}x$\lesssim${}0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold xc$\sim${}0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold xc.

We report a detailed investigation of $R$CoPO ($R$ $=$ La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zero-field measurements show that the electrons localized on the Pr${}^{3+}$ ions do not play any role in the static magnetic properties of the compounds. Magnetism at the local level is indeed fully dominated by the weakly itinerant ferromagnetism from the Co sublattice only. The increase of the chemical pressure triggered by the different ionic radii of La${}^{3+}$ and Pr${}^{3+}$, on the other hand, plays a crucial role in enhancing the value of the magnetic critical temperature and can be mimicked by the application of external hydrostatic pressure up to 24 kbar. A sharp discontinuity in the local magnetic field at the muon site in LaCoPO at around 5 kbar suggests a sizable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by ab initio density-functional-theory calculations. DOI: http://dx.doi.org/10.1103/PhysRevB.87.064401 ©2013 American Physical Society