Pressure effects in cuprate and iron-based superconductors studied by muon spin rotation*

Pressure effect (PE) studies of physical parameters of solid state systems allow one to investigate the properties of a material as a function of tuned inter-atomic distances. Such studies are performed on the same material with well defined composition and microstructure which is often advantageous, since {\em e.g.} chemical tuning of material properties (chemical pressure) may give rise to a number of misleading experimental artefacts. Muon-spin rotation ($\mu$SR) is a powerful and highly sensitive tool for probing static and dynamic magnetic fields in solids on the atomic scale. In type-II superconductors the nanoscale variation of the local magnetic field in the vortex state can be detected by $\mu$SR from which the magnetic penetration depth (superfliud density) can be extracted. Furthermore, $\mu$SR is a unique microscopic technique to explore magnetic ordering phenomena and various magnetic phases in solids. At the Paul Scherrer Institute (PSI) a high-pressure set-up was realized which allows to perform $\mu$SR experiments at hydrostatic pressures up to 25 kbar and low temperatures ($\simeq 0.3$~K) [1]. Such experiments open a wide spectrum of new possibilities for investigating the superconducting and magnetic properties of novel materials, such as high-temperature superconductors and related magnetic materials. Here, we present some representative examples of such $\mu$SR pressure studies carried out at PSI: Iron-based superconductors turned out to exhibit a rich and complex phase diagram which strongly depends on pressure [2,3]. $\mu$SR pressure experiments have significantly contributed to a better understanding of these novel class of superconductors [1,2]. In a further $\mu$SR study the PE on the magnetic penetration depth in cuprate superconductors was investigated and found to exhibit an interesting relation to the observed isotope effect [4]. Very recently, we also investigated the PE on the magnetic penetration depth in the heavy fermion system CeCoIn$_{5}$, revealing a strong increase of the superfluid density with pressure [5].\\[4pt] [1] A. Maisuradze {\it et al.}, arXiv:1211.3584 (2012); M. Bendele {\it et al.}, Phys. Rev. B {\bf 85}, 064517 (2012). \\[0pt] [2] R. Khasanov {\it et al.}, Phys. Rev. Lett. {\bf 104}, 087004 (2010). \\[0pt] [3] M. Bendele {\it et al.}, Phys. Rev. Lett. {\bf 104}, 087003 (2010). \\[0pt] [4] A. Maisuradze {\it et al.}, Phys. Rev. B {\bf 84}, 184523 (2011). \\[0pt] [5] L. Howald {\it et al.}, submitted for publication.