Left the grey volumes represent the isoenergy surfaces for a muon for three relative minima, A, B and C, in a unit cell of LaCoPO. The energy of these isosurfaces increases from left to right an when it reaches the value appropriate for site C (right) a path opens to the A stabler site, demonstrating that C is unstable. Muon Spin Rotation Spectroscopy (µSR) is a powerful technique to investigate superconductors and magnetic materirials. Muons are implanted in interstitial sites and, whenever there are sizeable internal magnetic fields at these sites, theµSR experiment yields their distribution. Such is typically the case of a type II superconductors with a votex lattice, and of a magnetically ordered compound. In the second case the internal fields may be used to extract the magnetic moments of the ions if the implantation site is known.Recently we learned how to determine a-priori the implantation site from DFT calculations that include the muon, simulated as a light hydrogen isotope. For example in Phys. Rev. B 87 064401 we use the site stability for LaCoPO to understand the changes that this material, isostructural to a high Tc iron superconductor, undergoes with pressure.
