1 mössbauer studies of iron-based superconductors

This contribution attempts to be a review concerned with the microscopic characterization of complex materials by using transmission Mössbauer spectroscopy – mainly 14.4-keV resonant transition in Fe. An attention is focused on the novel superconductors, i.e. iron-based superconductors, the latter being extensively investigated in our Mössbauer laboratory primarily versus sample temperature. Iron-based superconductors make four major families based on the corrugated nearly twodimensional sheets of either strongly bound iron-pnictogen or iron-chalcogen atoms. Usually, superconductivity is induced by doping or applying pressure to the parent compound except the simplest compounds of the ‘11’ family. One can dope any kind of atom within the compound in isovalent, hole-doping or electron-doping fashion. Parent compounds exhibit itinerant magnetic order of the 3d (iron) character. It appears as spin density wave (SDW) of the antiferromagnetic type incommensurate with the respective lattice period and of the complex shape. For majority of cases it is longitudinal SDW propagating along the a-axis of the orthorhombic unit cell being created at the magnetic order from the tetragonal cell – due to the magneto-elastic forces. On the other hand, the 3d magnetism and orthorhombic distortion are gone for superconductors as shown by Mössbauer spectra obtained versus temperature, and by spectra obtained in the strong external magnetic field at low temperatures – stronger than the first critical field for these second kind superconductors. However, superconductivity is intimately related to these layered structures with the electronic charge modulation leading to the charge density wave (CDW) on iron nuclei – observed as variation of the isomer shift. What is more, one observes closely related modulation of the electric field gradient on iron nuclei called electric field gradient wave (EFGW). The shape of these modulations changes rapidly at the superconducting gap opening and relaxes back once the bosonic system of Cooper pairs is fairly well separated from the rest of the electronic system. It was found that localized 4f magnetic moments order within superconducting phase in a similar fashion as in the normal phase.