Flux Dynamics in Iron-Based Superconductors

The newly discovered iron-based superconductors share a common layered structure. After the iron pnictide (1111) family, other iron-based superconductors such as the iron chalcogenides
$ \hbox{FeTe}_{1 - {\rm x}}\hbox{Se}_{\rm x}$
(11) family have been synthesized. The latter system, characterized by a simple crystal structure, represents an interesting reference system. Indeed, all iron-based superconductors have a stacked structure composed of a layer of iron atoms linked by tetrahedrally coordinated pnictogens (P, As) or a chalcogen (Se, Te) anion: the active layer. The latter is either simply stacked together, as in the FeSe (11), or separated by spacer layers with alkali (e.g., Li), alkaline earth (e.g., Ba), or rare earth oxides/fluorides. In this contribution, we present an ac multiharmonic susceptibility study of the flux dynamic in representative 1111 and 11 iron-based superconductors. Data analysis has been performed in the glass-weak pinning scenario. In particular, the comparison of the third harmonic components vs. temperature and magnetic field returned information on pinning strength and dimensionality. Although in the presence of large thermal fluctuations, because of the high
$Tc$
, susceptibility data points out a three-dimensional flux dynamic and, unexpectedly, an increase of the pinning amplitude in the Fe-based superconductor systems where the spacer layer is present.