Abstract
The London penetration depth, , is directly related to the density, , of the Cooper pairs and its variation with temperature provides valuable insight into the pairing mechanism. Here we study the evolution with doping of the temperature dependence of the in-plane and out-of-plane penetration depths in single crystals of electron-doped . As is the case for other pnictides, over the whole doping range and this behavior extends down to at least , setting a very small upper limit on the gap minimum. Furthermore, in the overdoped regime: (1) the exponent becomes substantially smaller than 2, which is incompatible with the models that explain power-law behavior to be due to scattering; (2) the exponent becomes anisotropic, with showing a clear -linear behavior over a large temperature interval. These findings suggest that in the overdoped regime the superconducting gap in iron-based pnictide superconductors develops nodal structure, which unlike in the cuprates, cannot be understood within a two-dimensional picture.
- Received 13 January 2010
DOI:https://doi.org/10.1103/PhysRevB.81.060505
©2010 American Physical Society