Anisotropic superconducting order parameters in the iron pnictide superconductors

Abstract

The symmetry and structure of the superconducting gap in the Fe-based superconductors are the central issues for understanding these novel materials. So far the experimental data and theoretical models have been highly controversial. Some experiments favor two or more isotropic gaps, others indicate strong anisotropy and even nodes. In this paper we present the experimental data of low temperature specific heat to illustrate that the superconducting gap is anisotropic. In the case of optimally doped 122, the gap minimum may appear as point nodes or small segment of nodal lines, while in the FeTe0.55Se0.45 it may appear on the electron pockets with the gap minimum along the Γ-Z (Fe-Fe bond) direction.