The Order-Parameter Symmetry and Fermi Surface Topology of 122 Fe-Based Superconductors: A Point-Contact Andreev-Reflection Study

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1−x}Co_{x} )_{2}As_{2} single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1−x}Co_{x})_{2}As_{2} single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder–Tinkham–Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the Density-Functional Theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})_{2}As_{2} unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})_{2}As_{2}, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of “hot spots” in the gaps of 122 compounds.