Evidence for two-gap superconductivity in ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ from directional point-contact Andreev-reflection spectroscopy

Directional point-contact Andreev-reflection spectroscopy measurements on the ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ single crystals are presented. The spectra show significant differences when measured in the $ab$ plane in comparison with those measured in the $c$ direction of the crystal. In the latter case only a reduced point-contact conductance around zero bias has been revealed persisting well above $T_c$ and probably related to the structural and magnetic transitions in the system. Within the $ab$ plane two superconducting energy gaps are detected below $T_c$. Here a reduced conductance above $T_c$ could also be found. The fits of the $ab$-plane data to the superconducting $s$-wave two-gap model indicate that the smaller gap has a size below the BCS value while the large gap reveals much higher coupling strength.

Figure 1

Typical PCAR spectra measured with the PC current mostly within the $ab$ plane of the ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ crystal showing two superconducting gaps indicated by arrows (curves 1, 4, and 5) and in the $c$ direction showing reduced PC conductance near zero bias (curves 2 and 3). For the curves 4 and 5 the right ordinate applies.

Figure 2

Spectrum measured in the $c$ direction showing a reduced conductance even above $T_c$. Inset—zero-bias conductance as a function of temperature.

Figure 3

Normalized PCAR spectra of the ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ single crystal in the $ab$ plane showing two superconducting gaps. Lower curves are vertically shifted for clarity. Circles represent the best fits to the two gap BTK formula. Left inset—raw data taken at 4.4 and 27 K, right inset—temperature dependence of two gaps, solid lines show BCS-type behavior of energy gaps.

Figure 4

PCAR spectra of the ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ single crystal showing the gap as well as the pseudogap features. The spectra have been divided by their value at 60 mV.