Point contact spectroscopy in the superconducting and normal state of $\mathrm{NaF}{\mathrm{e}}_{1-\text{x}}\mathrm{C}{\mathrm{o}}_{\text{x}}\mathrm{As}$

Abstract

We use point contact spectroscopy to probe the superconducting and normal state properties of the iron-based superconductor $\mathrm{NaF}{\mathrm{e}}_{1-\text{x}}\mathrm{C}{\mathrm{o}}_{\text{x}}\mathrm{As}$ with $\text{x}=0,0.02,0.06$. Andreev spectra corresponding to multiple superconducting gaps are detected in the superconducting phase. For $\text{x}=0.02$, a broad conductance enhancement around zero bias voltage is detected in both the normal and the superconducting phase. Such a feature is not present in the $\text{x}=0.06$ samples. We suspect that this enhancement is caused by orbital fluctuations, as previously detected in underdoped $\mathrm{Ba}(\mathrm{F}{\mathrm{e}}_{1-\text{x}}\mathrm{C}{\mathrm{o}}_{\text{x}}{)}_2\mathrm{A}{\mathrm{s}}_2$ (Phys. Rev. B 85, 214515 (2012)). Occasionally, the superconducting phase shows a distinct asymmetric conductance feature instead of Andreev reflection. We discuss the possible origins of this feature. NaFeAs (the parent compound) grown by two different techniques is probed. Melt-grown NaFeAs shows a normal state conductance enhancement. On the other hand, at low temperatures, flux-grown NaFeAs shows a sharp dip in the conductance at zero bias voltage. The compounds are very reactive in air and the different spectra are likely a reflection of their different oxidation and purity levels.