Point contact spectroscopy in the superconducting and normal state of $\mathrm{NaFe_{1-\textit{x}}Co_\textit{x}As}$

TL;DR

This study uses point contact spectroscopy to investigate the superconducting and normal states of NaFe_{1-x}Co_xAs, revealing multiple superconducting gaps and conductance features possibly linked to orbital fluctuations and sample purity.

Contribution

First application of point contact spectroscopy to NaFe_{1-x}Co_xAs, identifying multiple gaps and conductance anomalies related to orbital fluctuations and sample growth methods.

Findings

Detection of multiple superconducting gaps via Andreev spectra.

Observation of conductance enhancement around zero bias in certain doping levels.

Different spectral features linked to sample growth techniques and oxidation levels.

Abstract

We use point contact spectroscopy to probe the superconducting and normal state properties of the iron-based superconductor $\rm{NaFe_{1-\textit{x}}Co_{\textit{x}}As}$ with $\rm{\textit{x} = 0, 0.02, 0.06}$. Andreev spectra corresponding to multiple superconducting gaps are detected in the superconducting phase. For $\rm{\textit{x} = 0.02}$, 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 $\rm{\textit{x} = 0.06}$ samples. We suspect that this enhancement is caused by orbital fluctuations, as previously detected in underdoped $\rm{Ba(Fe_{1-\textit{x}}Co_\textit{x})_2As_2}$ (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.