Point Contact Spectroscopy of Fe Pnictides & Chalcogenides In The Normal State

TL;DR

This review discusses how point contact spectroscopy reveals orbital fluctuations and electronic nematicity in the normal state of iron-based superconductors, highlighting its sensitivity to density of states and its diagnostic capabilities.

Contribution

It provides a comprehensive overview of point contact spectroscopy findings in iron pnictides and chalcogenides, emphasizing its role in detecting orbital fluctuations and nematicity.

Findings

Conductance enhancement linked to orbital fluctuations in certain compounds.

PCS sensitive to excess density of states at the Fermi level.

Correlation between conductance enhancement and in-plane resistive anisotropy.

Abstract

We review the current status of point contact spectroscopy on the iron based superconductors, focusing on their normal state. Point contact spectroscopy is generally used to study superconductors via Andreev reflection, but in recent years it has also proved to be a useful bulk probe of strongly correlated electron systems. Point contact spectroscopy picks up a conductance enhancement in the normal state, above the structural phase transition, of certain iron based compounds. These include Co doped $\rm{BaFe_2As_2}$, $\rm{SrFe_2As_2}$, $\rm{Fe_{1+y}Te}$ and F doped $\rm{SmFeAsO}$ and $\rm{LaFeAsO}$. Two materials which do not show this conductance enhancement are $\rm{CaFe_2As_2}$ and K doped $\rm{BaFe_2As_2}$. This conductance enhancement is thought to be tied to orbital fluctuations. Orbital fluctuations in the normal state of these compounds increases the single particle density of states at the Fermi level, indicating that PCS is sensitive to this excess density of states. The enhancement is only observed at those temperatures and dopings where an in-plane resisitve anisotropy in the detwinned compounds is known to occur. Thus point contact spectroscopy provides strong indications of electronic nematicity in such materials. We also present diagnostics on how to judge if a junction is impacted by joule heating or not. We conclude with the outstanding challenges in the field and the new experiments that need to be carried out.