Point contact spectroscopy of Fe pnictides and chalcogenides in the normal state

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 BaFe2As2, SrFe2As2, Fe1+yTe and F doped SmFeAsO and LaFeAsO. Two materials which do not show this conductance enhancement are CaFe2As2 and K doped BaFe2As2. 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 resistive 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.