Iron Based Superconductors

The focus of this project is to compare the properties of the pseudogap regions close to a classic critical point, where thermal superconducting fluctuations dominate, to that close to a quantum critical point which is driven by quantum superconducting fluctuations. For this we will investigate FeSe thin films which undergo a thickness-driven superconductor-insulator-transition and relate the results to those which will be obtained on a BCS superconductor (Pb and Sn).

For this, we propose to use two experimental probes designed to study the superconducting fluctuation regime. The first is tunneling spectroscopy by which the pseudogap properties can be investigated and the second is the Nernst effect which is ideally suited to capture the effect of vortices (phase fluctuations). Our group has used these probes on nominally BCS superconductors upon inclusion of disorder and found both a quantum-fluctuation induced pseudogap and quantum scaling of the magneto-transport properties associated with vortices. In this proposal, we will perform similar experiments on FeSe superconductors which is a system that can be tuned from a thermal fluctuation regime to a quantum fluctuation regime. The results obtained in this work will help elucidate open questions related to the role of quantum vortices, as opposed to the better understood thermal vortices, as well as the nature of the density of states suppression in the two regimes.

We anticipate that the outcomes of this research will also have ramifications on the still open questions related to the pairing mechanism in FeSe and in Iron based superconductors in general.