London Penetration Depth in Iron - based Superconductors

TL;DR

This paper reviews how London penetration depth measurements reveal complex, doping-dependent multi-gap superconductivity in Fe-based superconductors, highlighting the non-universal nature of their gap structures and the influence of doping.

Contribution

It introduces a self-consistent Eilenberger model for analyzing superfluid density and applies it to Fe-based superconductors, elucidating their gap structures and doping effects.

Findings

Evidence for two distinct superconducting gaps.

Development of strong in-plane gap anisotropy with doping.

Presence of gap nodes in overdoped regimes.

Abstract

Measurements of London penetration depth are a sensitive tool to study multi-band superconductivity and it has provided several important insights to the behavior of Fe-based superconductors. We first briefly review the "experimentalist - friendly" self-consistent Eilenberger model that relates the measurable superfluid density and structure of superconducting gaps. Then we focus on the \BaFe -derived materials, for which the results are consistent with 1) two distinct superconducting gaps; 2) development of strong in-plane gap anisotropy with the departure from the optimal doping; 3) appearance of gap nodes along the $c-$direction in a highly overdoped regime; 4) significant pair-breaking, presumably due to charge doping; 5) fully gapped (exponential) intrinsic behavior at the optimal doping if scattering is removed (probed in the "self-doped" stoichiometric LiFeAs); 6) competition between magnetically ordered state and superconductivity, which do coexist in the underdoped compounds. Overall, it appears that while there are common trends in the behavior of Fe-based superconductors, the gap structure is non-universal and is sensitive to the doping level. It is plausible that the rich variety of possible gap structures within the general $s_{\pm}$ framework is responsible for the observed behavior.