Anomalous sharp peak in the London penetration depth induced by the nodeless-to-nodal superconducting transition in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

TL;DR

This paper explains the sharp peak in London penetration depth in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ as a nodeless-to-nodal superconducting transition linked to a quantum critical point, based on linear response theory and electronic structure analysis.

Contribution

It introduces a theoretical framework connecting the penetration depth peak to a pairing transition driven by three-dimensional electronic properties.

Findings

The peak corresponds to a nodeless-to-nodal transition in superconductivity.

The transition is driven by three-dimensional electronic effects.

The results align with experimental observations of the quantum critical point.

Abstract

The issue of whether the quantum critical point (QCP) is hidden inside unconventional superconductors is a matter of hot debate. Although a prominent experiment on London penetration depth has demonstrated the existence of the QCP in the isovalent-doped iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$, with the observation of a sharp peak in the penetration depth in the vicinity of the disappearance of magnetic order at zero temperature, the nature of such an emerging QCP remains unclear. Here, we provide a unique picture to understand well the phenomena of the QCP based on the framework of linear response theory. Evidence from the density of states and superfluid density calculations suggests the nodeless-to-nodal pairing transition accompanied the appearance of a sharp peak in the penetration depth in BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Such a pairing transition originates from the three-dimensional electronic properties with a strong interlayer superconducting pairing. This finding provides a significant insight into the understanding of the QCP observed in experiment in BaFe$_2$(As$_{1-x}$P$_x$)$_2$.