Infrared probe of the gap evolution across the phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

TL;DR

This study investigates how the superconducting gap evolves in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ across different doping levels using infrared spectroscopy, revealing a possible transition from nodal to nodeless gaps influenced by magnetism.

Contribution

It provides detailed optical conductivity measurements across the phase diagram, highlighting the interplay between magnetism and superconductivity and suggesting a gap structure transition.

Findings

Superfluid density decreases in underdoped samples.

Unpaired carrier weight increases with underdoping.

Evidence for a nodal-to-nodeless gap transition.

Abstract

We measured the optical conductivity of superconducting single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with $x$ ranging from 0.40 (optimal doping, $T_c = 39$ K) down to 0.20 (underdoped, $T_c = 16$ K), where a magnetic order coexists with superconductivity. In the normal state, the low-frequency optical conductivity can be described by an incoherent broad Drude component and a coherent narrow Drude component: the broad one is doping-independent, while the narrow one shows strong scattering in the heavily underdoped compound. In the superconducting state, the formation of the condensate leads to a low-frequency suppression of the optical conductivity spectral weight. In the heavily underdoped region, the superfluid density is significantly suppressed, and the weight of unpaired carriers rapidly increases. We attribute these results to changes in the superconducting gap across the phase diagram, which could show a nodal-to-nodeless transition due to the strong interplay between magnetism and superconductivity in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$.