Doping evolution of the superconducting gap structure in the underdoped iron arsenide Ba1-xKxFe2As2 revealed by thermal conductivity

TL;DR

This study investigates how the superconducting gap structure in Ba1-xKxFe2As2 evolves with doping, revealing the development of gap minima associated with antiferromagnetic order through thermal conductivity measurements.

Contribution

It provides the first detailed doping-dependent analysis of the superconducting gap structure in Ba1-xKxFe2As2 using thermal conductivity, linking gap minima to Fermi surface reconstruction.

Findings

No nodes in the superconducting gap across all dopings.

Gap minima deepen as doping decreases into antiferromagnetic coexistence.

Superconducting gap minima are linked to Fermi surface reconstruction.

Abstract

The thermal conductivity kappa of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping (x = 0.34; Tc = 39 K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity (x = 0.16; Tc = 7 K). In zero field, there is no residual linear term in kappa(T) as T goes to 0 at any doping, whether for in-plane or inter-plane transport. This shows that there are no nodes in the superconducting gap. However, as x decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing x. We propose that the minima in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is reconstructed by the antiferromagnetic order.