Doping evolution of the superconducting gap structure in heavily hole-doped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$: heat transport study

Abstract

We performed systematic thermal conductivity measurements on heavily hole-doped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ single crystals with 0.747 $\le x\le$ 0.974. At $x$ = 0.747, the $\hat{I}{º}_0/T$ is negligible, indicating nodeless superconducting gap. A small residual linear term $\hat{I}{º}_0/T$ ($\approx$ 0.035 mW/K${}^2$ cm) appears at $x$ = 0.826, and it increases slowly up to $x$ = 0.974, followed by a drastic increase of more than 20 times to the pure KFe${}_2$As${}_2$ ($x$ = 1.0). This doping dependence of $\hat{I}{º}_0/T$ clearly shows that the nodal gap appears near $x=0.8$, likely associated with the change of Fermi surface topology. The small values of $\hat{I}{º}_0/T$ from $x$ = 0.826 to 0.974 support a "$⋎$"-shaped nodal $s$-wave gap recently revealed by angle-resolved photoemission spectroscopy experiments at $x$ = 0.9. Furthermore, the drastic increase of $\hat{I}{º}_0/T$ from $x$ = 0.974 to 1.0 is inconsistent with a symmetry-imposed $d$-wave gap in KFe${}_2$As${}_2$, and the possible nodal gap structure in KFe${}_2$As${}_2$ is discussed.

5 pages, 3 figures