Doping evolution of the quasiparticle excitations in heavily hole-doped Ba1−xKxFe2As

Abstract

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity κ and penetration depth λ in the heavily hole-doped regime of Ba 1−x K x Fe 2 As 2 . The residual thermal conductivity (κ/T ) T →0 K and T dependence of λ consistently indicate the fully gapped superconductivity at x = 0.76 and the (line) nodal superconductivity at higher hole concentrations. The magnitude of κ T T c | T →0 K and the slope of λ(T ) at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual nonmonotonic x dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed x dependence is naturally explained by a doping crossover of the gap function between the s-wave states with and without sign reversal between -centered hole pockets.

Multiple phase transitions in Pauli-limited iron-based superconductors

“…However, d-wave symmetry can be also observed [60,61]. Hole doping of IBSC can lead to the transition of the gap symmetry from s ± -wave to d-wave [62][63][64]. Our main result is not affected, with similar results for other symmetries than s-wavethe FFLO is the ground state in the LTHM regime in every band.…”

Multiband Superconductivity in KFe2As2: Evidence for One Isotropic and Several Lilliputian Energy Gaps

“…In addition, our specific-heat measurements show that a significant increase of κ(H)/T is also to be expected for H/H c2 < 0.1 due to these small gaps. This feature was not observed in Refs., 949 or in the more recent data of Watanabe et al 50 while it clearly appears in many other multiband superconductors including MgB 2 , CeCoIn 5 and PrOs 4 Sb 12 . 18,20,21 Thus, the origin of the finite κ 0 /T cannot be attributed to d-wave superconductivity in KFe 2 As 2 in these experimental conditions.…”

Universal V-shaped temperature-pressure phase diagram in the iron-based superconductorsKFe2As2,RbFe2As2, andTafti1, Ouellet2, Juneau-Fecteau3 et al. 2015Phys. Rev. B

“…Laser ARPES experiments suggest a s ± state with eight line nodes on the ξ band and no nodes on the α and β bands. 13,14 By contrast, bulk measurements of thermal conductivity, 15,16 , heat capacity 17,18 and penetration depth 19,20 point to a d-wave state, with four line nodes on each of the Γ-centered Fermi pockets (α, ξ, and β).…”