Nonexponential London penetration depth of external magnetic fields in superconducting Ba_1−xK_xFe₂As₂ single crystals

We have studied the in- and out-of-plane magnetic penetration depths in the hole-doped iron-based superconductor Ba_1−xK_xFe₂As₂ (T_c≈30 K). Single crystals grown from different fluxes and by different groups showed nearly identical results. The in-plane London penetration depth λ_ab is not exponentially saturating at low temperature, as would be expected from a fully gapped superconductor. Instead, λ_ab(T) shows a power-law behavior, λ∝Tⁿ (n≈2), down to T≈0.02T_c, similar to the electron-doped Ba(Fe_1−xCo_x)₂As₂. The penetration depth anisotropy γ_λ=λ_c(T)/λ_ab(T) increases upon cooling, opposite to the trend observed in the anisotropy of the upper critical field, γ_ξ=H_c2^⊥c(0)/H_c2^∥c(0). These are universal characteristics of both the electron- and hole-doped 122 systems, suggesting unconventional multigap superconductivity. The behavior of the in-plane superfluid density ρ_ab(T) is discussed in light of existing theoretical models proposed for the iron pnictide superconductors.