Nonexponential London penetration depth of external magnetic fields in superconducting Ba1-xKxFe2As2 single crystals

We have studied the in- and out-of-plane magnetic penetration depths in the hole-doped iron-based superconductor Ba1-xKxFe2As2 (T-c approximate to 30 K). Single crystals grown from different fluxes and by different groups showed nearly identical results. The in-plane London penetration depth lambda(ab) is not exponentially saturating at low temperature, as would be expected from a fully gapped superconductor. Instead, lambda(ab)(T) shows a power-law behavior, lambda proportional to T-n (n approximate to 2), down to T approximate to 0.02T(c), similar to the electron-doped Ba(Fe1-xCox)(2)As-2. The penetration depth anisotropy gamma(lambda)=lambda(c)(T)/lambda(ab)(T) increases upon cooling, opposite to the trend observed in the anisotropy of the upper critical field, gamma(xi)=H-c2(perpendicular to 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 rho(ab)(T) is discussed in light of existing theoretical models proposed for the iron pnictide superconductors.