Two-Gap Superconductivity in Ba_1-xK_xFe₂As₂: A Complementary Study of the Magnetic Penetration Depth by Muon-Spin Rotation and Angle-Resolved Photoemission

We investigate the magnetic penetration depth λ in superconducting Ba_1-xK_xFe₂As₂ (T_c≃32  K) with muon-spin rotation (μSR) and angle-resolved photoemission (ARPES). Using μSR, we find the penetration-depth anisotropy γ_λ=λ_c/λ_ab and the second-critical-field anisotropy γ_Hc2 to show an opposite T evolution below T_c. This dichotomy resembles the situation in the two-gap superconductor MgB₂. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth λ_ab around 7 K. The complementarity of μSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T evolution of λ_ab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate.