Dependence of penetration depth, microwave surface resistance and energy gap of MgB₂ thin films on their normal-state resistivity

The dependences of magnetic field penetration depth at zero temperature λ(0), microwave surface resistance R_s and π-band energy gap at zero temperature Δπ (0) on the normal-state resistivity right above the critical temperature, ρ₀, were studied for MgB ₂ thin films prepared by different techniques by employing a sapphire resonator technique. We found that the zero-temperature penetration depth λ(0) data could be well fitted by λ_L (1 + ξ₀/l)^1/2 yielding a London penetration depth λ_L of 34.5 nm, where ξ₀ is the coherence length, and l is the mean free path determined from ρ₀. The surface resistance R_s at 15 and 20 K increases roughly linearly with ρ₀. The observed increase of Δπ (0) with ρ ₀ and the decrease of T_c indicate the expected effects of interbank impurity scattering within an extended BCS approach. The low values of R_s and γ(0) in conjunction with the large coherence length for epitaxial films are potentially attractive for applications in electronics and microwave technology.