The microwave surface impedance, Zs = Rs + jωμ0λ, was measured using dielectric resonator techniques for 2 c-axis-oriented thin films. The temperature dependence of the penetration depth, λ, measured with a sapphire resonator at 17.93GHz could be well fitted from 5K to close to TC by using the standard BCS integral expression; assuming that the reduced energy gap, Δ(0)/kTC, was as low as 1.13 and 1.03. From these fits, the penetration depth at zero temperature was determined to be 102 and 107nm. The results clearly indicated the s-wave nature of the order parameter. The temperature dependence of the surface resistance, Rs, measured with a rutile dielectric resonator, revealed an exponential behavior below about TC/2; with a reduced energy gap that was consistent with that determined from the λ data. The Rs-value at 4.2K was found to be as low as 19μΩ at 7.2GHz. A higher-order mode at 17.9GHz was used to determine the frequency-dependence: Rs  fn(T). The results revealed a decrease in n with increasing temperature: ranging from 2, below 8K, to unity between 13 and 34K.

Microwave Surface Impedance of MgB2 Thin Film. B.B.Jin, N.Klein, W.N.Kang, H.J.Kim, E.M.Choi, S.I.K.Lee, T.Dahm, K.Maki: Superconductor Science and Technology, 2003, 16, 205-9