Microwave properties of MgB₂ thin films

The microwave surface impedance Z_s = R_s + jωμ₀λ of MgB₂ thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth λ measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to T_c by the standard BCS integral expression assuming the reduced energy gap Δ(0) / kT_c to be as low as 1.0-1.1 assuming λ(0) = 100-110 nm. These results clearly indicate the s-wave nature of the order parameter. Similar good fits were achieved by an anisotropic one gap and an isotropic two-gap model. Second, the temperature dependence of surface resistance R_s, as measured with a rutile puck, indicates an exponential behavior below about T_c/2 with a reduced energy gap being consistent with the one determined from the λ data. The R_s value at 4.2 K was found to be as low as 19 μΩ at 7.2 GHz, which is comparable with that of a high-quality high temperature thin films of YBa₂Cu₃O₇. A higher-order mode at 17.9 GHz was employed to investigate the frequency f dependence of R_sαf^n(T). Our results revealed an decrease of n with increasing temperature ranging from n = 2 below 8 K to n = 1 close to T_c. Finally, the microwave power handling of MgB₂ films was deduced and compared with values for YBa₂ Cu₃O₇ films. We found that the power handling of MgB₂ is comparable or even better than that of YBa₂Cu₃O₇ films for temperature below 30 K.