Microwave Properties of MgB2 Thin Films

The microwave surface impedance = + 0 of MgB2 thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to by the standard BCS integral expression assuming the reduced energy gap (0) to be as low as 1.0–1.1 assuming (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 , as measured with a rutile puck, indicates an exponential behavior below about 2 with a reduced energy gap being consistent with the one determined from the data. The value at 4.2 K was found to be as low as 19 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 dependence of ( . Our results revealed an decrease of with increasing temperature ranging from = 2 below 8 K to = 1 close to . Finally, the microwave power handling of MgB2 films was deduced and compared with values for YBa2Cu3O7 films. We found that the power handling of MgB2 is comparable or even better than that of YBa2Cu3O7 films for temperature below 30 K.