Microwave properties of MgB/sub 2/ thin films

The microwave surface impedance Z/sub s/=R/sub s/+j/spl omega//spl mu//sub 0//spl lambda/ of MgB/sub 2/ thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth /spl lambda/ measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to T/sub c/ by the standard BCS integral expression assuming the reduced energy gap /spl Delta/(0)/kT/sub c/ to be as low as 1.0-1.1 assuming /spl lambda/(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/sub s/, as measured with a rutile puck, indicates an exponential behavior below about T/sub c//2 with a reduced energy gap being consistent with the one determined from the /spl lambda/ data. The R/sub s/ value at 4.2 K was found to be as low as 19 /spl mu//spl Omega/ at 7.2 GHz, which is comparable with that of a high-quality high temperature thin films of YBa/sub 2/Cu/sub 3/O/sub 7/. A higher-order mode at 17.9 GHz was employed to investigate the frequency f dependence of R/sub s//spl alpha/f/sup 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/sub c/. Finally, the microwave power handling of MgB/sub 2/ films was deduced and compared with values for YBa/sub 2/Cu/sub 3/O/sub 7/ films. We found that the power handling of MgB/sub 2/ is comparable or even better than that of YBa/sub 2/Cu/sub 3/O/sub 7/ films for temperature below 30 K.