Microwave surface impedance of MgB2 thin film

The microwave surface impedance Z(s) = R-s + jomegamu(0)lambda was measured with dielectric resonator techniques for two c-axis-oriented MgB2 thin films. The temperature dependence of the penetration depth, measured with a sapphire resonator at 17.93 GHz can be well fitted from 5 K close to T-c by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kT(c) to be as low as 1.13 and 1.03 for the two samples. From these fits the penetration depth at zero temperatures was determined to be 102 nm and 107 nm, respectively. The results clearly indicate the s-wave nature of the order parameter. The temperature dependence of surface resistance R-s measured with a rutile dielectric resonator, shows an exponential behaviour below about T-c/2 with a reduced energy gap being consistent with the one determined from the, data. The R-s value at 4.2 K was found to be as low as 19 muOmega at 7.2 GHz, which is comparable with that of a high-quality high-temperature thin film of YBa2Cu3O7. A higher-order mode at 17.9 GHz was employed to determine the frequency f dependence of R-s alpha f(n(T)). Our results revealed a decrease of n with increasing temperature ranging from n = 2 below 8 K to n = 1 from 13 to 34 K.