Energy gap, penetration depth and surface resistance of MgB2 thin films determined by microwave resonator mesurements

We have measured the temperature dependence of the microwave surface impedance Zs = Rs + i\Omega\mu_0 \Lamda of two c-axis oriented MgB2 films at a frequency \Omega /(2\pi) of 17.9 GHz employing a dielectric (sapphire) resonator technique. The temperature dependence of the magnetic field penetration depth \Lamda can be well fitted from 5 K close to Tc by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kTc to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 110 nm and 115 nm were determined from the fit. Our results clearly indicates the s-wave character of the order parameter. The temperature dependence of the surface resistance Rs below Tc/2 is consistent with the low value of the energy gap. The surface resistance below 8 K was found to be below the resolution limit of 100 microOhm of our measurement technique.