Microwave surface impedance and nonlinear properties of MgB2 films

We have measured the temperature dependence of the microwave surface impedance and the nonlinear response of high-quality MgB₂ films on c-cut sapphire at temperatures below 40 K. MgB₂ films with surface resistance (R_s) as low as 0.02 mΩ at 8 K and 8.5 GHz were prepared in an ex-situ process by annealing a boron layer in magnesium-rich environment. The nonlinear properties of the MgB₂ films were investigated as a function of temperature by means of higher-order harmonic-generation measurements in patterned coplanar waveguide devices. We find that the nonlinear responses of devices fabricated from MgB₂ films were substantially higher than the nonlinear responses of similar devices fabricated from YBa₂Cu₃O_7-δ thin films at the same reduced temperature. We also compared the nonlinear responses of surface ion-milled MgB₂ films with as-grown films and found that the ion-milled films gave lower nonlinear responses at temperatures below 24 K. The consequences of these measurements for passive microwave device applications of MgB₂ films are also discussed.