Nonlinear microwave response of MgB₂ thin films

Thin films of the recently discovered superconductor MgB2 show promise for a number of different electronic applications. In order to evaluate the suitability of this new material for communication applications at microwave frequencies, we have measured both the linear and nonlinear microwave response of ex situ fabricated thin films of MgB2 on sapphire substrates patterned into coplanar-waveguide (CPW) transmission lines. Linear measurements yield the surface resistance and absolute value of the penetration depth, as well as the characteristic impedance of our MgB2 transmission lines. The nonlinear response of the same transmission lines was then measured by harmonic generation. Assuming that the measured nonlinear response is due to kinetic inductance effects, we were able to directly determine the relevant pair-breaking current density in our MgB2 thin films by combining results from our linear and nonlinear measurements. Because the resulting pair-breaking current density is an intrinsic material property independent of sample geometry, we can quantitatively compare the nonlinear responses of MgB2 thin films and YBa2Cu3O7-delta (YBCO) thin films at comparable reduced temperatures. We find that for sufficiently low reduced temperatures the pair-breaking current density in MgB2 thin films rivals that in YBCO.