Measurements were made of both the linear and non-linear microwave response of ex situ fabricated thin films of MgB2 on sapphire substrates patterned into co-planar waveguide transmission lines. Linear measurements yielded the surface resistance and absolute value of the penetration depth, as well as the characteristic impedance of MgB2 transmission lines. The non-linear response of the same transmission lines was then measured by harmonic generation. By assuming that the measured nonlinear response was due to kinetic inductance effects, it was possible to determine directly the relevant pair-breaking current density in the MgB2 thin films by combining results from linear and non-linear measurements. Because the resulting pair-breaking current density was an intrinsic material property independent of sample geometry, the non-linear responses of MgB2 thin films and YBa2Cu3O7-δ (YBCO) thin films could be quantitatively compared at equivalent reduced temperatures. It was found that, for sufficiently low reduced temperatures, the pair-breaking current density in MgB2 thin films rivaled that in YBCO.

Nonlinear Microwave Response of MgB2 Thin Films. J.C.Booth, K.T.Leong, S.Y.Lee, J.H.Lee, B.Oh, H.N.Lee, S.H.Moon: Superconductor Science and Technology, 2003, 16, 1518-22