Correlation of microwave surface impedance of MgB₂ thin film with material parameters and a temperature niche for microwave applications

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Two issues related to the microwave surface impedance Zₛ of MgB₂ thin film are discussed in this Letter, both being significant for potential microwave applications. At first, a correlation between Zs and Alpha = Xi/l was found, where Xi is the coherence length, and l is the mean free path. The surface resistance Rs decreases with Alpha at moderate and large values of Alpha and saturates when Alpha approaches one. The values of the penetration depth at zero temperature Lamda(0) for different films could be well fitted by Lamdaₗ (1+Alpha)(1/2), yielding a London penetration depth Lamdaₗ of 33.6 nm. The second issue is to find a temperature niche for possible microwave applications. Between 10K and 15K, our best MgB₂ films possess the lowest Rs values compared with other superconductors such as NbN, Nb3Sn and the high-temperature superconductor YBCO.