Microwave and terahertz surface resistance of MgB ₂ thin films

The knowledge of the surface resistance R _s of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the R _s of MgB ₂ films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between R _s and normal-state resistivity right above T _c, ρ ₀, i.e., R _s decreases almost linearly with the decrease of ρ ₀. (2) A low residual R _s, less than 50 μΩ at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB ₂ has the lowest R _s compared with two other superconductors Nb ₃Sn and the high-temperature superconductor YBa ₂Cu ₃O _7-δ(YBCO). (3) From THz measurement it is found that the R _s of MgB ₂ up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of R _s follows ω ⁿ , where ω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB ₂ thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.