The knowledge of the surface resistance Rs 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 Rs of MgB2 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 Rs and normal-state resistivity right above Tc, ρ0, i.e., Rs decreases almost linearly with the decrease of ρ0. (2) A low residual Rs, less than 50 μ at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB2 has the lowest Rs compared with two other superconductors Nb3Sn and the high-temperature superconductor YBa2Cu3O7−δ(YBCO). (3) From THz measurement it is found that the Rs of MgB2 up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of Rs follows ωn, 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 MgB2 thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.

The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB2. Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by band structure calculations. In addition, the observation of a coherence peak indicates that the band is in the dirty limit and dominates the total conductivity of our films.

We have measured the temperature dependence of the microwave surface impedance Zs=Rs+iωμoλ of two c-axis oriented MgB2 films employing dielectric resonator techniques. The temperature dependence of the magnetic-field penetration depth λ determined by a sapphire dielectric resonator at 17.9 GHz can be well fitted from 5K close to Tc by the standard BCS integral expression assuming the reduced energy gap △(0)/kTc to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 102 and 107 nm were determined from the fit. Our results clearly indicate the s-wave character of the order parameter. A similar fit of the penetration depth data was obtained with an anisotropic s-wave BCS model. Within this model we had to assume a prolate order parameter, having a large gap value in the c-axis direction and a small gap within the ab plane. This is in contrast to recent fits of the anisotropic s-wave model to upper critical-field data, where an oblate order parameter had to be used, and raises interesting questions about the nature of the superconducting state in MgB2. A rutile dielectric resonator was employed to obtain the temperature dependence of Rs with high accuracy. Below about Tc/2, Rs (T) 2Rs (5K) exhibits an exponential temperature dependence with a reduced energy gap consistent with that determined from the penetration depth data. The Rs value at 4.2K was found to be as low as 19 mVat 7.2 GHz, which is comparable with a high-temperature superconducting copper oxide thin film.

The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB2. Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by band structure calculations. In addition, the observation of a coherence peak indicates that the band is in the dirty limit and dominates the total conductivity of our films.

We have measured the temperature dependence of the microwave surface impedance Zs5Rs1ivm0l of two c-axis oriented MgB2 films employing dielectric resonator techniques. The temperature dependence of the magnetic-field penetration depth l determined by a sapphire dielectric resonator at 17.9 GHz can be well fitted from 5 K close to Tc by the standard BCS integral expression assuming the reduced energy gap D(0)/kTc to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 102 and 107 nm were determined from the fit. Our results clearly indicate the s-wave character of the order parameter. A similar fit of the penetration depth data was obtained with an anisotropic s-wave BCS model. Within this model we had to assume a prolate order parameter, having a large gap value in the c-axis direction and a small gap within the ab plane. This is in contrast to recent fits of the anisotropic s-wave model to upper critical-field data, where an oblate order parameter had to be used, and raises interesting questions about the nature of the superconducting state in MgB2 . A rutile dielectric resonator was employed to obtain the temperaturedependence of Rs with high accuracy. Below about Tc/2, Rs(T)2Rs(5 K) exhibits an exponential temperature dependence with a reduced energy gap consistent with that determined from the penetration depth data. The Rs value at 4.2 K was found to be as low as 19 mV at 7.2 GHz, which is comparable with a high-temperature superconducting copper oxide thin film.