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We propose a modified two-tone method that could be used for sensitive measurements of the intrinsic microwave surface impedance (Zs) of thin superconductor films and the tand of a low-loss dielectric. An open-gap resonator scheme is used to measure the penetration depth of thin superconductor films and extract the intrinsic Zs of the superconductor films from its measured effective surface resistance and penetration depth. We use a very small gap of 10 um between the top plate and the rest parts of the resonator. The loss tangent of rutile in the low 10^-7 range and the dielectric constant as high as ~110 are observed at temperatures below 10 K at ~15.2 GHz, which enable to measure the Rs of the 10 mm-in-diameter YBCO films as low as \~100 micro-ohm at the same frequency (f). The discrepancy between the effective surface resistance at ~15.2 GHz and that at ~8.5 GHz scaled to ~15.2 GHz appears less than 6 % when the relations of Rs ~ f^2 and loss tangent ~ f are used. We describe usefulness of our measurement method for measuring the intrinsic microwave properties of various superconductor samples., Comment: 6 pages, 7 figures. To be published in IEEE Transaction on applied superconductivity June (2005)

The surface resistance (Rs) and the real part (sigma_1) of the microwave complex conductivity of a ~380 nm-thick polycrystalline MgB2 film with the critical temperature (Tc) of 39.3 K were investigated at ~8.5 GHz as a function of temperature. Two coherence peaks were observed in the sigma_1 versus temperature curve at temperatures of ~0.5 Tc and ~0.9 Tc, respectively, providing a direct evidence for the two-gap nature of MgB2. The film appeared to have a pi-band gap energy of 1.8 meV. For the MgB2 film ion-milled down to the thickness of ~320 nm, two coherence peaks were still observed with the first conductivity peak at ~0.6 Tc. Reduction of Tc by 3 K and reduced normal-state conductivity at Tc were observed along with an enhanced pi-band gap energy of 2.1 meV and a reduced Rs at temperatures below 15 K for the ion-milled film. Calculations based on the gap energies from the weak coupling Bardeen-Cooper-Schrieffer theory and the strong coupling theory suggest that both the sigma-band and the pi-band contribute to sigma_1 of the polycrystalline MgB2 films significantly. Our results are in contrast with the observation of single coherence peak at ~0.6 Tc and dominant role of the pi-band in the microwave conductivity of c-axis oriented MgB2 films as reported by Jin et al. [Phys. Rev. Lett. 91, 127006 (2003)]. Variations in the inter-band coupling constants with the level of disorder can account for the different Tc and sigma_1 behavior for the as-grown and ion-milled films. Our results suggest that enhanced inter-band scattering can improve microwave properties of MgB2 filims at low temperatures due to the larger pi-band gap despite the reduction of Tc., Comment: 8 pages, 9 figures (cond-mat/0403644 is replaced by this version)

The new superconductor MgB2 may prove useful for microwave applications at intermediate temperatures. MgB2 films with the thickness of 300 - 400 nm have surface resistance Rs less than 0.01 mOhms at a frequency (f) of 8.5 GHz and 7 K,and ~ 1.5 mOhms at 87 GHz and 4.2 K. The critical temperature (Tc) of these films is ~ 39 K when they are prepared under optimum conditions. The Rs appears to scale as f^2 up to 30 K. After surface ion-milling, a reduction of the Tc and an enhanced resistivity rho(Tc) are observed consistently at 8.5 GHz and 87 GHz along with a reduced Rs at low temperatures. The observed rho(Tc) - Tc relation and the uncorrelated behavior between rho(Tc) and Rs values measured at low temperatures are well explained in terms of the two-gap model, with the observed rho(Tc) - Tc relation attributed to the properties of the large gap, and the Rs at lower temperatures reflecting the properties of the small gap, with an enhanced small gap energy due to increased interband scattering. This study suggests that the interband scattering should be enhanced to improve the low temperature microwave properties of MgB2 films and that the ion-milling process must be performed with great care to preserve the high quality of MgB2 films., Comment: v3 of cond-mat/0208304 with typos corrected (attribution of the small and the large gap to the pi-band and the sigma-band, respectively, the Rs at 8.5 GHz and 7 K corrected to 0.01 mOhms), and a few words added in the abstract and the conclusion. 5 pages, 5 figures, presented at 2002 Applied Superconductivity Conference, Houston, U.S.A., Aug. 4-9, 2002

The microwave surface resistance Rs of MgB2 films with the zero-resistance temperature of - 39 K was measured at 8.0 - 8.5 GHz. The MgB2 films were prepared by deposition of boron films on c-cut sapphire, followed by annealing in a vaporized magnesium environment. The Rs appeared significantly reduced by ion milling of the as-grown MgB2 film surface, with the observed Rs of ~ 0.8 mohm at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the corresponding as-grown MgB2 film. The reduced Rs of the ion-milled MgB2 films is attributed to the effects of the Mg-rich metallic layer existing at the surfaces of the as-grown MgB2 films., Comment: 3 pages, 4 figures